Paleomagnetic evidence for repeated glacial lake missoula floods from sediments of the Sanpoil River Valley, Northeastern Washington

QUATERNARY

RESEARCH

35, 197-207 (1991)

Paleomagnetic Evidence for Repeated Glacial Lake Missoula from Sediments of the Sanpoil River Valley, Northeastern Washington

Floods

WILLIAM K. STEELE Department

of

Geology, Mail Stop 70, Eastern Washington University, Cheney, Washington 99004 Received December 13, 1989

Recent explanations of widespread rhythmically layered sediments in eastern Washington as the result of repeated great floods from glacial Lake Missoula implicitly suggest a paleomagnetic test for validity. If each conjectural flood layer is separated by years or decades, as hypothesized, a sequence of several such flood beds should record measurable secular variation in geomagnetic field direction. In the Sanpoil River valley where the rhythmite sequences are thought to have been deposited in glacial Lake Columbia, the paleomagnetic test consists of measuring remanent magnetization (RM) directions for the thick, upwardly fining beds inferred to be sediments deposited by the influx of flood waters from glacial Lake Missoula into glacial Lake Columbia. Laboratory measurements of samples from three widely spaced sections along the Sanpoil River yield RM vectors with erratic inclinations, apparently affected by varying contributions of inclination error and (or) compaction shallowing, but with declinations that generally differ statistically from one flood to the next and that show the same west-to-east trend at all three locations. The rates of declination change inferred from these data are consistent with modem rates, thus providing the first geophysical evidence supporting the timing in the tens-of-floods theory. 0 1991 University of Washington.

INTRODUCTION From sedimentological evidence, Waitt (1984, 1985) and Atwater (1984, 1986, 1987) have inferred that the rhythmic recurrence of thick sandy beds separated by thin varves in valleys in northern Washington and Idaho was caused by great floods from glacial Lake Missoula invading late Pleistocene glacial lakes. Previous work had ascribed the sandy graded layers either to a draining of the depositing lake (Flint and Irwin, 1939; Walker, 1967) or to turbidity currents within the lake (McKenzie and Stemen, 1983), although both Flint and Irwin (1939) and McKenzie and Stemen (1983) suggested the possibility that the sandy graded beds were due to periodic flood waters from other lakes. The matter has become part of a larger controversy affecting the whole region. Waitt (1985) attempted to answer criticism of the tensof-floods idea that had been published from 1979 to 1984, but some doubt still remains

(Baker and Bunker, 1985). Spencer (1989) recently reported a study of small mammal fossils from the Touchet Beds of southern Washington that supports the many-flood theory for the origin of those rhythmites. The thick conjectural flood layers in the Sanpoil River valley (Fig. 1) are separated by tens of varve-like rhythmites, and the rate of geomagnetic secular variation (SV) is on the order of IOYcentury (Holcomb et al., 1986). Therefore, if the rhythmites do represent annual deposits and the floods were spaced decades apart, the flood beds should record a measurable amount of SV in their remanent magnetization (RM). Alternatively, if the sandy layers result from turbidity currents recurring at a much smaller time interval, their magnetic directions should be indistinguishable. The present work is an application of this paleomagnetic test of the tens-of-floods hypothesis. Indeed, Baker and Bunker (1985) suggested the potential usefulness of paleomagnetic studies in this matter. 197 0033-5894191 $3.00 Copyright 0 1991 by the University of Washington. All rights of reproduction in any form reserved.

198

WILLIAM

I

I I

K.

STEELE

RHYTHMITE

199

PALEOMAGNETISM

The age of the Sanpoil flood sediments is somewhat tenuous. A combination of stratigraphic evidence and ash layer and radiocarbon dating indicates that glacial Lake Missoula is of Fraser age (last glaciation) and originated after 19,000 yr B.P. (Atwater, 1987), but the absolute age of the Sanpoil River valley deposits rests on a single radiocarbon date of 14,490 + 290 yr B.P. (USGS-1860) for detrital wood from the middle of the Manila Creek section (Atwater, 1986). Because the wood might be as much as 2500 years older than its enclosing sediments, Atwater (1986) recognizes that the sediment ages, which are based on the age of the wood and on varve counts, may be as much as 2500 years too old. In southern Washington, the ages of similar, related rhythmites are constrained by the presence of the Mount St. Helens set-S tephra couplet in the upper part of the sections (Waitt, 1980, 1985) and by the dates of shells, 14,060 -+ 450 yr B.P. (USGS-684; Waitt, 1985, p. 1284) and 13,330 + 190 yr B.P. (WSU-2714; Baker and Bunker, 1985, p. 20), from the third rhythmite below the setS couplet. SAMPLING

