Relative-age dating of transported regolith and application to study of landform evolution in the Appalachians

Geomorphology 67 (2005) 63 – 96 www.elsevier.com/locate/geomorph

Relative-age dating of transported regolith and application to study of landform evolution in the Appalachians Hugh H. Mills* Department of Earth Sciences, Tennessee, Technological University, Cookeville, TN 38505, USA Received 10 September 2003; received in revised form 2 August 2004; accepted 16 August 2004 Available online 7 January 2005

Abstract Surficial deposits in the Appalachians generally are thin, discontinuous, and difficult to date. In the absence of numerical dates, particularly for older deposits, relative-age indices based on degree of weathering and soil development have been used to distinguish and correlate deposits of different ages. A favorite locale of relative-age studies has been stream terraces, where chronosequences have been developed to shed light on the relationship between time and weathering/soil development. The lack of applicable numerical dating techniques has led some researchers to establish brelativeQ chronosequences in which height of stream or fan terraces above modern river level (ARL) serves as a proxy for time. Approximate incision rates can then be used to estimate numerical ages of the deposits. Relative-age correlations between the weathering indices on the terraces and comparable indices on dated deposits on the Coastal Plain and in nearby glaciated regions suggest that ages so estimated are the right order of magnitude. Relative-age indices show correlations with height ARL that are as high as those with time, perhaps because more data sets are available for relative chronosequences than for true chronosequences. Analogous relative-age sequences can be established for alluvial fans and even for hillslope colluvium. Many weathering indices have been employed, but weathering-rind thickness on mafic-to-intermediate clasts has proved to be a particularly consistent and useful parameter. In addition to showing how weathering and soil development progress over time, relative-age dating has shed light on the manner in which landforms evolve. Along some rivers, for example, the areal extent of old, highly weathered alluvium far exceeds that of younger alluvium, suggesting that formerly floodplains and low terraces were much more extensive than at present. In addition, the manner in which weathering indices on some river terrace sequences vary with height ARL suggests that incision rate has changed through time. Relative-age mapping of alluvial fans produces a map pattern of older and younger fan surfaces that allows the sequence of fan development to be inferred, in one case suggesting a scenario different from those previously described for fans. Mapping also shows that the relative abundance of young, intermediate, and old fan surfaces varies greatly from one area to another. Several studies have demonstrated clustering of relative-age values into a small number of groups, suggesting temporal grouping and thus episodic deposition on fans. Regression of weathering-rind thickness against height above modern drainageways of remnant fan surfaces in two areas suggests that fans are being incised more rapidly in one area than in another. Relative age of hillslope colluvium also aids geomorphic interpretation. Young brown colluvium is much more common than old red colluvium near the glacier border, where red colluvium rarely occurs at the surface but typically is covered

* Fax: +1 931 372 3363. E-mail address: [email protected]. 0169-555X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2004.08.015

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by brown colluvium probably mobilized during the late Pleistocene. Distant from the ice margin, however, red colluvium is extensively exposed on interfluves. Relatively stable hillslopes can also be distinguished from active ones in this manner. For example, colluvium on hillslope noses is somewhat more weathered than that on hollow floors and sideslopes. D 2004 Elsevier B.V. All rights reserved. Keywords: Relative-age dating; Transported regolith; Appalachians; Landscape evolution; Weathering

1. Introduction Surficial deposits in the Appalachians (Fig. 1) generally are only a few meters thick, patchy, and difficult to date. Concerning dating, Colman et al. (1987) suggested that the term numerical-age be used for dating methods that independently produce quantitative estimates of age, relative-age for those that provide only an age sequence and perhaps some measure of the magnitude between members of a sequence, and calibrated-age for methods that produce

quantitative estimates of age by means of calibration by independent chronologic control; many relative-age techniques lend themselves to such calibration. This terminology will be used herein. Because of the (until recently) lack of applicable numerical dating techniques, relative-age indices based on degree of weathering and soil development have been used to distinguish and correlate deposits of different ages. Such relative-age differences are commonly studied as a chronosequence, defined as a group of related soils that differ primarily as a result of differences in time as a

Fig. 1. Map of physiographic boundaries in the southeastern United States. The Appalachian provinces include the Piedmont, Blue Ridge, Ridge and Valley, Cumberland Plateau, Kanawha Appalachian Plateaus, and the Allegheny Mountains.

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soil-forming factor (Soil Science Society of America, 1996). The most common type of chronosequence is the postincisive one (Vreeken, 1975), in which there is a sequence of deposits of different age and the soils related to each deposit have formed from the end of the time of deposition to the present. Skepticism has been voiced concerning the utility of chronosequences, for factors of soil formation other than time may have been different for older soils in a chronosequence than for younger ones (e.g., Daniels and Hammer, 1992). Hunt and Sokoloff (1950), for example, in a study of deep red soils of the Rocky Mountains, concluded that former climate was the major factor affecting these soils, and considered time to be of minor importance. Birkeland (1999), however, argued that time commonly is a major factor in soil development. Where deposit ages are unknown, as is commonly the case in the Appalachians, a brelativeQ chronosequence can be determined by relating soil characteristics to a measure of relative deposit age independent of weathering and soil development characteristics. The most common example is height above modern river level (ARL) of stream terraces. The relatively flat gradient of rivers and their floodplains allows measurement of terrace height ARL to be readily determined. Analogously, relative chronosequences can also be established for alluvial fans and even for hillslope colluvium. The purpose of this paper is to discuss relative-age indices based on weathering and soil-development properties that have been used in the unglaciated Appalachians and adjacent physiographic provinces, compare both true and relative chronosequences from different locales within the region, and show how such relative-age data have promoted understanding of landform evolution in the region. The discussion is arranged according to type of landform and deposit. Study locations will be referred to by 1:24,000-scale quadrangle name. Relative-age indices based on weathering or soildevelopment properties used in the southeastern US for the most part are simply a subset of those used elsewhere in the world. As the general nature of these indices has been discussed elsewhere (e.g., Birkeland and Noller, 1998; Birkeland, 1999), discussion will concentrate on the indices commonly used and their particular applications and problems in the Appalachians.

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2. Stream terraces and relative-age dating Chronosequence construction for stream terraces is facilitated by the fact that the order of terrace ages is already known (i.e., higher terraces are older than lower terraces, at least within a given reach) and because stream terraces are more readily correlated than many other landforms. In the Appalachians, numerical ages obtained for terraces have until very recently been confined to terraces no older than late Pleistocene, with older terraces left largely unstudied (e.g., Delcourt, 1980; Leigh, 1996). Owing to the continuity of terraces and the physical relation of terraces to paleontologically dated marine units, more complete and better dated chronosequences have been developed for the southeastern Coastal Plain (e.g., Markewich et al., 1987, 1988, 1989; Markewich and Pavich, 1991). For this reason, and to help resolve the question of how weathering and soil development on Coastal Plain terraces compares with that on Appalachian terraces, the Coastal Plain studies are included herein. Recently progress has been made in dating older terraces in the Appalachians. Terraces of the Susquehanna River in the Piedmont and Valley and Ridge of Pennsylvania, with ages extending back into the Tertiary, have been dated petrographically by correlating them with dated Coastal Plain deposits (Pazzaglia and Gardner, 1993). The use of cosmogenic isotope burial dating also promises numerical dating of older deposits (Granger et al., 1997; Mills and Granger, 2002). These developments are making possible chronosequences with much older known ages than was previously possible in this region. 2.1. Stream terrace chronosequences Fig. 2 presents a plot of values of common indices of weathering and soil development that have been reported for dated terraces in the unglaciated Appalachians and southeastern Coastal Plain. One of the major problems in comparing such data is that researchers have used a diverse assortment of indices, so that there are few indices that have been measured in a substantial number of studies. It is thus difficult to compare the indices in Fig. 2 as to which provides the best correlation with age, since the incorporated studies vary from plot to plot. A productive way to examine the plots is to look at the trends for individual locales,

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Fig. 2. Soil development/weathering relative-age indices for stream terraces in the Appalachians and southeastern Coastal Plain plotted as a function of age: (A) Munsell hue; (B) Buntley–Westin color index; (C) thickness of B horizon; (D) maximum percent clay in the B horizon; (E) citrate-dithionate extractable iron (maximum for each soil profile). Ages are from radiocarbon dates for young terraces and from correlation with dated Coastal Plain terraces for old ones, except for the old New River terraces of Mills and Wagner (1985), which was dated by an incision rate determined from cosmogenic isotope burial dating of nearby cave sediments (Granger et al., 1997). The R 2 values were obtained using the logarithm of age. All parameters except maximum percent clay showed a correlation with age significant at the pb0.05 level.

rather than to compare the absolute values between locales. Note, however, that for several of the parameters, there is in fact reasonable consistency among locales as well. The most commonly measured index is color, and this index does seem to correlate fairly well with age (Fig. 2A and B). As pointed out by Leigh (1996), the

Buntley–Westin index (Buntley and Westin, 1965), which incorporates chroma as well as hue, seems to give better results than hue alone. The thickness of the B horizon (Fig. 2C) also seems to show a reasonable correlation. Thickness of the solum (i.e., combined thickness of the A and B horizon) and of the Bt horizon appear to correlate fairly highly with the B thickness,

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and give similar results. One advantage of using the Bt instead of the B thickness is that young soils frequently show a thickness of zero—i.e., no Bt. This often results in a higher correlation with age than in the case of the B and solum thickness, at least where the chronosequence includes young soils. For very old surfaces, with ages approaching 10 Ma or greater, however, Bt thickness may decrease. Howard et al. (1993), for example, reported this finding for high terraces of the James River in Virginia. They suggested that thinning of the Bt might result from the exceptionally high clay contents of these horizons (about 60% by weight), which so plugs the system that illuviation into the horizon terminates and degradation follows. The maximum percent clay in the B horizon might seem an improper index, as the most relevant criterion is how the clay percentage compares with that in the parent material. However, because of thin deposits and deep weathering, at many sites there is really no R horizon in the transported regolith, for weathering extends completely through the section to bedrock (or, more typically, to saprolite). Therefore, this index has been used out of necessity. As Fig. 2D shows, this parameter shows a low (not significant) correlation with age. If the Holocene soils from the Alabama Coastal Plain were omitted, correlation would be improved somewhat. Probably, the best way to estimate clay development at a given area, given the restraints discussed, is to compare the clay percentage in old terraces with that in young ones in the same terrace sequence. The maximum extractable iron (citrate-dithionite method), which presumably is the main cause of color change with age appears to show more scatter than do the color indices, although they do show a reasonably consistent increase with age (Fig. 2E). It might seem surprising that most parameters would show such consistency between study sites. The reason may be that the climate at all sites is relatively humid (though actual rainfall does vary substantially), so that climate is less of a factor than it would be if dry-climate sites were included. Whittecar and Duffy (2000) have expressed caution in comparing the characteristics of soils and weathering in the Appalachians with those in the Coastal Plain. This admonition seems prudent, but Fig. 2 suggests that there is no consistent difference between Coastal Plain and Appalachian soils, at least for the indices considered here.

