Saginaw Lobe tunnel channels (Laurentide Ice Sheet) and their significance in south-central Michigan, USA

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Saginaw Lobe tunnel channels (Laurentide Ice Sheet) and their significance in south-central Michigan, USA Timothy G. Fishera,, Harry M. Jolb, Amber M. Boudreaua a

Department of Earth, Ecological, and Environmental Sciences, MS#604, 2801 W. Bancroft St., University of Toledo, Toledo, OH 43606, USA b Department of Geography and Anthropology, University of Wisconsin-Eau Claire, 105 Garfield Avenue, Eau Claire, WI 54702-4004, USA Received 19 January 2004; accepted 24 November 2004

Abstract A network of tunnel channels in southern Michigan records substantial subglacial meltwater activity beneath the Saginaw Lobe of the Laurentide Ice Sheet. The channels are incompletely filled with outwash, contain eskers, and in many places crosscut and continue beyond upland ridges previously mapped as recessional moraines. The presence of the tunnel channels and drumlins on these upland ridges indicate that the ridges are not recessional moraines. Instead, outwash fans and minor ridges record deglaciation in the area. A palimpsest relationship between buried bedrock valleys and tunnel channels records the presence of multiple generations of tunnel channels. Evidence for a subglacial meltwater sheetflow consists of sheets of boulder gravel in upland 415 km wide with a hummocky upper surface. The hummocks transform down flow into erosional remnant drumlins; possibly the result of flow acceleration on a negative slope. Tunnel channels at the distal end of the drumlin swarm suggest collapse of the sheetflow into a channelized flow. The tunnel channels then end at the Sturgis Moraine at the heads of large outwash fans. The observed geomorphic relationships between tunnel channels, moraines, and drumlins in south-central Michigan are applicable to glacial landform studies elsewhere, and indicate the important role of meltwater in landscape evolution. r 2005 Elsevier Ltd. All rights reserved.

1. Introduction The recognition that numerous, modern-day valleys have their origin as subglacial tunnel channels provides a different perspective and constraint on the evolution of subglacial and ice-marginal landscapes. Within the last few years tunnel channels (valleys) on the Lower Peninsula of Michigan have been recognized by Kehew et al. (1999), Kozlowski (1999), Fisher and Taylor (2002), Kozlowski et al. (2003), Kozlowski (2004). Similar landforms elsewhere in Michigan have been described by Johnson (1999), Schaetzl (2000), and Schaetzl and Weisenborn (2004). Until recently, glacial research in Michigan had not advanced much beyond the pioneering work of Leverett and Taylor (1915) with Corresponding author. Tel.: +1 419 530 2883; fax: +1 419 530 4421. E-mail address: timothy.fi[email protected] (T.G. Fisher).

0277-3791/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2004.11.019

most upland areas referred to as moraines and mapped according to their texture (Farrand, 1982). In the last few years, research has focused on understanding the glacial landscape using a process and sediment-landform assemblage approach (Blewett and Winters, 1995; Kehew et al., 1999; Schaetzl, 2000; Fisher and Taylor, 2002; Fisher et al., 2003; Larson et al., 2003; Stone et al., 2003; Kozlowski, 2004; Schaetzl and Weisenborn, 2004; Kozlowski et al., 2005; Kehew et al., 2005). These studies have improved our understanding of the glacial landscape including landform and sediment genesis. Here we propose that most of the lowland valleys in the study area are tunnel channels, some of which predate the last glaciation. The recognition of tunnel channels associated with the Saginaw Bay Lobe in south-central Michigan (Fig. 1) indicates that many of the uplands previously interpreted as moraines instead record subglacial rather than ice marginal landscapes.

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2. Aims The aims of this paper are two fold. First, to identify and document the tunnel channels and their sediment over a broader area of south-central Michigan than were first described by Fisher and Taylor (2002). Second, it is the relationship of the tunnel channel with other landforms that is further explored in this paper. Through the identification of other subglacial landforms and their spatial and stratigraphic relationships with tunnel channels, we discuss multigenerational tunnel channels, their antecedent role in draining subglacial meltwater and their relationship to bedrock lithology and buried bedrock valleys. Although the landforms described here are only from Michigan, similarity in form are drawn from glacial landscapes elsewhere, and evolutionary processes are applicable universally.