Because of their small grain size, clayey and silty horizons within the upper part of the inferred flood layers were deemed likely carriers of stable RM and were sampled for the paleomagnetic investigation. Collections of oriented paleomagnetic specimens were made at three of Atwater’s (1984, 1986) localities in the Sanpoil River valley: Old Keller, Ranger Station, and French Johns (Fig. 1). These sections were selected because they contain several flood beds that Atwater had correlated over a significant horizontal distance, and a congruence of the magnetic record among the localities would therefore support both Atwater’s correlations and the reliability of the magnetic record. Atwater kindly identified the flood layers for the author in the field so as to avoid possible misidentification of the former’s designated strata (Atwater, 1986).

Paleomagnetic specimens were collected in the form of rectangular blocks carved from clay-rich horizons within the flood layers with a knife and were placed in 25 x 25 X 19-mm plastic boxes. The orientation of the square top surface of the specimens was determined with magnetic compass and clinometer. Ten specimens from each flood layer were taken to constitute a paleomagnetic site, and generally these specimens were distributed over a horizontal distance of 1 to 3 m so as to average out local systematic depositional and bedding effects. Most of the sampling was carried out in September-October, 1985; erratic laboratory results apparently related to disturbed bedding led to resampling of one site in October, 1986. The specimens were stored over water in a glass container to prohibit desiccation and were measured during the 6 months following collection. LABORATORY

Measurements of RM were carried out by standard paleomagnetic procedures (Collinson, 1983). The natural remanent magnetization (NRM) of all specimens was measured with a Schonstedt SSM-IA spinner magnetometer, and one pilot specimen per site was remeasured after serial demagnetizations in peak alternating fields of 5, 10, 15,20,25,30,40, and 50 mT. Plotting of orthogonal demagnetization (Zijderveld) diagrams revealed that most pilot specimens had a small component of viscous remanent magnetization (VRM) along the present geomagnetic field, and that this VRM was generally removed in a peak alternating field of 5 to 10 mT. The original intensity of remanence of most specimens was in the range 2.4 x 10-l A/m to 1.2 x lop2 A/m. The specimen magnetizations were quite stable with median destructive fields ranging from 27 to over 50 mT. The average median destructive field for the pilot specimens was approximately 40 mT. Ail remaining specimens were cleaned by demagnetization in a peak field of 30 mT. Isothermal remanent magnetization (IRM) studies

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were carried out on three randomly selected specimens, two from the Ranger Station locality and one from the Old Keller locality. The IRM was found to saturate in an applied field of approximately 200 mT. Alternating field demagnetization of the saturated IRM caused a more rapid decrease in intensity of magnetization with peak demagnetizing field than for the NRM. This fact, together with the high median destructive field, suggests that the NRM is carried in single-domain and (or) pseudo-single-domain magnetite (Lowrie and Fuller, 1971; Bailey and Dunlop, 1983). TABLE

Flood-bedb Old Keller F14 F13 F12 Fll FlO (?) F9 (?) F8 (?) Ranger Station F14 F13 F12 Fll FlO F9 French Johns F14 F13 F12 Fll FlO F9 F8 F7 F6 F5 F4 F3 F2

Site

K. STEELE

Because one specimen from each of sites 1985029 and 1985032 and three specimens from site 1985030 showed wildly different paleomagnetic directions compared to the rest of the specimens from their respective sites, and since the field notes indicated that some specimens from these sites may have been from rotated blocks, these data were discarded. Magnetic directions from site 1985012 were highly dispersed (A,, = 62.6”) and were from a region of flood layer F14 that had contorted bedding and hematite stain; therefore, this site was recollected the following year as site 1986005.