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2.2. Terrace chronosequences based on terrace height as proxy for age The amount of data available for comparison of terrace sequences is substantially increased by including studies in which terrace height ARL is used as a proxy for age, and plotting the relative-age indices against height ARL. Fig. 3 is analogous to Fig. 2, except that the abscissa is height ARL rather than numerical age. One might expect that this plot would show more scatter than Fig. 2, as stream incision rate is not controlled for. Surprisingly, however, the plots show correlations that are approximately as high as those in Fig. 2. This result leads one to the conclusion that height ARL must be at least a fair approximation of age, and that incision rates do not vary greatly. (In fact, a plot of stream incision rates by Mills, 2000a suggests that this rate is similar throughout the eastern United States.) The only alternative explanation would be that somehow height ARL affects weathering and soil development. This possibility is probably true to an extent, for higher terraces are topographically situated to have better drainage than do lower terraces. This difference may be promoted by the bedge effectQ (Daniels and Gamble, 1967; Markewich et al., 1989), especially as the older terraces become dissected. However, this phenomenon would probably have less effect on weathering criteria less dependent on pedogenic processes and likely would not be able to mask the vast differences in terrace age. Dunford-Jackson (1978) has described soils on terraces of the Rappahannock River in the Piedmont of Virginia with apparent weathering intensities even greater than that shown in Fig. 3. For example, for terraces ranging from 25 to 60 m ARL, B horizons show a reddest hue of Munsell 10R and a maximum clay content of 93%. 2.3. Additional relative-age techniques The weathering and soil-development indices shown in Figs. 2 and 3 are by no means necessarily the best ones, but simply the ones that have been used by the largest number of researchers. Other, more sophisticated indices, particularly some of those used by Markewich et al. (1987, 1988, 1989) and Howard et al. (1993, 1995), may well prove to be superior and should be considered by those planning chronose-

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Fig. 3. Soil development/weathering relative-age indices for stream terraces in the Appalachians and southeastern Coastal Plain plotted as a function of height above modern river level: (A) Munsell hue; (B) Buntley–Westin color index; (C) thickness of B horizon; (D) maximum percent clay in the B horizon; (E) citrate-dithionate extractable iron. To simplify the plot of data from 35 sites of Mills and Wagner (1985), the sites were grouped by elevation into 10 categories. The R 2 values were obtained using the logarithms of height ARL. All parameters showed a correlation with height ARL significant at the pb0.05 level.

quence studies. A brief discussion of indices other than those shown in Figs. 2 and 3 follows. Markewich et al. (1987) found that the clay mass of the solum (weight percent clay times bulk density times horizon thickness of each horizon, summed for the pedon) showed a better correlation with age than did percent clay for Coastal Plain soils between 104 and 106 years in age. This technique requires

measuring density in addition to the particle size distribution. No other researchers in the region have reported these data for terrace chronosequences. Several studies have used the mineralogical composition of the sand fraction as determined with a petrographic microscope as a measure of degree of weathering. Mills and Wagner (1985), for example, found that for terraces of the New River the following

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parameters have a correlation of N0.75 (either positive or negative) with height ARL: feldspar/quartz ratio and percents amphibole, rutile, staurolite, quartz, and plagioclase. In the same area, Harris et al. (1980) also reported fairly high correlations of percents feldspar, quartz, mica, and heavy minerals with height ARL. Howard et al. (1993), investigating terraces on the inner Coastal Plain dating back to the Miocene, found that for terraces older than Pleistocene, only quartz and a small amount of zircon and tourmaline remained in the sand fraction. Making such mineral counts is demanding and time consuming, and is probably not worth the effort considering modern alternatives. In particular, bulk chemistry, expressed as weight percent of oxides, provides a much easier approach to the description of differential weathering of minerals with time. Markewich et al. (1987, 1989), for example, found that the (Fe2O3+Al2O3)/SiO2 ratio showed a good correlation with age in the 104 to 106 year BP range for Coastal Plain soils, and that this index appears to be the best chemical indicator of soil age. Leigh (1996) reported good correlations for both the bases/Al2O3 and FeO/ Fe2O3 ratios with terrace height ARL in the Blue Ridge province. These measurements can be made for each horizon and the maximum value selected as the index (Markewich et al., 1987), or (more rapidly) just for the most well-expressed part of the B horizon (Leigh, 1996). Another potentially useful chemical approach is to use the ratio of the citrate-dithionite extractable iron and the total iron, which partially controls for variation of iron in the parent material (e.g., Engel et al., 1996). Clay mineralogy using X-ray diffraction provides another approach. Harris et al. (1980), for example, found that in terraces of the New River, vermiculite and mica decrease with height ARL, with the reverse being true for hydroxy interlayered vermiculite (HIV), which increases with height. Markewich et al. (1987, 1989) reported that, for Coastal Plain soils, HIV increases relative to kaolinite and mica with increasing soil age. For soils exceeding 1 Ma, however, kaolinite increases, whereas the percentages of HIV and gibbsite decrease. For soils exceeding 10 Ma, Howard et al. (1993) reported that for terraces of the James River on the inner Coastal Plain of Virginia, kaolinite drops dramatically, accompanied by a marked increase in total Fe and Fe-oxide content.

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These observed tendencies of clay mineralogy to continue to change even with great age agree with observations by Harris et al. (1980), who noted that the mineralogy of the clay fraction appears to continue to change for even the highest (oldest) soils, in contrast to the sand mineralogy, which attains an equilibrium somewhat more rapidly with age and thereafter shows little change. Hence, clay mineralogy may be a particularly useful index for differentiating very old soils, although exactly which trends to expect appears uncertain and awaits further research. The use of clay mineralogy for relative-age dating also has other difficulties. Markewich et al. (1989) noted that clay mineralogy is strongly affected by provenance and climate, so that clay minerals are probably of limited use in making age correlations between different regions, such as between the Coastal Plain and the Appalachians. A new technique for studying relative age is the use of scanning electron microscopy of quartz sand. This technique has been used for decades to discriminate between depositional environments and sedimentary processes, but Howard et al. (1995) have shown that it can be used to distinguish terraces of different age, including very old terraces. Studying terraces of the James River on the Virginia Coastal Plain, they found that for terraces 60–120 ka in age, quartz grains showed little or no etching; for terraces 0.7–1.6 Ma, grains showed etching and pitting, with abundant reprecipitated silica; for 3.4–5.3 Ma, grains showed strong grain rounding, with solution pits interconnected by a network of grooves and crevasses; for 10.8 Ma, grains showed large, deep solution cavities and channels, with large-scale grain decomposition; and for 13.0 Ma, there was very large-scale decomposition, with deep grooves and channels, and perforated grains permeated with tubes and holes, with bulk grain decomposition occurring. (Interestingly, the features associated with grains on very old terraces had previously been considered to be produced only in the humid tropics.) Although SEM study is difficult to quantify, the observed qualitative differences appear dramatic. Note, however, that the method seems to be really useful only for terraces with ages of 1 Ma or greater. Pebble- or cobble-sized clast weathering has received little attention as a technique for relativeage dating of stream terraces in this region, although it has been used extensively in fan studies (see

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discussion below). Peltier (1949), p. 33, noted differences in sandstone weathering rinds in Pleistocene terrace deposits of the Susquehanna River. He reported that in Wisconsin deposits, clasts commonly have a single buff-colored rind about 3 mm thick. In older (Illinoian?) deposits, there is commonly an outer yellowish-brown rind about 3 mm thick underlain by a dark-brown or reddish-brown layer about 6 to 13 mm thick. In still some older clasts, he observed up to four zones of weathering. Peltier stated that the successive zones could be taken as a record of the weathering history of the pebble. However, these rinds have apparently received no further study. New River alluvial deposits show a systematic increase in the percent of decomposed sandstone clasts with increasing height ARL (Fig. 4). In the range of height ARL considered (90 m or less), quartz or quartzite clasts rarely show evidence of significant weathering. Howard et al. (1995) examined the weathering of quartzite clasts on dated Coastal Plain terraces of the James River, Virginia. On terraces of age 3.4–5.3 Ma, they found clasts to be largely intact with thin weathering rinds. On terraces of age 10.8 Ma, some clasts were disintegrating, and on terraces of age 13.0 Ma most quartzite clasts were disintegrat-

Fig. 4. Percent of weathered (breakable by hand) sandstone clasts in New River terrace deposits plotted against height of terrace above modern river level, southwestern Virginia (Mills, unpublished data).

ing. They also looked at quartzite clasts on the highest (undated) terraces of the New River in southwestern Virginia (same location studied by Harris et al., 1980) and of the Shenandoah River in the Elkton area in northwestern Virginia. They found the weathering state of quartzite clasts to be similar to that on the 3.4– 5.3-Ma terraces of the James River. Bell (1986) has provided an excellent discussion of the effect of bedrock lithology on terrace preservation. Alluvial deposits are preserved from surface erosion best on limestone (e.g., Houser, 1981; Mills and Wagner, 1985), on which most drainage is into minor karst features, thereby reducing the amount of surface runoff, but the terrace form is destroyed by sinkhole growth and solution collapse. Shale, on the other hand, preserves terrace form for long periods of time, but owing to the large amount of surface runoff associated with it, surficial deposits are removed relatively rapidly. Bell (1986) reported that the best lithology for terrace preservation is silty dolomite, which weathers into a saprolite that offers optimum resistance to both solutional lowering (i.e., much lower rate than pure carbonate) and to surface wash (in that surface drainage is into karst features). The best evidence for existence of a former stream, however, is probably the presence of alluvial materials, for other processes than stream action can produce flat, terrace-like surfaces. Commonly the last remainder of a stream deposit, after most of the alluvium has been stripped by erosion, is scattered quartz pebbles and cobbles overlying residual soils on the underlying bedrock. Just how long can such evidence survive after abandonment by the depositing stream? One approach to this question is to see how high ARL such gravels can be found, and then use regional incision rates to estimate ages. This was done by searching the literature and asking colleagues for unpublished data on the highest elevations ARL of quartz gravels in the Appalachian region. In Fig. 5, these maximum heights are plotted against location altitudes. The reason for such a plot was the expectation that stream incision might be more rapid where relief was greater, resulting in gravels at greater heights ARL, and I used altitude as a rough approximation of relief. As Fig. 5 shows, the locations with the highest altitudes do indeed show the greatest height ARL, but most of the points fall along a horizontal line defining an envelope at about 140 m ARL, with only the two New

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2.4. Evidence for past stream behavior from relative-age terrace studies

Fig. 5. Height above modern river level (ARL) for the highest reported gravel deposits in the Appalachians, plotted against altitude: (1) Susquehanna River, Holtwood, PA, Piedmont province (Pazzaglia and Gardner, 1993); (2) Green River, Flint Ridge, KY, Interior Low Plateau (Miotke and Palmer, 1972); (3) Cumberland River, Burristown quad, TN, Interior Low Plateau (Lusk, 1928); (4) Tennessee River, S. Cleveland quad, TN, Valley and Ridge (Swingle, 1959); (5) S. Fork Shenandoah River, Elkton, VA, Valley and Ridge/Blue Ridge (King, 1950); (6) French Broad River, Hot Springs quad, NC, Blue Ridge (Wiener, unpublished data); (7) Jackson River, Falling Springs quad, VA, Valley and Ridge (Sherwood and Campbell, unpublished data); (8) French Broad River, Weaverville quad, NC, Blue Ridge (Mills, unpublished data); (9) New River, Peterstown quad, WV, Appalachian Plateau (Mills, unpublished data); (10) New River, Eggleston quad, VA, Valley and Ridge (Houser, 1981).