3. Study area Deglaciation of the Saginaw Lobe of the Laurentide Ice Sheet was presumably influenced by the regional topography of the Great Lakes Basins. The Saginaw Lobe developed between the Michigan and Huron Erie Lobes (Fig. 1B). The chronology of ice marginal positions is weakly constrained using drift stratigraphy in the Tekonsha and Kalamzoo Moraines (Fig. 1C) to 15,000 14C yrs BP (Monaghan and Larson, 1986). The topography of the study area is generally of low relief (rarely exceeding 40 m) and can be divided into nongenetic lowland and upland regions. The lowlands are shallow valleys that cross-cut the upland surfaces. Upland regions in the southwest corner of the study area have drumlinized surfaces, and elsewhere form irregular blocks, some of which are aligned approximately west–east (Figs. 1C, D). Many of the upland regions were interpreted by previous workers as recessional moraines with intervening lowlands dominated by outwash deposits (Leverett and Taylor, 1915; Ekblaw and Athy, 1925; Zumberge, 1960; Farrand, 1982). However, as briefly mentioned in Fisher and Taylor (2002), many of the ‘recessional moraines’ are overprinted by drumlin fields and cross-cut by sequences of tunnel channels lacking outwash fans on their distal sides (Figs. 1D, E). It is the relationship of the tunnel

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channel with other landforms that is further explored in this paper.

4. Background Before a brief review of tunnel channels, it is important to discuss their terminology. In North America the term ‘tunnel valley’ was first used in Minnesota (Wright et al., 1964; Wright, 1973). By 1983 large sinuous channels up to 1 km wide that cross-cut drumlins and hosted eskers were being described as ‘channels’ because they are assumed to be bankfull (Shaw, 1983). Wright’s (1973) formative mechanism for the valleys was a catastrophic flow in a channel. The term tunnel valley was subsequently reinforced by Mooers (1989) who interpreted the same features to have formed from the lateral and vertical movement of a subglacial channel creating a subglacial valley (Fig. 2). We adopt the Clayton et al. (1999) use of the term channel, which implies the lateral depression in the landscape (often the modern valley) formed from a subglacial, bankfull discharge, where the size of the channel matches the size of the valley. Thus the term ‘tunnel channel’ is a genetic rather than descriptive term, reflecting subglacial hydrologic conditions implying bankfull conditions. Tunnel channels in Minnesota and Wisconsin are relatively narrow (180 m–1 km) with an average width of 300 m and are up to 70 m deep, with 10 m being the average (Wright, 1973; Clayton et al., 1999). Elsewhere tunnel channels often exist as individual channels (e.g. Rains et al., 2002) while others form networks (e.g., in Minnesota, Wright, 1973; on the Nova Scotian continental shelf, Boyd et al., 1988; southern Ontario, Brennand and Shaw, 1994; Sharpe et al., 2004; northern Germany, Ehlers and Linke, 1989; Piotrowski, 1994, 1999; Piotrowski et al., 1999; and the North Sea, Huuse and Lykke-Andersen, 2000). Rains et al. (2002) described discrete tunnel channels that abruptly start and stop, are up to 20 km long and 1 km wide with undulating long-profiles, incised on the slopes of the foothills in Alberta. Further southeast in Alberta, Beaney (2002) described tunnel channels 100 m deep and up to 5 km wide, usually with convex-up longitudinal profiles that cut across a proglacial drainage

Fig. 1. Location map of (A) east-central North America. Boxed numbers 1 and 2 refer to study sites of Kor et al. (1991) and Kor and Cowell (1998), respectively. (B) Study area affected by the Saginaw Lobe with the Lake Michigan Lobe to the west and Huron-Erie Lobe to the east. (C) Labeled moraines as mapped by Farrand (1982). (D) Color DEM constructed from USGS. seamless 1 arcsec data with 30 m horizontal resolution and synthetic hillshaded relief from 315 at 451 elevation. Colors from white to green indicate high to low topography. Letters refer to place names: C— Coldwater; K—Kalamazoo; BC—Battle Creek; A—Albion; J—Jackson; L—Lansing. End moraines of various texture from Farrand (1982) are shown as transparent white overlays. (E) Close-up of the area mapped by previous researchers as the Tekonsha Moraine (Fig. 1C). Note that the drumlin field (circled D’s) is continuous across the mapped moraine and is cut by tunnel channels (black dashed lines), with fans lacking at the distal end of the tunnel channels. Therefore, the upland ridge with drumlins should not be considered a recessional moraine. Some fans did develop on this upland during ice recession: C—Climax fan; DH—Dixon Hill kame; S—Stony Point Hill fan.