1. PALEOMAGNETICDATA~

Mean declination (“I

Mean inclination (“)

hD (“I

A t

N

1985023 1985024 1985025 1985026 1985030 1985031 1985032

0.0 10.6 350.9 349.4 337.5 342.0 333.6

55.0 58.5 25.9 31.8 40.7 48.7 40.8

7.9 >27.6 1.3 1.8 3.1 8.2 3.3

5.7 17.4 1.5 1.9 3.0 6.8 3.2

10 10 10 10 7 10 9

1985027 1985028 1985033 1985029 1985034 1985035

353.4 350.0 346.9 348.6 342.6 335.8

36.7 33.7 25.1 33.6 34.5 35.7

3.9 1.5 2.0 1.8 2.3 1.3

4.0 1.6 2.3 1.9 2.4 1.3

10 10 10 9 10 10

1986005 1985013 1985014 1985015 1985016 1985017 1985021 1985007 1985008 1985009 1985010 1985011 1985022

351.7 349.9 348.0 346.8 338.4 340.9 320.5 332.2 344.6 324.6 316.1 336.1 349.5

26.7 28.1 36.0 28.3 34.5 41.7 36.1 36.5 41.7 35.6 45.1 38.5 31.4

4.4 1.4 2.4 2.6 3.5 13.6 2.7 13.3 8.5 11.3 10.9 10.1 3.1

5.0 1.6 2.5 2.9 3.6 12.8 2.8 13.5 8.1 11.6 9.7 10.0 3.3

10 10 10 10 10 10 10 10 10 10 10 10 10

’ hD is half-width of the 95% confidence interval for the declination, A,, is the radius of the circle of 95% confidence for the site mean direction, and N is the number of specimens for each site. Declination, inclination, AD, and A,, are in degrees. The statistics describe the RM after cleaning in a peak alternating field of 30 mT. The question marks following three floods at Old Keller indicate a possible hiatus below Fll b Flood beds numbered as by Atwater (1986, Fig. 6).

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201

PALEOMAGNETISM

RESULTS

The measurements of RM directions after alternating-field demagnetization of all specimens in a peak alternating field of 30 mT are presented in Table 1 and are shown in stereographic (Wulff net) projection with 95% confidence circles in Figure 2. The quantity AD in Table 1 is the half-width of the 95% confidence interval for the declination (Demarest, 1983, eqs. 1 and 5). Intercomparisons among the three localities of the directions and 95% confidence circles for the seven flood layers sampled at more than one locality are shown in Figure 3. Inspection of Table 1 and Figure 3 shows that in general there is poor agreement between paleomagnetic directions from the correlated flood beds at the Old Keller, Ranger Station, and French Johns localities. Moreover, at each individual locality,

the record of the magnetic directions (Fig. 2) does not show a smooth, consistent, systematic change as would be expected for geomagnetic SV. Paleomagnetic declinations and their 95% confidence intervals are plotted in Figure 4. These plots show a conspicuous trend of eastward change upsection at all three localities. INTERPRETATION

Declinations

in Sanpoil Sediment

In general, overall paleomagnetic directions from the correlated flood beds are in poor agreement, but Figure 4 shows a clear trend of eastward change upsection in the direction of RM. Although the declinations are not everywhere statistically different from one flood layer to the next, the trend is

Old Keller Loca.7-

F9

Ranger Station Locality

-60

y--

FIG. 2. RM directions and their 95% confidence circles in stereographic projection. Flood layers are designated by number at each locality.

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FIG. 3. Intercomparisons

of flood layer RM directions at different localities.

unmistakable. Moreover, the eastward drift is consistent for all three localities. Where abnormally large differences in declination are observed between one flood and the next, these differences are not significant because of the large dispersion of magnetic directions from one or both of the flood layers. The congruity of the paleomagnetic declinations from correlative floods among the three localities and the presence of the eastward trend at all three places constitute very strong evidence for the authenticity of the remanent declination as an indicator of the declination of the ancient geomagnetic field. Estimation