River points from southwestern Virginia differing greatly. The most logical interpretation is that there is no correlation between altitude and height ARL, but rather there is something unique about the New River locations. As the New River incision rate is average for the Appalachians (Granger et al., 1997), some other factor must be involved. Perhaps, it is the fact that carbonate, which favors preservation of deposits, occurs at unusually high elevations along the course of the New River in southwestern Virginia. However, point 9 occurs on sandstone. Thus, there is a quandary concerning the New River points, but it is probably most reasonable to ignore these points and focus on the envelope at 140 m ARL. The fact that maximum elevations ARL seem independent of altitude suggests that incision is fairly constant throughout the area. Using a regional denudation rate of 30 m/my (e.g., Hack, 1979; Matmon et al., 2003), we find that quartz gravel deposits in the Appalachians have a maximum age on the order of 4.67 Ma, although the New River deposits may exceed 10 ma.

Weathering indices measured at 35 sites along the New River in the Radford North and Eggleston quadrangles in southwestern Virginia suggest that the incision rate has varied over time during the past 50 m of incision. A statistical analysis of terrace elevations showed that alluvial deposits are more abundant at 0– 12 and 30–49 m ARL, whereas the 12–30-m interval has relative few deposits (Mills and Wagner, 1985). Weathering/soil development indices indicate a dramatic increase in weathering intensity between about 20 and 30 m ARL, corresponding to the break between lower and higher terraces. Three examples are shown in Fig. 6. The concentration of alluvial surfaces at 30– 49 m ARL may represent a time of relative stability during which broad floodplains and terraces were formed. The gap in terrace-elevation frequency between 12 and 30 m similarly may correspond to a subsequent interval of downcutting during which only a few narrow terraces were formed. Elevations below 12 m ARL correspond to the modern floodplain and to the relatively young low terraces (Fig. 7). The extent of high-level terraces is even more widespread in some other areas in the Appalachians. For example, in the Valley and Ridge of western Polk County, southeastern Tennessee, an area of about 100 km2 is covered by alluvial soils, with 85% consisting of the Waynesboro series, which forms on intensely weathered high-level terrace deposits (Newton and Moffitt, 2001). The area of modern floodplains and low terraces is miniscule relative to the area of the highlevel deposits (Fig. 8). Field inspections show rounded vein-quartz clasts, which could have been derived only from the Blue Ridge province to the east (i.e., they could not have been supplied from local bedrock) to be very common in this soil, supporting the alluvial interpretation. The terrace deposits are not flat, but overlie rolling topography underlain by carbonates. Perhaps, these extensive terraces reflect past tectonic and climatic conditions that favored much broader floodplains than in more recent times. Such a past history of relative stability, if true, might help explain why there appears to be a long-term increase in stream incision over the past few million years (Mills, 2000a). On the other hand, perhaps the high terraces are so widespread simply because they represent a

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much greater age range than do the lower terraces, the greater time span allowing greater geographic range in the course of the rivers. The spatial distribution of soils in Fig. 8 also suggests possible changes in drainage courses. Note that the divide between streams draining directly to the Gulf of Mexico and those draining to the Mississippi via the Tennessee River passes across high-level terraces linking the Ocoee and the Conasauga Rivers. Although this interfluve area today is drained only by small tributaries of the Ocoee and Conasauga, the high-level deposits suggest that a much larger river flowed here in the past. Perhaps the Conasauga once flowed north to join the Ocoee and Hiwassee rather than following a southwestward course to the Gulf as it does today.

3. Alluvial fans/pediments and relative-age dating 3.1. Fans and pediments in the Appalachians Alluvial and debris-flow deposits, ranging in area from less than a hectare to several square kilometers in area, occur on many mountain piedmonts and make up perhaps the most prominent surficial deposits in the unglaciated Appalachians. Some of these deposits are alluvial fans, although a substantial portion of them would best be termed pediments, not in Howard’s (1942) sense of consisting of bare rock or being mantled by a layer of alluvium not exceeding in thickness the depth of stream scour during flood, but in the more inclusive sense of Schumm (1977, 1980) and Carter (1981). Carter, for example, described pediment remnants along the Book Cliffs, Utah, with gravel caps 40 m thick. Gravel caps are rarely this thick on Appalachian piedmont slopes. For brevity, all such deposits will herein be referred to as fans. Appalachian fans are commonly irregular in shape relative to their larger cousins in the Basin and Range province, for example, owing to greater topographic constraint than in the latter setting (Fig. 9) Fan thickness ranges from several meters to tens of meters, Fig. 6. Examples of New River weathering indices as function of terrace height above modern river level for the New River in southwestern Virginia. (A) Buntley–Westin color index; (B) heavy mineral weathering index; (C) feldspar/quartz ratio index. Break between younger and older terraces is shown by dashed line.

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Fig. 7. Schematic cross section of New River Valley in Radford North and Eggleston quadrangles, showing broad, high-level terraces contrasted with narrow, low-level terraces. Note terraces are unpaired.

and, in the large majority of fans, deposits consist mainly of debris-flow sediments. Fans have been most studied in the Blue Ridge province, or in the immediately adjacent Piedmont province to the east

or Valley and Ridge province to the west (e.g., Kochel, 1990). Fans in the latter setting, particularly where the Central Blue Ridge sheds voluminous debris into the Shenandoah Valley, are nearly unique,

Fig. 8. Alluvial deposits in western Polk County, southeastern Tennessee, as interpreted from NRCS soils map.

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Fig. 9. Topographic maps of representative fans/pediments in the unglaciated Appalachians. Altitudes are in feet. Black indicates bedrock outcrop or saprolite. Uphill direction is toward top of page. After Mills (2000b).

comprising the largest fans in the Appalachians and consisting largely of alluvium or hyperconcentrated flows, in contrast to the debris flows that cover piedmont slopes elsewhere in this province. Appalachian fans were described and mapped by bedrock mappers before being studied by geomorphologists (e.g., Hamilton, 1961; Hadley and Goldsmith, 1963; King, 1964; Neuman and Nelson, 1965). More than 30 fans were included on Hadley and Goldsmith’s (1963) map of the Dellwood quadrangle, North Carolina. The first geomorphologists to study these features were Hack and Goodlett (1960) and Hack (1965) and Michalek (1968) in the southern Appalachians. An understanding of the genesis of the fans has been a major goal of researchers. Observing that many of the fans appeared inactive, earlier researchers assumed that they were relicts of the ice ages. However, the

catastrophic flooding in central Virginia associated with Hurricane Camille in 1969 showed that fans can undergo deposition today (Williams and Guy, 1973). By radiocarbon dating, Kochel and Johnson (1984) and Kochel (1987) demonstrated that events similar to that set off by Hurricane Camille had occurred several times during the Holocene in Nelson County, Virginia, indicating a recurrence interval for deposition of not more than 3000–4000 years. Kochel (1987) suggested that catastrophic floods and concomitant flood deposition might even be largely confined to the Holocene. The most intense rainfalls are usually associated with tropical air masses, and Delcourt and Delcourt (1984) suggested that during glacial climates the polar front is so far south that tropical air masses can only rarely penetrate to the latitude of even the southern Appalachians, so that large floods during these intervals are probably rare. Kochel (1987) reported that the concept of Delcourt (1980) and Delcourt and Delcourt (1985) (that maximum deposition on mountain piedmonts occurs at the transition from glacial to interglacial intervals) was compatible with the evidence from Nelson County, where radiocarbon dates from the bases of fan sequences range from 13,170 to 10,800 year BP. However, more detailed work involving extensive dating in Madison County, Virginia, by Eaton et al. (2001, 2003) showed that debris flows were as common during the late Pleistocene as during the Holocene. Further, the latter authors reduced the recurrence interval of debris flows in the region, reporting it to be not greater than 2500 years. Although glacial/interglacial cycles probably dominate landform evolution during glacial intervals, over longer time spans other processes may become important. Although a fan surface may be aggraded over limited intervals of time resulting in burial of older deposits by younger, over intervals of several hundred thousand years or more degradation inevitably exceeds aggradation, given the relatively low rate of sediment supply in the Appalachians. Surfaces of fans thus become permanently abandoned as streams incise so deeply that deposition becomes impossible even during the most intense precipitation. Lateral erosion of these relict fan surfaces then produces new fan surfaces at lower levels (e.g., Mills and Allison, 1995b,c). These long-term changes occur despite the superimposition of shorter-term changes produced by extreme floods and climatic cycles.

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Research on sedimentology and depositional processes of Appalachian fans has focused on the most recent generation of fans, probably of late Pleistocene and Holocene age. The surfaces of these fans generally stand no higher than several meters above present drainageways and show brown soils and hard clasts. Organics for radiocarbon dating are rare in these fans. In addition, however, there are other fan

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surfaces that stand well above modern drainageways and show red soils and decomposed clasts. These relict surfaces for the most part are beyond the range of radiocarbon dating. Such older surfaces were first mapped by Hadley and Goldsmith (1963). More recent mapping (Mills, 1983; Whittecar and Ryter, 1992; Whittecar and Duffy, 2000; Mills and Allison, 1995a, 1995c; Bierman et al., 2002; Eaton et al.,

Fig. 10. Stereo aerial photos of fan/pediment surfaces in Zionville study area, Watauga County, North Carolina. Surfaces labeled L are lower, younger surfaces; those labeled H are higher, older surfaces.