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Fig. 2. Sketch to illustrate the difference between tunnel channels and valleys. Adapted from Clayton et al. (1999).

divide. It is not known whether there is an equilibrium form for tunnel channels. They appear to exist in various development stages with forms representative of flow duration and intensity of erosion, with weakly developed forms referred to as incipient tunnel channels (Sjogren et al., 2002). Often eskers are found within the tunnel channels and the channels cross-cut drumlins (cf. Shaw, 1983; Rains et al., 1993; Brennand and Shaw, 1994; Shaw, 1996; Rains et al., 2002). The characteristics of tunnel channels that can aid in their identification and which are used in this study are: (1) channels that often abruptly start or stop; (2) anabranching of the channels, sometimes giving the landscape a rectilinear appearance; (3) convex-up longitudinal profiles; (4) undulating long profile, with reverse gradients with respect to the ground surface; (5) occupation by elongated lakes and underfit streams; (6) often hosting eskers; and (7) cross-cutting drumlins. Although by no means are all of these characteristics required to identify a tunnel channel. Occasionally, tunnel channels and their infills are preserved within the sedimentary record (e.g., Piotrowski et al., 1999; Russell et al., 2003), but more often they are recognized by their morphology and topographic relationships. The formation of tunnel channels is most often attributed to incision by channelized, high velocity, subglacial meltwater flow under hydrostatic pressure (Wright, 1973; Shaw and Gilbert, 1990; Brennand and Shaw, 1994; Piotrowski, 1994; Barnett et al., 1998;

Clayton et al., 1999; Beaney, 2002; Cutler et al., 2002; Rains et al., 2002; Sjogren et al., 2002). Piotrowski (1994) suggested that some tunnel valleys and their infills record continued development by successive glaciations. Time transgressive development during retreat may be indicated by recessional moraines with outwash fans at the distal ends of tunncl valleys (Mooers, 1989; Clayton et al., 1999; Cutler et al., 2002). Wright (1973) inferred release of stored water from the Lake Superior basin and speculated that a supraglacial water source might be important but was unable to explain how supraglacial meltwater could penetrate cold ice. Drainage of supraglacial lakes from the Greenland Ice Sheet has been observed (e.g., Russell, 1993), and incipient tunnel channels from Michigan (Sjogren et al., 2002) consisting of linked circular depressions may best be explained by supraglacial conduits (moulins) connected to subglacial streams. Cutler et al. (2002) and Piotrowski (1994) discuss the role permafrost may play at the ice margin for water storage preceding outburst flooding. Breemer et al. (2002) modeled basal water pressures beneath the Lake Michigan Lobe and determined that subglacial aquifers were not capable of draining excess basal meltwater. Kehew et al. (2005) discuss the role of excess meltwater and fast ice flow of the Lake Michigan Lobe in the Michigan basin. Flow within a tunnel channel in southeast Alberta was modeled and it was determined that energy losses from friction constrain the maximum discharge, which for channels in southeastern Alberta was 107 m3 s 1 (Beaney and Hicks, 2000). The water source for tunnel channels continues to be an important research issue. Shoemaker (1999) hypothesized that many of the Great Lake basins could have acted as reservoirs. Sedimentological studies by Munro-Stasiuk (2003) have documented a subglacial reservoir in southern Alberta based on the spatial distribution of glaciolacustrine sediment interbedded with till. The plausibility of subglacial Laurentide lakes is supported by the presence of subglacial lakes beneath the Antarctic ice sheets (e.g. Siegert, 2000). However, the spatial and temporal relationships to ice streams (e.g. Siegert and Bamber, 2000) and outburst floods are only conjectural at this time. The 1996 jo¨kulhlaup from Grı´ msvo¨tn was recently modeled in an attempt to explain a hydrograph that peaked over a short time interval followed by declining flow, the opposite of the normal hydrograph (Bjo¨rnsson, 2002). A coupled sheetflow and conduit flow geometry was able to simulate the major features of that flood, including the unchannelized and conduit flows emerging at the ice margin (Flowers et al., 2004). The presence of tunnel channels implies that large meltwater flows, presumably from subglacial reservoirs, were important glacial processes for the development of glacial landscapes and sediments.