K. STEELE

of Sanpoil SV Rates

To obtain an estimate of SV rates, leastsquares regression lines were fitted to the declination data under the hypothesis, supported by Atwater’s (1984, 1986) varve counts, that the time interval between

floods was approximately constant. These lines give average declination changes of 5.4”/flood at Old Keller, 3. lo/flood at Ranger Station, and l.S”/flood at French Johns. The relatively low value at French Johns arises because many additional older flood layers are exposed at this locality and not at the others, and the data from some of these older deposits are considerably more dispersed. If the older floods from the French Johns data are excluded, and only the uppermost seven floods are used for French Johns, as at the other two localities, then the rate becomes 4.3”/flood, similar to those at the other two localities. Assuming that on average 50 rhythmites were deposited between floods, consistent with Atwater’s (1984, 1986) counts, and that each rhythmite represents 1 yr’s sedimentation (i.e., it is a varve), the respective rates of SV in declination are 6.5, 3.7, and 5.2 min/ yr; these values are comparable to the

RHYTHMITE lntercomparlson 7:.

PALEOMAGNETISM

Among Locahes:

FIG.

3-Continued.

present rate of 6.8 min/yr for this region (Fabian0 and Peddie, 1980). Inclinations

203

in Sanpoil Sediment

The previously mentioned statistical differences in paleomagnetic directions for flood correlatives at the three localities at first appear to suggest that Atwater’s (1984, 1986) correlations are incorrect. The correlations of the flood beds are based on recognition of the distinctive lithologies of certain varves in the varved intervals between the flood layers. Such correlations are often less secure than geological correlations based on traditional stratigraphic markers such as index fossils and volcanic ash layers. But attempts to improve the agreement of magnetic directions by hypothetically displacing one section relative to another do not significantly enhance the overall concordance of magnetic directions; no match position between sections at any two

localities can be found that produces statistical agreement between magnetization directions for all corresponding flood layers. The possibility that some flood layers are missing altogether at one or more localities is not suggested by field observations, except perhaps at the Old Keller locality where a hiatus may exist between flood layers Fll and FlO; this would make layers FlO and lower at Old Keller older than their designations indicate. The data presented in the present study cannot entirely rule out incompleteness of the sedimentological record at one or more localities. Inspection of Table 1 and Figure 3 shows that the discordance among magnetic directions is confined largely to the paleomagnetic inclination. The laboratory studies of the RM indicate that it is carried in finegrained magnetite and is quite stable. Various mechanisms are known, however, that can affect the inclination of detrital or de-

204

WILLIAM Declinations

K. STEELE

at Old Keller Locality

Declination,

Declinations

Declination,

degrees

Declinations 14

at French

Johns

13 :

degrees

Locality

l-=-l

11

w

10 zl

Station Locality

t-l

12

f

at Ranger

H

9-

I

8-

I

H

t t $

m 20 lL

I

7

6543-

I I

I

I

I

i

2 1 300

H

310

320

330

Declination,

FIG. 4. Declinations of flood-layer intervals shown are 2ALI.

340

350

360

degrees

RM at the three localities. The widths of the 95% confidence

positional remanent magnetization (DRM) as the sediment is deposited or in very early diagenesis. These effects include inclination error, bedding error, current rotation error, and compaction shallowing. The best known of these is inclination error, which laboratory experiments have shown may cause magnetic inclinations of sediments to be as much as 20” too shallow (Johnson et al., 1948). The inclination error arises during deposition as round magnetic carriers rotate after touching the substrate or flattened carriers settle into their preferred orientation (Verosub, 1977). Bedding error, which occurs in deposi-

tion on a slope, can also prevent the initial DRM of a sediment from accurately recording the direction of the ambient geomagnetic field (King, 1955). Because the flood sediments studied here were deposited nearly horizontally, bedding error is not likely to be a problem. Current rotation error (King, 1955; Granar, 1958) can affect declination as well as inclination, but because only the clay-sized sediments at the tops of the flood layers were used in this study, water currents were probably not strong enough to create significant current rotation errors. An effect recently described that can al-