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2003) has shown such old surfaces to be very common. Examples of fan surfaces at different levels are shown in Fig. 10. 3.2. Relative-age dating of fans and pediments As numerical dating of Appalachian fans has remained limited owing to a paucity of applicable techniques, particularly for older deposits, relative-age measurements based on degree of weathering and soil development have provided a next-best means of studying changes in fans through time. This method allows fan surfaces, at least within a given area of relatively homogeneous topography and geology, to be placed approximately in order of numerical age. This in turn allows mapping of fan surfaces and inferences concerning long-term fan development. In some cases, relative-age dating allows rough estimates of numerical age by comparison of weathering or soildevelopment on deposits of unknown age with those on deposits of known age (e.g., Whittecar and Duffy, 2000; Mills and Allison, 1995b). Weathering and soil-development indices used for relative-age studies of fans are, in general, similar to those used for stream terraces. One technique little used for terraces but extensively used for fans is clast weathering. (This technique should work as well for terraces as for fans, but probably has been used more in fan studies owing to the abundance of clasts in most fan deposits.) For lithologies that decompose into constituent grains during weathering (e.g., granite), an appropriate technique for describing the degree of weathering is to classify each clast into one of several predefined weathering categories. A five-part classification (described by Whittecar and Duffy, 2000) is used here, in which a clast is assigned a rating of 1 if it is hard and fresh, 2 if it shows signs of weathering but is difficult to break with a hammer, 3 if it can be broken easily with a hammer but not by hand, 4 if it can be broken into small pieces by hand, and 5 if it is so decomposed that it can be removed from the outcrop only with difficulty. Other techniques used on this type of rock are listed by Birkeland and Noller (1998). For lithologies that develop weathering rinds, weathering-rind thickness provides a readily quantifiable weathering index. Basalt and andesite rinds have been extensively used for relative-age dating in the

western United States (e.g., Colman and Pierce, 1981). These lithologies are not available in much of the Appalachians, but coarse-grained mafic metamorphic rocks such as amphibolite also show good rind development, although rinds are not as sharp as those on fine-grained rocks (Fig. 11). Table 1 summarizes data on relative-age indices associated with soil-profile descriptions that have been most widely used for fan studies in the Appalachians, and Table 2 summarizes relative-age data measured at other sites where soilprofile descriptions were not made. These data can be compared to those in Fig. 2 for stream terraces. For the old fan sites, hue, maximum percent clay, thickness of B horizon, and maximum extractable iron generally

Fig. 11. Photograph of amphibolite weathering rinds. (A) In-place amphibolite clast from Bakersville quadrangle, Mitchell County, North Carolina. Arrow points to inner boundary of rind. Matrix consists of intensely weathered fan debris with 54% clay in b2-mm fraction. (B) Rinds from Zionville quadrangle, Watauga County, North Carolina.

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Table 1 Soil profile and weathering index summaries for Appalachian fans Location (quadrangle)

Relative age of surface

n

Reddest Munsell hue (YR)

Maximum % clay in B horizon

Thickness of Bt horizon (cm)

Clast weathering (scale 1–5)

Stuarts Draft and Big Levels, VA (Whittecar and Duffy, 2000, 1992) Sherando, VA (Ryter, 1989; Whittecar and Ryter, 1992) Sunset Village and Warm Springs, GA (Markewich et al., 1994) Bakersville, NC (Mills and Allison, 1995b)

Y I O Y O O

4 14 4 4 4 2

8.1 5.5 3.0 5.6 3.1 2.5

(1.4) (1.2) (1.1) (1.1) (1.1) (0.0)

17.3 32.7 33.3 24.0 36.8 41.0

(7.4) (9.0) (10.5) (1.6) (4.3) (15.1)

98 104 158 63 51 80

(56) (32) (73) (15) (13) (4.2)

1.6a (0.3) 2.7 (0.3) 4.5 (0.3) – – –

Y I O Y I O Y I O

9 5 6 3 1 3 2 7 2

7.6 5.5 5.0 6.7 7.5 4.2 7.5 6.1 2.5

(2.0) (1.1) (1.6) (1.4) (–) (1.4) (0) (1.3) (0)

30.7 43.0 52.2 18.7 39.0 56.0 21.5 23.7 40.0

(14.1) (12.2) (8.8) (16.7) (–) (8.7) (4.9) (9.3) (4.2)

56 114 133 19 47 89 44 14 94

(54) (56) (48) (33) (–) (27) (20) (19) (118)

– – – – – –

Zionville and Sherwood, NC (Mills and Allison, 1995c) Alarka, Franklin, and Bryson City, NC (Liebens and Schaetzl, 1997; Liebens, 1999)

2.6d (0.11) 3.4 (0.1) 4.1 (0.4)

Weathering rind thickness (mm)

Maximum % free iron

– – –

– – – 1.6 (0.1) 2.2 (0.4) 4.1 (0.3)

4.3b (1.1) 9.0 (2.2) –

0.9c (0.7) 2.4 (0.8) 9.5 (3.0) 0.1c (0.0) 1.6 (–) 8.9 (2.0) –

4.8 5.8 6.8 1.9 3.8 4.7 1.12 2.28 3.26

(1.7) (1.2) (2.0) (0.2) (–) (0.8) (0.17) (1.50) (0.80)

Standard deviations are in parentheses. a Antietam quartzite. b Greenstone. c Amphibolite. d Scale of 1–3 was used, and scores have been transformed into comparable scores on a 1–5 scale.

suggest ages between 105 and 106 year BP, although, as discussed above for terraces (and discussed elsewhere for fans—see Whittecar and Duffy, 2000, and Mills and Allison, 1995b), such estimates are tenuous. Concerning maximum numerical ages for fans, Mills and Allison (1995c) reported reversed magnetism in one old fan remnant in Zionville quadrangle, North

Carolina. As this magnetism appeared to reside in secondary hematite, a minimum age on the order of 1 Ma was suggested. More recently, cosmogenic isotope burial dating indicated an age of 1.4 Ma for this fan (Mills and Granger, 2002). For the very old fans at Pine Mountain, Georgia, Markewich et al. (1994) suggested an age of latest Miocene to Pliocene, no

Table 2 Additional soil/weathering data from North Carolina fans Location (quadrangle)

Relative age of surface

Reddest Munsell hue (YR)

Maximum clay in B horizon

Mean clast weathering (scale 1–5)

Weathering rind thickness (mm)

Dellwood, NC (Mills, 1982 and unpublished data) Hazelwood, NC (Mills and Allison, 1995a)

Y O

9.8 (0.8), n=7 5.7 (2.8), n=7

23.0 (9.3), n=7 32 (9.4), n=21

2.0 (0.4)a, n=14 3.7 (0.5), n=7

– –

Y I O Y I O

8.3 6.1 3.0 8.2 6.4 5.0

18.6 30.9 33.1 22.9 35.4 50.3

1.9 (0.7)a, n=7 2.5 (0.5), n=14 3.7 (0.5), n=13 – – –

– – – 0.3 (0.2)b, n=26 1.4 (0.7), n=26 5.0 (2.2), n=28

Bakersville, NC (Roan Mountain) (Mills, 1983, 1982)

(1.3), (1.6), (0.8), (1.1), (1.3), (1.6),

n=9 n=20 n=25 n=26 n=26 n=28

(7.9), n=9 (7.5), n=13 (9.4), n=26 (11.8), n=26 (6.7), n=26 (11.4), n=28

Parameter values shown are mean values. Standard deviations are in parentheses. a Schist and gneiss. b Amphibolite.

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younger than early Pleistocene. If this estimate is correct, it suggests that ages inferred from Fig. 2 probably do not overestimate the actual ages of the oldest fan deposits. Not only are comparisons between the fans and stream terraces tenuous, but even comparisons among the fan areas must be considered with caution, given the differences in bedrock lithologies, in criteria used to designate the relative age of a surface, and in measurement techniques employed. In particular, the fans near Stuarts Draft in the Shenandoah Valley (Whittecar and Duffy, 2000) and near Pine Mountain (Markewich et al., 1994) contain large amounts of quartzite, whereas fans in other areas contain materials derived from metamorphic lithologies with somewhat lower quartz contents and higher contents of labile minerals. In addition, the fans at Stuarts Draft are fluvially dominated, whereas fans at most other locations are debris-flow dominated. Given these differences, the mean thickness of the argillic horizon (158 cm) on the old surfaces in the Stuarts Draft and Big Levels area, the very red mean hue (3.0 YR), and the highly weathered state of the quartzite clasts at this locale (mean of 4.5 on a scale where 5 is totally decomposed) suggest that this surface has undergone more intense weathering and soil development than the other sites in Tables 1 and 2, with the possible exception of the old fans at Pine Mountain which show very red hues and very thick B horizons. The data thus probably indicate a somewhat greater age for the fans at these two study areas. Other differences between fans that stand out are the high amounts of free iron measured at Bakersville (Roan Mountain) relative to the Sherando and Zionville/Sherwood study areas. This might be attributed to the high content of mafic rocks in the fan debris, but in fact the amphibolite content is much higher in the Zionville/Sherwood than in the Bakersville fans. An explanation is not obvious. The large difference in weathering rind thicknesses at the Bakersville study area shown between Tables 1 and 2 reflects a change in the method of measurement. In the earlier measurements (Table 2), the average thickness of the rind was measured. In the later measurements associated with the soil profiles (Table 1), the thickness on the side of the clast having the thickest rind (excluding corners) was measured.