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5. Methodology The digital elevation model (DEM) used in this study was built in ARCVIEW V3.2 from seamless data (http:// seamless.usgs.gov/) with a 30 m horizontal resolution artificially illuminated from 3151 azimuth, 451 above the horizon. Shapefiles of the Quaternary sediments (Farrand, 1982) were downloaded from the Michigan geographic data library (www.mcgi.state.mi.us/mgdl/). Our DEM analysis is a continuation of previous detailed studies in the area (Fisher and Taylor, 2002; Fisher et al., 2003) that examines a larger part of the Saginaw Lobe outlined in Fig. 1. The GPR survey followed the methodology outlined in Jol and Bristow (2003). A pulseEKKOTM 100 unit with 100 MHz antennae and a 400-V transmitter was used to conduct the survey on a boulder-gravel hummock. The upper 1–2 m thick conductive till was cleared as much as possible from the boulder gravel to allow for greater signal penetration into the hummock. Based on earlier projects in this area (Fisher et al., 2003), the above configuration worked well in the region and provides high-resolution data (dm scale) of the subsurface sedimentary architecture. The traces were collected at 0.25 m intervals along parallel lines of 23–31 m length and spaced 0.5 m apart. This resulted in a grid of 20  25 m. The lines were oriented approximately perpendicular to the strike of the gravel beds. Data were collected in step mode with the antennae operated in a perpendicular broadside configuration with a separation of 1 m. The traces were stacked 32 times with a sampling rate of 800 ps. A common midpoint survey provided a near surface velocity of 0.09 m/ns to determine the thickness of the sediment. The profile was corrected for topography using a laser-level survey. For further information on GPR surveys in glaciated areas see Fisher et al. (1995) or Jol et al. (1996). After acquisition, data were processed and converted into a 3D volume using EKKOTM 3D software. The volume was imported, viewed and interpreted using a 3D visualizing software package, T3DTM. The software enables a cube of data to be displayed in a variety of orientations, including vertical sections in the dip direction or in the strike direction and horizontal sections (planview) called time slices.

6. Results 6.1. Saginaw lobe tunnel channels Previous workers described parts of the study area as a dissected upland landscape surrounded by lowlands incompletely filled with outwash (Fisher and Taylor, 2002; Fisher et al., 2003). The lowlands are flat-bottom valleys, some with terraces. Many upland surfaces host

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drumlins without the upland itself being streamlined. The lowlands have a reticulate pattern and are divided into two groups based upon their relative age determined by cross-cutting relationships. The youngest set exhibits a radial divergence from the Saginaw Bay area and is parallel with drumlins, while the older lowlands are oblique to the younger set, and their southwesterly valley sides are in places drumlinized and indented with reentrants. Because most of the lowlands are interpreted as tunnel channels (cf., Fisher and Taylor, 2002; Fisher et al., 2003), they will now be referred to as tunnel channels. The younger set of tunnel channels radially diverges across the study area, and is especially evident in the northern part of Fig. 3. There is an average spacing between the channels of 6 km with a standard deviation of 2.7 km. The distribution of mapped tunnel channels decrease in density but increase in width in a southwest direction. In the southern part of the study area the channels diverge to the southwest towards the Sturgis Moraine. Many of the tunnel channels are best developed on the proximal side of the upland ridges giving the landscape its dissected appearance, with the numerous reentrants giving it an indented appearance (black and white arrows, Fig. 4A). The younger tunnel channels do not have a consistent morphology across the study area. However, they usually have a northeast–southwest orientation. In places the tunnel channels are incompletely developed where they are inscribed across the uplands. Elsewhere, they are larger channels (modern valleys) deeply incised into upland blocks and concordant with valleys that run west–east. Across one upland, mapped as the Tekonsha Moraine (Leverett and Taylor, 1915), tunnel channels are narrow and crenulated, such low areas consisting of amalgamated circular depressions surrounded by drumlins were interpreted to be incipient tunnel channels (Sjogren et al., 2002). An older set of valleys (tunnel channels) are observed and shown as dashed and thin black lines on Fig. 3. Tunnel channels indicated by dashed black lines are most common in the south central area of the map, with a north–south orientation. The southernmost of these channels extend into the interlobate morainal landscape of Indiana and have been described by Brown (1999). The longest tunnel channel of this group is in the centre of the field (Fig. 3). Tunnel channels represented by the thin black lines (Fig. 3) have a west to east orientation that forms the rectilinear and dissected appearance of the landscape (some junctions indicated by circles— Fig. 4B). Modern rivers occupy many of these channels. Locally, within the study area, the bedrock is near the surface. For example, immediately west of Albion, in the Kalamazoo valley (A on Fig. 1D), Marshall Sandstone (Harrell et al., 1991) is exposed in the bottom of a shallow gravel pit.