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205

PALEOMAGNETISM

ter the magnetic inclination of sediments after deposition is compaction shallowing (Anson and Kodama, 1987; Deamer and Kodama, 1988). Anson and Kodama (1987) obtained inclination shallowing of as much as 12” by compressing a synthetic sediment consisting of magnetite and kaolinite. Most of this effect was produced by a compression of 0.14 MPa, and additional compression to 2.53 MPa added only slight further shallowing. These investigators explain the shallowing with an electrostatic model in which magnetite grains adhere to negatively charged clay grains that become reoriented during compaction. Evidence of compaction shallowing has been reported in natural sediments from California and Alaska (Levi, 1988; Stamatakos et al., 1988). Compaction-induced inclination shallowing could well have affected the magnetization of the Sanpoil sediments, either as they became loaded by the weight of overlying sediments or as a result of variation of the water depth in glacial Lake Columbia. An increase in water depth of only 14 m is enough to increase the loading on a buried sediment by 0.14 MPa; such waterlevel variations are likely to have occurred in glacial Lake Columbia. An additional small but. significant error in inclination is introduced by measuring samples of noncubic geometry in a spinner magnetometer (Steele, 1989), but for specimens of the dimensions used in this study in the Schonstedt spinner, the error should be no more than 4”. This value is minor compared with the inclination dispersions among localities observed for most of the flood beds. The changes in paleomagnetic inclination from one flood to the next are frequently far greater than would be expected solely from geomagnetic SV, and these changes are not consistent in magnitude or sense. At present, the global maximum rate of secular change in geomagnetic inclination at latitudes above 45” is only 8.3” per 50 yr (Parkinson, 1983, p. 94) compared with changes of up to 15” per 50 yr inferred from the San-

poil data under the assumption that the flood layers are separated by 40 to 55 varves (Atwater, 1986, p. 8). Thus, the change in inclination from one flood layer to the next is generally far larger than would be expected from geomagnetic SV alone, which suggests that one or more of the inclination-shallowing processes, most likely inclination error and compaction shallowing, have had an effect. Incompleteness in the stratigraphic sequence could also explain the erratic variations in the inclination record, but in that case, a similar irregularity would be expected in the declination instead of the consistent trend that is observed. CONCLUSIONS

The magnetic inclinations from the Sanpoil flood sediments are quite “noisy,” either from a combination of DRM inclination error and postdepositional compaction shallowing or because of incompleteness in the stratigraphic sequence, and are not consistent among the localities studied. The latter explanation is less likely because stratigraphic incompleteness should also cause irregularities in the declination record, but the declination log is actually much smoother than the inclination record. Unlike the inclinations, the paleomagnetic declinations from the flood sediments are concordant among the three localities and show a consistent upsection trend from west to east with time at all three localities sampled. The agreement of RM declinations among the localities and the similarity of the upsection west-to-east trend provide evidence that the flood sediments contain a reliable record of the paleomagnetic declination. The rate of change of the magnetic declination with time, calculated assuming that the rhythmites between floods are varves, yields a value comparable to present rates of secular variation in magnetic declination. This constitutes the first nonsedimentological evidence that the rhythmites do represent annual deposits and that the conjec-

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WILLIAM

tural flood layers result from events spaced at intervals of 35-55 yr. These paleomagnetic data bear only on the timing of the depositional phenomena and contribute no information about the nature of the events that produced the conjectural flood layers or about the source of the possible floods. The record of secular change in magnetic declination from the flood layers supports Atwater’s (1986) contention that the interflood-layered mud beds are varves, and the data are hostile to any model for their origin not allowing sufficient time for this change to have occurred.

I am grateful to Brian Atwater for spending considerable time guiding me around the outcrops identifying his flood units so that even I, as a geophysicist, could not confuse them. The Colville Confederated Tribes graciously allowed me to conduct the field investigation on their land.

REFERENCES Anson, G. L., and Kodama, K. P. (1987). Compaction-induced inclination shallowing of the postdepositional remanent magnetization in a synthetic sediment. Geophysical Journal of the Royal AstroSociety

88, 673-692.

Atwater, B. F. (1984). Periodic floods from glacial Lake Missoula into the Sanpoil arm of glacial Lake Columbia, northeastern Washington, Geology 12, 464467. Atwater, B. F. (1986). “Pleistocene glacial-lake deposits of the Sanpoil River valley, northeastern Washington.” U.S. Geological Survey Bulletin 1661. Atwater, B. F. (1987). Status of glacial Lake Columbia during the last floods from glacial Lake Missoula. Quaternary Research 27, 182-201. Bailey, M. E., and Dunlop, D. J. (1983). Alternating field characteristics of pseudo-single-domain (2-14 pm) and multidomain magnetite. Earth and Planetary

Science

Letters

63, 335-352.