Given that the fan age assignment is largely subjective, probably the best use of the data in Tables 1 and 2 is to evaluate differences between Y (young), I (intermediate), and O (old) surfaces at each study area. The magnitude of these differences can then be compared between study areas. For example, the greatest degree of rubification seems to have occurred at the Stuarts Draft site (Table 1: 8.1 YR on the Y surface to 3.0 YR on the O surface). The greatest increase in clay is seen on the Zionville/Sherwood fans (Table 1: 18.7% on the Y to 56.0% on the O). However, the latter result seems to be misleading, probably owing to the small sample size, for the larger Zionville/Sherwood sample in Table 2 shows a much smaller difference. Using data from both tables, the increase in clay appears to be greatest in Bakersville. This and several other study areas show an approximate doubling of percent clay from Y to O. Note, however, that for sites with three categories of surface age, several show only a small increase in clay from I to O. In Table 1, the percent clay goes only from 32.7 to 33.3 for Stuarts Draft, and in Table 2, it goes from 30.9 to 33.1 for Hazelwood and from 35.7 to 36.1 for Zionville/Sherwood. How can this lack of increase with age be explained? One observation that may bear on this problem is that with increasing age, large numbers of clasts decompose. This decomposition produces sand-sized material which, as it mixes with the surrounding soil matrix, decreases the percentage of clay in the soil by increasing the percentage of sand. Because clasts comprise perhaps as much as half or more of the volume of the deposits, this effect is a major one. Comparing clast weathering based on the 1–5 scale, typically the mean index approximately doubles from Y to O surfaces. The greatest increase, from 1.6 to 4.5, is seen at the Stuarts Draft location. Comparing weathering rinds, thickness increases at least by a factor of 10 at two sites, but only by a factor of 2 at Sherando. The 4.3-mm rind thickness on Y surfaces suggests that even the Y surfaces have a substantial age at this location. Colman and Pierce (1981) have described a simple means of determining limiting age ratios from weathering rind thickness. It is based on two simple and reasonable assumptions. First, a linear relationship between rind thickness and time, d=kt (where d is rind thickness, t is time, and k is a constant) provides a

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minimum estimate of age, for the rate of rind thickening almost certainly decreases with time. Second, an exponential relationship, d=kt 1/n (or t=kd n ), where n=2, probably provides a maximum age as indicated by empirical data on weathering reactions. Thus, the rind ratio squared is probably near the upper end of the age range. For example, from the rind data in Table 1, for Bakersville the age of the O surface is at least 4.0 times older than the Y (9.5/2.4) and not more than 16 times older (42). 3.3. Fan chronosequences based on height as proxy for age Only two studies have attempted to relate height of Appalachian fan surfaces above the modern drainageways to weathering and soil-development indices in a manner analogous to that discussed for stream terraces. For stream terraces, the method is straightforward: simply subtract the elevation of the stream low-water level from the elevation of the terrace. There is some measurement error, but this difference can be measured with an error of less than 1 m. For alluvial fans and pediments, the method is less obvious. There are two approaches. The first applies to fans that debouch into the valley floor of a higher-order bmasterQ stream. Modern fan surfaces generally are graded to the master stream, or at least to the master-stream floodplain. The height of older fan surfaces can be measured by projecting the longitudinal profile of the fan surface over the present-day master stream, and then measuring the vertical distance between the low-water elevation of the master stream and the profile (Mills and Allison, 1995b). This method does not work for fans that terminate before reaching the master stream. For these fans, all that can be measured is the height of the surface above the lowest laterally adjacent drainageway, albeit this drainageway may contain only an ephemeral stream. This approach, on the face of it, seems inferior to the first, in that instead of measuring heights relative to a single (or a small number of) master streams, one is measuring heights relative to a large number of streams of diverse sizes. The first study relating weathering and soildevelopment indices to fan height was done in the Bakersville quadrangle, using a bheight indexQ that

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essentially is the mean of the two above methods. At this locale, 20 soil profiles were described, so that it is possible to plot weathering indices against height ARL in a manner analogous to that done for stream terraces (Fig. 12). One difference between these data and those for stream terraces in Fig. 3 is that whereas the stream-terrace weathering parameters showed a higher correlation with the log of the height, the fan parameters latter showed a higher correlation with the untransformed height. All the correlations shown in Fig. 12 are significant at the pb0.05 level, although generally correlations tend to be lower than those shown in Fig. 3. Note that the weathering-rind thickness has by far the best correlation with height. One parameter shown here but not in the tables is mean particle size, based on all particles 256 mm or smaller. This parameter probably increases with height and age due to the increased weathering of clasts, which decreases the number of large clasts and increases the amount of fines. Further comparing Fig. 12 with Fig. 3, if we look at a height of, say, 20 m ARL, the fan weathering indices appear to indicate greater age than do those of the stream terraces. Whether this difference stems from the different parent materials in the Bakersville fans and the stream terraces, or whether it results from slower downcutting of the fan-associated streams is unknown. A second effort to compare fan height and weathering indices was carried out in the Zionville quadrangle (Mills and Allison, 1995c). In this setting, most of the fans terminate short of the master stream of the lowland into which they debouch, and height ARL was measured by the second method only. That is, transverse profiles were surveyed across each fan surface and the maximum vertical distance between the fan surface and the laterally adjacent drainageway then measured. Rind thickness shows a correlation with height ARL analogous to that in Bakersville, although the relationship is weaker. One other relative-age index not shown in the tables or plot is the maximum surface boulder size, found by measuring the intermediate diameter of the five largest boulders and taking the mean. This was measured at Bakersville (Mills and Allison, 1995b) and showed a fairly good negative correlation with height ARL. Basically, this correlation reflects the tendency of surface boulders to weather and break down over time.

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Fig. 12. Soil development/weathering indices as function of height above modern drainageways for alluvial fans along the southern and western slopes of Roan Mountain (Bakersville), North Carolina (from Mills and Allison, 1995b): (A) Munsell hue; (B) maximum percent clay in the B horizon (or horizon if no B); (C) thickness of Bt horizon; (D) maximum citrate-dithionate extractable iron; (E) mean particle size (in /, where / is the negative log to the base 2 of the particle diameter in millimeters); (F) mean weathering rind thickness on amphibolite clasts, in millimeters. In contrast to Figs. 2 and 3, height was not log transformed when computing R 2 values. All correlations are significant at the pb0.05 level.

Harden (1982) developed a quantitative index of soil development which she successfully used as a relative-age indicator that can be correlated with numerical age (e.g., Harden and Taylor, 1983; Harden and Matti, 1989). Mills and Allison (1995b) devised an analogous index (which included weathering indices as well as soil-development indices) for use with the Bakersville fans. It showed a R 2 coefficient of 0.688 with fan-surface height ARL. However, this value was actually lower than the coefficient for rind thickness alone (0.740). In addition to changes in weathering and soil development, fan landform changes also occur over time. Peterson (1981) observed that in the Basin and Range province, abandoned fan surfaces at first are

relatively flat with steep risers. With increasing age, however, these remnants become rounded. Their erosional shoulders eventually meet from either side to form a broadly and continuously rounded crest. Peterson (1981) used the term ballenas (Spanish for whales) for such features (Fig. 13). Birkeland (1999) used Peterson’s (1981) sequence in his discussion of soil age, in which he points out that the maximum age of the soil is related to how long the landform upon which the soil forms can survive. On the fan sequence, younger soils are formed on relatively flat and stable surfaces. With time, these surfaces become slightly dissected, sufficient to erode the upper parts of the soils in places on the surface. Finally, the ballena stage is reached, in which all original depositional surfaces are

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Fig. 13. Sketch of alluvial fan complex development in Great Basin (Peterson, 1981).

gone, and the soils are highly eroded. Erosion becomes more rapid than soil development, and the apparent development thus reverses as the soil thins (regressive mode of Johnson and Watson-Stegner, 1987).

A similar erosional sequence can be seen on Appalachian fans. Fig. 14 is a transverse profile surveyed across a fan complex in the Hazelwood quadrangle, North Carolina, showing that older fan

Fig. 14. Transverse topographic profile across Hazelwood fan remnants, showing weathering/soil indices for three different fan levels.

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remnants are not only higher ARL but also more convex. Properties of three soil profiles are shown on the figure (from Mills and Allison, 1995a). As the older surfaces become more convex, the soils undergo greater surface erosion, with the result that the soil at site 41, the highest remnant, is actually thinner than that at sites 42 and 115. This example illustrates why soil thickness (solum, B, or Bt) may not be as good a relative-age index as some other techniques that are less affected by erosion. In contrast, for example, the hue in the B horizon becomes redder with increasing height of the three fan remnants, in accordance with probable increasing age. Of the areas studied by the author, the area with the most dramatic changes in transverse profiles is the Zionville quadrangle in Watauga County, North Carolina (Fig. 15). Note that age (as indicated by weathering rind thickness) increases with convexity. On the most convex surface, all fan debris has been stripped off, revealing saprolite. Only a small number of soil profiles were described in this area, so the

Fig. 15. Transverse topographic profiles of fan surfaces compared with relative age of surface as indicated by mean weathering-rind thickness on crest, Zionville and Sherwood quadrangles, North Carolina and Tennessee. Rind-thickness class for each surface is shown. The saprolite surface had no fan debris, the mantle of debris apparently having been removed by erosion. Vertical exaggeration is 3.0 (after Mills and Allison, 1995c).

relationship between topography and soil characteristics cannot be addressed. Profile convexity may increase faster than elsewhere because of two characteristics of fan deposits here. First, the deposits are relatively thin. The result is that the flanks of ballenas commonly consist largely of saprolite, which is probably more subject both to lateral erosion by streams and to hillslope erosion than is bouldery fan debris. Second, the size of the boulders in the deposits is somewhat smaller than at many other fan areas in the Appalachians. This may allow hillslope processes such as creep and gelifluction to operate more effectively than where boulders are large. Fan remnants in the Bakersville quadrangle, which have relatively thick debris mantles containing large boulders, show much less tendency to profile rounding and maintain relatively flat surfaces for long intervals of time. 3.4. Evidence for fan/pediment evolution from relative-age studies (1) Data on fan surface heights ARL and weathering rind thickness can be used to compare the relative rate of fan incision in the Bakersville quadrangle with that in Zionville/Sherwood, even though the absolute rate must await further numerical dating. The only assumption is that the rate of weathering rind thickening on amphibolite is the same in the two areas. Fig. 16 shows regressions of rind thickness on height ARL for each of the areas. These show that for a given height ARL, rinds are thinner in Zionville. For example, for a height of 20 m, the regression equations yield a 6.4-mm rind thickness for Zionville/Sherwood, vs. 11.6 mm for Bakersville. This indicates that downcutting and abandonment of fan surfaces takes place more rapidly at Zionville/Sherwood than in Bakersville. However, although both correlations are significant at the pb0.05 level, a statistical test (Kleinbaum and Kupper, 1978) shows that the regression line slopes are not significantly different, so that this finding can be considered suggestive only. (2) Analysis of relative-age data suggests that fan surfaces tend to cluster into distinct groups, rather than forming a continuum from least weathered to most weathered. For example, Mills and Allison (1995b) used cluster analysis to demonstrate the

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Fig. 16. Mean weathering rind thickness on amphibolite clasts as a function of height above modern drainageways for both Bakersville (Roan Mountain) and Zionville/Sherwood fans, North Carolina (from data in Mills and Allison, 1995b,c, and Mills, unpublished data).

existence of three groups at Bakersville quadrangle, North Carolina, and Whittecar and Duffy (2000) used canonical discrimination to do the same for fans in Stuarts Draft and Big Levels quadrangles, Virginia. Liebens and Schaetzl (1997) likewise used discriminant analysis to demonstrate the existence of three groups of fans in Alarka, Franklin, and Bryson City quadrangles, North Carolina. It is likely that some of these groups consist of multiple ages and that there are actually more than three groups, but at least these results indicate that formation of fan surfaces is episodic. (3) Mapping of relative-age of fan/pediment surfaces provides evidence for the manner in which fan and pediment complexes evolve. For example, Fig. 17 shows age patterns that have been reported for fan complexes. A is a btelescopicQ pattern produced by fanhead incision and unidirectional fan progradation (Dorn, 1988). Note the paired terraces. B is a concentric pattern with decreasing age toward the apex. Bull (1964) attributed this pattern to intermittent uplift of the mountain front. Whittecar and Duffy (2000), however, have pointed out that it might also

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Fig. 17. Four hypothetical age patterns for alluvial fan surfaces produced by different fan development scenarios (from Mills, 2000b). The larger the number, the older the surface. See text for discussion.

be caused by a long-term decrease in tectonic activity. They observed this pattern on some fans in Augusta County, Virginia, and suggested that it might reflect a Miocene episode of crustal uplift that was greater than any subsequent uplift. C is a more-or-less random fan pattern produced by piedmont stream capture. This mechanism was first described by Rich (1935) for pediments along the Book Cliffs, Utah (Fig. 18). It operates where pediments formed on less-resistant rock lie adjacent to uplands composed of more

Fig. 18. Basic piedmont-stream capture mechanism, after Rich (1935) and Carter (1981).

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resistant rock. The upland stream, carrying coarse fragments from the resistant caprock, flows on a steeper gradient and at a higher level than adjacent stream A which heads on the soft-rock piedmont and carries a fine load. Because of the elevation difference between the two, stream A eventually captures the upland stream. Alluviation then begins in the A vicinity, and eventually A comes to stand higher than the adjacent piedmont, and likewise becomes subject to capture by piedmont streams such as Y. This mechanism has also been described by Hack (1960, 1965) and Denny (1967), and Mills (1983) and Whittecar and Duffy (2000) have recognized examples in the Appalachians. Pattern D in Fig. 17 has not been described before, but an example is found on the western slopes of Rich Mountain on the Zionville/Sherwood quadrangles (Fig. 19). Fan remnants are elongate downslope, approximately parallel to streams heading in the uplands, with fairly constant widths. The cross-slope order of surface ages is essentially random (i.e., no paired terraces). The pattern, together with observation of present erosional activity of fan streams, suggests that the process responsible for abandonment of a fan surface and establishing another at a lower level operates chiefly in the cross-slope direction. Apparently, new fan surfaces are created by a process of stream entrenchment accompanied by lateral

erosion and stream migration. The lack of paired terraces indicates that entrenchment takes place only at the margins of fan surfaces, not in the middle as would be required to produce paired terraces. The probable reason for this is that streams migrate away from the margins of young fan deposits (the unweathered boulders of which impede erosion) toward saprolitized bedrock or highly weathered old fan remnants. Much of the lateral erosion is probably accomplished by storms smaller than the catastrophic storms necessary to deposit or remove the boulder sediments that mantle the fan surfaces. Eventually, the stream at the margin of the fan surface becomes permanently entrenched, turning the active surface into a relict one. The entrenched stream subsequently begins forming a new active fan surface by lateral migration; plugging by a debris flow might also cause the position of the stream to shift. (4) Relative-age dating can aid in site selection for numerical dating. In the Zionville quadrangle, the goal was to date one of the oldest fans using cosmogenic isotope burial dating. Because this technique is expensive and time consuming, the consequence of poor site selection is significant. To accomplish this goal, we looked at the mean amphibolite-rind thickness for 118 samples. The site selected, besides meeting other criteria, had the third greatest mean thickness, 15.1 m. This procedure allowed us to be

Fig. 19. Map of fan/pediment surfaces on west slope of Rich Mountain, Zionville quadrangle, North Carolina. Surfaces have been grouped into six relative-age classes.

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reasonably sure that the date obtained, 1.4 Ma (Mills and Granger, 2002), set an approximate limit for fan age in this vicinity. (5) Individual fan complexes vary greatly in the relative areas of young, intermediate, and old fan surfaces. Mapping using relative-age indices allow a quantitative measure of this age distribution. For example, Zionville fans show 56% Y, 26% I, and 18% O, whereas Bakersville fans show 42% Y, 34% I, and 24% O. Deciding why fan complexes differ in this regard is difficult, given the many possible parameters involved. Two complexes in Haywood County, North Carolina, however, differ greatly in this regard despite being in close proximity, and invited investigation. Fans in the Dellwood quadrangle study area consist mainly of young surfaces, showing 82% Y, 9% I, and 9% O, whereas fans in the adjacent Hazelwood quadrangle study area show a greatly different distribution: 14% Y, 57% I, and 29% O. Y surfaces here are confined mainly to relatively narrow strips along modern drainageways (Mills and Allison, 1995a). One possible explanation might be a difference in geology such that the sediments in the Hazelwood area weather more rapidly and thus give the appearance of greater age than those in the Dellwood area. However, the geology is relatively similar. Both areas have highlands underlain by metasedimentary rocks and lowlands underlain by gneisses and schists. Another possibility is a difference in altitude, which would affect both modern and Pleistocene climate, with more rapid production of fresh rock debris by physical weathering taking place at higher altitude, resulting in larger and more numerous Y surfaces. There is little difference in altitude, however, and probably little difference in climate. Another explanation might be that the findings are an artifact of the technique. However, soil maps of the two areas show a similar difference, with Braddock and Dillsboro series (indicative of old fan and terrace surfaces) being common in the Hazelwood area but not in the Dellwood area. Topographic differences in addition to altitude were also studied. Fans in both study areas have slopes between 38 and 128. A slope map was analyzed to see if the Hazelwood piedmont area had a greater area of such gentle slopes, conducive to fan formation. The results showed 22.3% of the Hazelwood area to be underlain by such slopes and 21.2%

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of the Dellwood area—virtually the same. With regard to steep slopes that might be expected to yield fresher, less weathered material, 3.1% of the Hazelwood area is underlain by slopes of 358 or greater, vs. only 0.6% of the Dellwood area—the opposite of what might be expected. One topographic analysis gave a positive result. In the Dellwood area, the total area of fans was 9.96 km2 and the total area of basins tributary to the fans was 47.33 km2, for a ratio of 0.21. In the Hazelwood area, the total area of fans was 5.22 km2 and the total area of basins tributary to the fans was 7.10 km2, for a ratio of 0.74, more than three times the ratio in the Dellwood area. This relationship was examined in more detail by computing statistics on the relative proportions of relict surfaces (i.e., I or O) for each fan and comparing these data with the fan area/basin area ratio for each fan. A plot of the fraction of relict areas on each fan against the fan area/basin area for that fan showed a correlation of 0.634. Although this explains only 40% of the variance in the proportion of relict surface on a fan, the effect appears to be a real one. That such a relationship exists seems reasonable. The larger the tributary basin relative to the fan, the more discharge per unit area that occurs in the vicinity of the fans. The greater the discharge, the greater the probable erosion of abandoned fan surfaces. A large fan area/basin area ratio, on the other hand, should favor longer survival times for abandoned fan surfaces. In addition to providing evidence about fan evolution, relative-age mapping of fan surfaces also has value for hazard evaluation. Footslopes in the Appalachians, particularly in the Blue Ridge province, are rapidly being developed for vacation and retirement homes. These footslopes are prone to debris flows, which occur somewhere in the Blue Ridge every 2–5 years (Morgan et al., 1997), although at a given location recurrence intervals are probably greater than 1 ka, as discussed above. Because of their potentially devastating impact, it is of interest to determine what locations on mountain footslopes are most likely to be covered by debris flows, in a manner analogous to a flood-hazard map. Relative-age mapping can be applied to this problem. The basic rationale is that the hazard on a given footslope is inversely proportional to the length of time since that area was last covered by a debris flow.

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For example, fan surfaces with red soils and decayed clasts are relict surfaces that probably have not undergone debris-flow inundation for tens of thousands of years, and therefore are unlikely to be covered by flows in the near future. In contrast, surfaces with brown soils and fresh clasts probably remain locations of active deposition and must be considered hazardous, although recurrence intervals are probably somewhat greater than the 100-year interval used for determining flood-hazard areas along rivers. A more direct approach to determining debris-flow hazard is by evaluating the topography, for active fan surfaces are usually closer to the level of modern stream courses than are old, relict fan surfaces. However, elevation differences between young and old surfaces may be small, and detailed surveying of topography would be required. Relative-age dating of fan surfaces is a more rapid technique.

4. Hillslope colluvium and relative-age dating Colluvium refers to poorly sorted, sometimes stratified, mixture of rock fragments and fine material found on most hillslopes. Thickness may attain 10 m or greater at the base of some slopes. This material is moved downhill by creep, sliding, slope wash, windthrow, and, particularly on footslopes, debris flows. (There is a continuum from colluvium on footslopes to fans and debris-covered pediments, and classification as one or the other is thus to some degree arbitrary. We are here, however, concerned with locations farther upslope than we were above.) Colluvium covers large parts of every Appalachian province (e.g., Blue Ridge—Daniels et al., 1987; Valley and Ridge—Ciolkosz et al., 1979; Piedmont— McCracken et al., 1989). Maximum ages for colluvial deposits very likely are somewhat less than those for stream terrace and fan deposits because of the steepness of slopes involved. There do appear, however, to be large variations in the age of colluvial deposits, and this section seeks to illustrate that relative-age dating has potential for increasing our understanding of hillslope evolution. Interest in the age of colluvium has generally involved one of two questions. One concerns the

episodic nature of colluviation, particularly the effects of Pleistocene glacial climates on the movement and emplacement of colluvium. In the unglaciated Appalachians, mean annual temperatures were as much as 15 8C colder than today even at great distances from the ice front (e.g., Delcourt, 1979), and probably still colder close to the front. The greater depth of freezing undoubtedly increased creep rates in the colluvium, and in places probably induced gelifluction (e.g. Pollack et al., 2000). These considerations suggest that colluvium was much more mobile during glacial climates than during the Holocene, and that much of the colluvium in the Appalachians may be relict from the Pleistocene, remaining essentially stable during the warmer climate of today. Numerical-age dating of colluvium has been difficult owing to the paucity of uncontaminated organics in the colluvium. AMS dating promises to improve this situation, but until recently there were only a few numerical ages to demonstrate the Pleistocene origin of this material (Jordan et al., 1987; Snyder and Bryant, 1992). Recently, taking advantage of deep incision by floodwaters in 1995, Eaton et al. (2001, 2003) have obtained a number of radiocarbon dates on stratified slope deposits in Madison County, Virginia. These demonstrate that 6.5 m of slope deposits formed during late-glacial times between 24,570 and 15,800 YBP. In most areas, however, relative-age dating alone has been available to suggest strongly, if not prove, the ice-age origin of wide areas of colluvium. For example, the degree of soil development in Pennsylvania colluvium has been compared with that in glacial tills of Late Wisconsinan age (Ciolkosz et al., 1979, 1986, 1990) and found to be as great as or greater than that of the tills. The colluvial soils show, for example, argillic horizon development, fragipan formation, significant leaching, and clay mineral weathering (Ciolkosz et al., 1979, Table 2). Late Wisconsinan tills in the area generally show several of these characteristics but lack the argillic horizons, which could indicate an older age for the colluvial deposits, perhaps early Wisconsinan. Of course, the colluvium very likely includes more weathered parent material than does the till, which probably promotes the formation of argillic horizons. In any case, the soils on the colluvium appear to be at least Late Wisconsinan in age. Presumably additional quantitative techniques, perhaps including measures of clast

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weathering, may demonstrate the resemblance to Late Wisconsin tills more rigorously. The above discussion refers to the most-common, relatively byoungQ colluvium with brown color and hard clasts. Somewhat less common in areas near the Late Wisconsinan ice margin is boldQ colluvium, characterized by Munsell hues as red as 5 or 2.5 YR

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and many decomposed clasts. Although uncommon on the surface, Hoover (1983) and Hoover and Ciolkosz (1988) found that on many footslopes in the Valley and Ridge of Pennsylvania such colluvium occurs buried beneath 1–3 m of younger brown colluvium. Typically, the clay content increases abruptly below the brown/red boundary. Associated

Fig. 20. Diagram of two soil profiles on representative backslopes, southeastern Pennsylvania (Pollack et al., 2000).