Baker, V. R., and Bunker, R. C. (1985). Cataclysmic late Pleistocene flooding from glacial Lake Missoula: A review. Quaternary Science Reviews 4, l41. Collinson, D. W. (1983). “Methods in Rock Magnetism and Palaeomagnetism.” Chapman and Hall, London. Deamer, G. A., and Kodama, K. P. (1988). Experimental test of the electrostatic attraction model for compaction shallowing of inclination. Eos, Transactions

of the American

Geophysical

STEELE

Demarest, H. H., Jr. (1983). Error analysis for the determination of tectonic rotation from paleomagnetic data. Journal of Geophysical Research 88, 43214328. Fabiano, E. B., and Peddie, N. W. (1980). “Magnetic declination in the United States-Epoch 1980.” U.S. Geological Survey Map I-1283. Flint, R. F., and Irwin, W. H. (1939). Glacial geology of Grand Coulee Dam, Washington. Geological Society of America Bulletin 50, 661-680. Granar, L. (1958). Magnetic measurements on Swedish varved sediments. Art& fir Geofysik 3, l-40. Holcomb, R., Champion, D., and McWilliams, M. (1986). Dating recent Hawaiian lava flows using paleomagnetic secular variation. Geological Society of America

Bulletin

97, 829-839.

Johnson, E. A., Murphy, T., and Torreson, 0. W. (1948). Pre-history of the earth’s magnetic field. Ter-

ACKNOWLEDGMENTS

nomical

K.

Union

69, 334.

restrial

Magnetism

and Atmospheric

Electricity

53,

349-372. King, R. F. (1955). The remanent magnetism of artificially deposited sediments. Monthly Notices of the Royal Astronomical ment 7, 115-134.

Society,

Geophysical

Supple-

Levi, S. (1988). Evidence for inclination shallowing in some continental sediments. Eos, Transactions of the American Geophysical Union 69, 1163. Lowrie, W., and Fuller, M. (1971). On the alternating field demagnetization characteristics of multidomain thermoremanent magnetization in magnetite. Journal of Geophysical

Research

76, 6339-6349.

McKenzie, G. D., and Stemen, K. S. (1983). Late Quatemary history of southern Sanpoil Valley and sediments of glacial Lake Columbia, eastern Washington. Geological Society of America Abstracts with Programs

15, 640.

Parkinson, W. D. (1983). “Introduction to Geomagnetism.” Elsevier, New York. Rees, A. I. (1961). The effect of water currents on the magnetic remanence and anisotropy of susceptibility of some sediments. Geophysical Journal of the Royal

Astronomical

Society

5, 235-251.

Spencer, P. K. (1989). A small mammal fauna from the Touchet Beds of Walla Walla County, Washington: Support for the multiple-flood hypothesis. Northwest Science 63, 167-174. Stamatakos, J. A., Kodama, K. P., and Pavlis, T. L. (1988). The effects of compaction on the magnetization of the Arkose Ridge Formation: Experimental results. Eos, Transactions of the American Geophysical

Union

69, 339.

Steele, W. K. (1989). Directional errors in remanent magnetization of non-cubic soft sediment specimens measured with spinner magnetometers. Geophysical Journal %, 333-341. Verosub, K. L. (1977). Depositional and postdeposi-

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tional processes in the magnetization of sediments. of Geophysics and Space Physics 15, 129143. Waitt, R. B. (1980). About forty last-glacial Lake Missoula jokulhlaups through southern Washington. Reviews

Journal

of Geology

207

PALEOMAGNETISM

88, 653619.

Waitt, R. B. (1984). Periodic jijkulhlaups from Pleistocene glacial Lake Missoula-New evidence from

varved sediment in northern Idaho and Washington. Quaternary

Research

22, 46-58.

Waitt, R. B. (1985). Case for periodic, colossal jokulhlaups from Pleistocene glacial Lake Missoula. Geological Society of America Bulletin 96, 1271-1286. Walker, E. H. (1967). “Varved lake beds in northern Idaho and northeastern Washington.” U.S. Geological Survey Professional Paper 575-B.