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with the red layer is bright red rubification of sandstone rock fragments. These clasts commonly bear red weathering rinds with hues of 2.5 YR to 10 R. The authors concluded that the weathering associated with the red layer is at least pre-Wisconsinan, and Hoover (1983) suggested that this colluvium might have been deposited during the Illinoian and weathered during the Sangamon. They considered the brown layer to be the product of Late Wisconsinan periglacial gelifluction. A similar two-layer sequence of colluvium has been reported for the unglaciated Appalachian Plateau of Pennsylvania by Waltman et al. (1990). More recently, Pollack et al. (2000) have investigated colluvium in more detail in the Piedmont of southeastern Pennsylvania (Fig. 20). They noted that although a red color is usually an indication of an advanced state of soil weathering, the red colluvium in their study rarely showed other indications of great age such as thick clay films or well-developed structure. Because of features indicating movement in the soil, they instead interpreted the red soil as a material that was weathered during the pre-Wisconsinan time and then moved as colluvium during the Late Wisconsinan, the movement disrupting structure and clay film expression. They also found a number of periglacial features in both brown and red colluvium, including ice wedges, involutions, features indicative of upfreezing of stones, and evidence

of frost creep. Despite this overwhelming evidence for late Pleistocene activity, they were unable to find organics with late Pleistocene radiocarbon ages. Obviously, there is an opportunity for systematic relative-age dating to relate the red colluvium to numerically dated deposits elsewhere. The locations where this two-layer sequence of colluvium has been reported lie relatively close to the Wisconsinan ice margin. Appearance of the red layer at the surface here is rare, presumably due to the effect of the former periglacial climate which produces the upper brown layer, as well as mobilizing the underlying red layer. In the southern Appalachians, however, hundreds of kilometers from the ice margin, highly weathered red colluvium at the surface is common (by bat the surfaceQ I allow for an overlying brown-colored A horizon, and perhaps an E horizon, of several decimeters). An example is provided on the footslopes of the southeastern slope of Flat Top Mountain adjacent to Grandfather Mountain in North Carolina (Fig. 21). As in the northern locales, here we also see brown and red colluvium, but the brown (B) is confined to the floors of the hollows, and the red (R) is widely exposed on the interfluves. Note that R and B deposits show large differences in reddest hue, clast weathering, and in the distribution of large boulders. (The maximum percent clay is essentially the same for R and B; as discussed above for fans, this similarity may reflect the decomposition of sandstone

Fig. 21. Transverse topographic profile across footslope of southeastern flank of Flat Top Mountain, in Grandfather Mountain quadrangle, Avery County, North Carolina. Plot shows relationship among topography, boulder size, and degree of weathering as indicated by three weathering indices. View is uphill (northwest). Downhill slope ranges from 88 to 108. B indicates younger, brownish colluvium with hard clasts exposed in road cut downhill from profile, and R indicates older, reddish colluvium with decomposed clasts. Length of vertical line indicates intermediate diameter of clast measured at that point, expressed in phi units; absence of line indicates a clast size less than 8 mm. Inverted solid triangles indicate drainage channels, mainly ephemeral. Clast weathering was measured on arkose and subarkose fragments (from Mills, 1981b).

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clasts in the R colluvium, which increases the sand content and thereby lowers the percent clay.) The red colluvium appears to have been stable for some time, unlike that described by Pollack et al. (2000). In particular, decomposed clasts that cannot be removed intact from the outcrop (bghostsQ) obviously could not have undergone significant movement. Note that the boulders are associated mainly with the hollows (the apparent nose at about 70 m along the profile is actually a huge boulder). Whether the lack

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of boulders on the noses is due to a long period of weathering, or because boulders have been selectively carried in the drainageways, is unclear. The origin of the brown colluvium including the boulders in the hollows is also uncertain. Debris flow is a likely agent, but with an altitude of 1255 m, this site may well have been affected by gelifluction and other periglacial processes, albeit somewhat weaker than those closer to the glacial border. It would be of interest to compare the weathering of R with that of

Fig. 22. Photographs of Chilhowee Mountain, Oswald Dome quadrangle, Polk County, Tennessee. (A) Backslope and footslope; (B) dissected footslope, looking downhill; (C) dissected footslope, looking uphill.

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Fig. 23. Perspective plot of hollows and noses on south flank of Bean Field Mountain, east of Mountain Lake, Eggleston quadrangle, Giles County, Virginia.

the buried red colluvium in Pennsylvania. Another interesting study would be to determine the relative amounts of red colluvium at the ground surface as a function of latitude and/or distance from the glacial margin in the Appalachians. Other examples of multiple generations of colluvium have been reported. For example, Eargle (1940, 1977) reported examples from the inner Piedmont of South Carolina, and Whittecar (1985) reported them from the Piedmont of Virginia and North Carolina. Quaternary climatic change may well have been involved in this deposition. A second major question concerning colluvium involves long-term landscape evolution, in which deposits are abandoned on interfluves as the drainageways cut down in a manner analogous to the abandonment of stream terraces and fan surfaces. The downhill motion of the colluvium essentially ceases, and the deposits become relict, with weathering increasing with age. A section of Chilhowee Mountain, (a ridge northwest of and paralleling the Blue Ridge Mountains in east Tennessee) in Polk County, southeastern Tennessee, provides a good example. Fig. 22A shows

the northwest flank of the mountain. Note the break in slope, above which erosion is dominant and colluvium is thin and discontinuous (backslope) and below which colluvium is thicker and relatively continuous (footslope). Fig. 22B and C shows footslope surfaces that have been abandoned and dissected. These surfaces appear to be graded to high-level terrace remnants of the Hiwassee River. The very old footslope colluvium on these footslopes has undergone intense weathering. Hues as red as 2.5 YR commonly occur and clay percentage reaches 57%, although clast weathering is not as great as one might expect, averaging about 3.5 on the 5-point scale, perhaps reflecting the very resistant sandstone and conglomerate that comprise the clasts here. Large areas of the footslopes show such soils, obviously at least pre-Wisconsinan in age, strikingly in contrast with colluvium and fan deposits common in the northern North Carolina Blue Ridge province, where young deposits of probable Late Wisconsinan or Holocene age generally are far more widespread than are older deposits (with some exceptions—see discussion of the Hazelwood study area above). One possible explanation of this differ-

Table 3 Properties of colluvium on hillslopes in vicinity of Mountain Lake, Eggleston quadrangle, Virginia Soil property

Upslope noses

Downslope noses

Side slopes

Hollows

Percent clay Reddest hue (YR) Munsell % Decomposed clasts

29 (12), n=9 4.2 (1.2), n=8 25 (8), n=8

43 (14), n=16 5.0 (2.3), n=16 57 (17), n=13

23 (10), n=14 6.7 (1.8), n=14 4 (5), n=2

22 (6), n=15 7.3 (1.5), n=15 4 (7), n=14

Number in parentheses is standard deviation. Data from Mills (1987, 1988).

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ence is that Chilhowee Mountain and its footslopes are somewhat lower in altitude than areas studied in the North Carolina Blue Ridge (typical ridge crest altitude of 700 m for Chilhowee vs. 1600 m for mountain peaks in the latter), and so would have been less affected by the colluvium-mobilizing glacial climates of the Pleistocene. Another possibility is the low ratio of the depositional slopes to the uphill tributary slopes, analogous to the fan area/tributary area ratio discussed for fans. In this case, ratios of 1.5 are common—the depositional area is actually somewhat larger than the erosional area. Such a ratio probably results in a long residence time for the colluvium. An example of old colluvium on backslopes as well as on footslopes can be seen on ridges in the Valley and Ridge of southwestern Virginia (Mills, 1981a, 1987, 1988). The antidip flanks of these ridges are not planar, but have a corrugated form consisting of alternating hollows and noses (e.g., Hack and Goodlett, 1960; Hack, 1965) (Fig. 23). A dynamic equilibrium interpretation of this topography is that noses supply debris to hollows via creep, slope wash, and windthrow. This debris is temporarily stored in

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the hollows until it is episodically flushed out by rare, catastrophic rainstorms that set off slides and debris flows (e.g., Scott, 1972; Dietrich et al., 1986). According to this interpretation, noses and hollows maintain their positions through time, and colluvium on noses is no older than that in hollows, or perhaps slightly younger. An alternative hypothesis is that, under certain circumstances, the positions of noses and hollows may shift through time, and that topographic inversion of noses and hollows may even take place (Bryan, 1940; Mills, 1981a). In studying the way that hillslopes change through time, an important question is the relative age of colluvium in hollows and on noses. Weathering indices similar to those used for fans were used on these hillslopes (Mills, 1987, 1988). Table 3 shows that differences akin to those seen for different fan surfaces are shown by noses, hollows, and side slopes. Hue by itself is suspect, for noses and hollows differ greatly in drainage, and the better drainage on noses may account for the redder hue. However, the percent clay and percent decomposed clasts show apparent age differences. Another difference observed, not shown in Table 3, is that the

Fig. 24. Surficial map of area east of Mountain Lake, Eggleston quadrangle, Giles County, Virginia, showing distribution of young and old colluvial deposits (from Mills, 1988).

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percentage of cracked boulders was somewhat higher on noses than in hollows. Note that noses have been divided into those higher and those lower on the flank of the ridge. The latter show gentler longitudinal slopes than the former. Apparently downslope noses are more stable than upslope ones, as colluvium weathering seems somewhat more intense on the downslope ones. The observed differences between noses and hollows support a somewhat greater age of the deposits on noses than those in hollows, a difference compatible with topographic inversion. In addition to the weathering indices shown in Table 3, an effort was made to use sandstone-clast weathering rinds as a weathering index. However, these proved too complex and variable to use. It is possible, however, that these rinds may hold much potential information about the history of the deposits, and so are briefly described here to stimulate further research. Commonly, there are two or even three concentric rinds of different colors on the same clasts, in many different combinations of colors (Fig. 26 of Mills, 1988). Rinds were best developed and most likely to be multiple in the colluvium on older surfaces. Some change in rind character with depth is apparent. The multiple rinds on some clasts apparently formed sequentially, for an outer rind truncates an inner rind, indicating that the inner rind was formed first, the clast then broken, and the second rind subsequently formed. Different conditions may produce different-color rinds, so that multiple rinds may reflect a changing environment. Such change might be caused by climatic change or perhaps simply by the movement of clasts into a different soil horizon by windthrow or other disruption of the soil profile. By employing the weathering indices in Table 3, it was possible to map colluvium on the south slope of Bean Field Mountain (east of Mountain Lake, Eggleston quadrangle, Giles County, Virginia) into young and old colluvium (Fig. 24). As can be seen, the old colluvium is on noses, particularly downslope noses. Mapping showed that old colluvium occurs at angles up to 168, occasionally up to 238. A particular problem for relative-age dating of colluvium, more so than for stream terraces and even for alluvial fans, is that materials are of very local provenance. This can make between-site relative-age comparisons more difficult because of the problem of unlike parent materials.

5. Discussion and conclusions That relative-age dating can at least provide an ordinal ranking of Appalachian landforms age seems indisputable, and the technique also can commonly provide a semiquantitative estimate of the magnitude of age difference between two landforms. This information can assist greatly in evaluating theories of landform evolution. Just how well the relative-age data will lend themselves to calibrated-age dating, however, remains to be seen. From one perspective, the outlook is not too hopeful. The fact that stream terrace height ARL shows as good a correlation with the most-reported relative-age criteria as does terrace age (Figs. 2 and 3) suggests that we are dealing with low-resolution data. Markewich et al. (1989) considered their relative-age data for the Coastal Plain to be capable only of order-of-magnitude estimates of numerical age, and Appalachian relative-age data is certainly no better. Such age estimates are better than nothing, but do not help much in working out detailed late-Cenozoic geologic history. The above evaluation may be overly pessimistic, however. In the first place, just because many measures of weathering show only modest correlation with age does not mean that they all do, and only one parameter with a high correlation would be sufficient. Knuepfer (1988) pointed out that most calibrated-age studies have involved low-resolution techniques and landforms well separated in age. In contrast, Knuepfer studied stream terraces of late Pleistocene and Holocene age in a setting where many radiocarbon dates were available. By means of graywacke weathering rinds he was able to estimate ages of terraces with uncertainties of F5% to F40%, much better than the resolutions observed in the Appalachians. The key to improvement in the latter region, therefore, involves obtaining large amounts of calibration data and narrowing the age range of interest. Probably, Knuepfer’s (1988) level of uncertainty is about the best that can be hoped for, as older age ranges are likely to have fewer calibration data. With regard to the use of indices that incorporate numerous measures of weathering or soil development (e.g., Harden, 1982), two considerations are important. First, the use of soil-profile data as the main relativeand calibrated-age parameters imposes a problem for any statistical procedure: in most cases there simply are

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not enough profiles. Many studies involving detailed soil-profile description, sampling, and analysis involve no more than a dozen sites. This problem is particularly severe for studies in which several geomorphic surfaces are being compared, as there may be only two or three described pedons per surface. Given the great site-to-site variability of most soil properties, the small number of sites makes reducing the effect of between-site variance difficult or impossible. The answer is either to greatly increase the number of profiles described (usually logistically difficult) or to use parameters not requiring soil profile description that can be collected and measured relatively quickly. A good candidate is weathering-rind thickness, studies of which usually involve more than 100 sites (e.g., Colman and Pierce, 1981; Mills and Allison, 1995c). The second consideration is that using a statistical technique which incorporates numerous parameters, many of which show poor or only modest correlations with age, is not necessarily the best approach. To be sure, usually the multiple correlation coefficient between the dependent and independent variables improves when more independent variables are used. However, if there is one parameter that shows a high correlation with age, it is better just to use it and forgo the rest—the addition of the other variables to the regression equation just diminishes the degree of correlation. While we are awaiting the accumulation of more numerical ages in the Appalachians and perhaps the advent of new more-readily applied numerical-age techniques, there are improvements that can be made now in designing relative-age dating studies. Certainly, efforts should be made to use more sophisticated measures of weathering and soil development, as discussed above. Probably, the main insight that can be gained from the research discussed here is an appreciation of which parameters work best for which age ranges (keeping in mind, of course, the effect of geologic province, particularly on parent material). A few broad generalizations follow. Although the age of surficial deposits in the Coastal Plain may extend well back into the Miocene, those in the Appalachians are rarely older than Pleistocene (Markewich et al., 1990; Mills and Granger, 2002), so attention will be confined to ages of about 2 Ma or younger. Soil color generally is variable and shows little trend for ages less than 10 ka. In the Appalachians, this

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parameter is most applicable to ages between 10 ka and 1 Ma. By 1 Ma, it has reached its reddest hue. Measures of free iron oxide probably follow a similar pattern. Measures of B thickness are probably most useful for Holocene soils; the Bt thickness in particular is good, and it appears that Bt horizons can form (in alluvium) in about 5 ka (Foss et al., 1981; Leigh, 1996). The B thickness apparently can continue to increase until well beyond 1 Ma, but shows a great deal of variability. The percent clay in the B horizon can show an increase between 1 and 10 ka due to translocation of clay; it can continue to increase beyond 1 Ma by in situ production of clay from weathering, but usually reaches an asymptote at a younger age. Sand mineralogy is best for ages of 10 ka to 1 Ma; at ages younger than 10 ka, little weathering has taken place, and by 1 Ma nothing remains but quartz and a small amount of the most resistant heavy minerals. Of the chemical indices, Leigh (1996) found the bases/ Al2O3 ratio useful in the Holocene to late Pleistocene range. Markewich et al. (1989) reported the oxide ratio (Fe2O3+Al2O3)/SiO2 to be most useful for ages of 200 to 800 ka in Coastal Plain soils; there are no reports on this ratio for older Appalachian soils. Harris et al. (1980) found clay mineralogy to show trends continuing well beyond an age of probably 1 Ma; there are few data on clay mineralogy changes in young soils, but apparently this parameter is best for older soils. Surface textures of quartz sand grains is applicable for ages greater than 0.7 Ma, but is probably best for great ages seen mainly in the Coastal Plain (Howard et al., 1995). Clast weathering, as shown by Knuepfer (1988), may become a very good calibrated-age technique, but in the Appalachians there are as yet very few numerical dates from areas in which clast-weathering measurements have been made. One useful study in the meantime would be to further investigate the degree of operator variance in classifying clast decomposition and in measuring weathering rinds (e.g., Colman and Pierce, 1981). This knowledge will be needed if clast weathering eventually is to be used for calibrated-age dating. If widely applicable numerical-dating techniques eventually do become available for the Appalachians, the need for relative-age dating is unlikely to disappear. It is difficult to imagine numerical-dating techniques ever becoming so inexpensive and rapid that it will not be necessary to use relative-age dating, particularly in

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the course of mapping. Such techniques will, to be sure, replace relative-age techniques for some landform evolution problems, such as stream incision rates, but for others, such as inferences about fan evolution drawn from patterns of surface ages, an ordinal measure of surface age is sufficient, and numerical ages will contribute only to our understanding of the rate of the process, not the basic sequence of changes which produce the patterns. References Bell, A.M., 1986. Morphology and stratigraphy of terraces in the upper Shenandoah Valley, Virginia. MS Thesis, West Virginia University, Morgantown, West Virginia, p. 160. Bierman, P.R., Pavich, M., Eaton, L.S., Finkel, R., Larsen, J., 2002. The boulders of Madison county. Abstr. Programs-Geol. Soc. Am. 34 (6), 127. Birkeland, P.W., 1999. Soils and Geomorphology, 3rd edition. Oxford, New York, p. 430. Birkeland, P.W., Noller, J.S., 1998. Rock and mineral weathering. In: Sowers, J.M., Noller, J.S., Lettis, W.R. (Eds.), Dating and Earthquakes: Review of Quaternary Geochronology and its Application to Paleoseismology, NUREGG/CR, vol. 5562. U.S. Nuclear Regulatory Commission, pp. 2.467 – 2.496. Bryan, K., 1940. Gully gravure—a method of slope retreat. J. Geomorphol. 3, 89 – 107. Bull, W.B., 1964. Geomorphology of segmented alluvial fans in western Fresno County, California. U. S. Geol. Surv. Prof. Pap. 352-E, 89 – 129. Buntley, B.T., Westin, F.C., 1965. A comparative study of developmental color in a Chestnut–Chernozem–Brunizem soil climosequence. Proc.-Soil Sci. Soc. Am. 29, 579 – 582. Carter, T.E., 1981. Pediment development along the Book Cliffs, Utah. MS Thesis, Colorado State University, Fort Collins, p. 92. Ciolkosz, E.J., Petersen, R.L., Cuningham, R.L., Matelski, R.P., 1979. Soils developed from colluvium in the Ridge and Valley area of Pennsylvania. Soil Sci. 28, 153 – 162. Ciolkosz, E.J., Cronce, R.C., Sevon, W.D., 1986. Periglacial features in Pennsylvania. Geomorphology 3, 245 – 261. Ciolkosz, E.J., Carter, B.J., Hoover, M.T., Cronce, R.C., Waltman, W.J., Dobos, R.R., 1990. Genesis of soils and landscapes in the Ridge and Valley province of central Pennsylvania. Geomorphology 3, 245 – 261. Colman, S.M., Pierce, K.L., 1981. Weathering rinds on andesitic and basaltic stones as a Quaternary age indicator, western United States. U. S. Geol. Surv. Prof. Pap. 1210, 56. Colman, S.M., Pierce, K.L., Birkeland, P.W., 1987. Suggested terminology for Quaternary dating methods. Quat. Res. 28, 314 – 319. Daniels, R.B., Gamble, E.E., 1967. The edge effect in some Ultisols in the North Carolina Coastal Plain. Geoderma 1, 117 – 124. Daniels, R.B., Hammer, R.D., 1992. Soil Geomorphology. Wiley, New York, p. 236.

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