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Fig. 3. Location of tunnel channels and eskers. The oldest tunnel channel set is indicated by the dashed black lines (north–south orientation) and thin black lines (west–east orientation). The thick black lines represent the youngest set. Eskers are shown as black triangles indicating presumed direction of flow.

The relative age of the two sets of tunnel channels can be determined by careful examination of channel edges. The older channels have their southwesterly channel sides drumlinized and indented with reentrants. For example, the proximal and distal ends of drumlins along the sides of channels (Figs. 4C, 4D) are not truncated by the older (dashed line) tunnel channel. This demonstrates the age relationship between these two landforms, with the drumlins postdating the older tunnel channel set. Therefore, the tunnel channels parallel with the drumlins (thick black lines, Fig. 4B) are assumed to be the younger set. There is another cross-cutting relationship observed usually along the margins of the older tunnel channel set cutting into the upland surface (e.g. black arrows on Figs. 4A, B). However, this crosscutting relationship is best explained by lateral planation associated with outwash deposition in a proglacial rather than subglacial setting.

Most of the lowland valleys are interpreted to be tunnel channels. Many of them have reverse gradients, with interfluves within them and widths up to 1 km are not uncommon. Often the valleys are straight, abruptly stop and start, giving the landscape a rectilinear appearance. The relationship of elongated lakes within the lowlands was established by Rieck and Winters (1980), and eskers are found within many lowlands. Not all of the valleys in the study area have all of these characteristics, but because most of the valleys are concordant and part of the rectilinear pattern, at this regional scale investigation they are interpreted to be tunnel channels. The relationship of the tunnel channels with buried bedrock valleys, eskers and drumlins is discussed next. There is a coincidence of modern streams, lakes and buried bedrock valleys in the southern part of the study area (inset—Fig. 5). Using 9000 well logs, Rieck and

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Fig. 4. Tunnel channels and dissected landscape. For legend, see Fig. 3: (A) Dissected upland ridge formerly mapped as the Kalamazoo Moraine. Note the tunnel channels that dissect it, with the same dissected landscape trending further southwest. The proximal side of the upland is heavily indented with numerous reentrants marked by black and white arrows. The flat lowland area is outwash in the middle of the image. Black arrows indicate lateral planation by proglacial outwash streams. (B) Close-up of drumlins associated with tunnel channels. Dashed lines are oldest tunnel channel set trending north–south. Circled areas indicate the junction of the westerly and southwesterly trending channels. Note in (C) and (D) that drumlin noses are part of the western tunnel channel side (an escarpment) indicating that the drumlins are younger than the tunnel channels.

Winters (1980) mapped buried bedrock valleys, with up to 90 m relief on the bedrock, underlying modern river courses and lakes. They also observed that organic material within logs was primarily found in buried valley fills and not in the sediment underlying interfluves. Depth to the organics is approximately 45 to o75 m, but on average about 30 m. Ages from the organic material ranged from mid-Wisconsinan to infinite, indicating minimal subsequent glacial erosion of the valleys. Alternatively, the dated material could be reworked, a possibility that cannot be discounted because detailed stratigraphy was not reported from the core logs. In Figure 5, the distribution of buried bedrock valleys (line symbol) is superimposed upon the DEM with white areas representing tunnel channels and lowland areas over part of the study area. In the northern half and southern part of the map, the bedrock valleys and the radiating tunnel channel set on the DEM

are well aligned. Also, the dimensions of the bedrock valleys are similar to the tunnel channels. In the southcentral map area, the bedrock valleys are aligned with the tunnel channels although there is a greater area of lowlands than bedrock valleys. 6.2. Eskers Eskers on Fig. 3 can be divided into two sets, either associated with tunnel channels or not. The northwestern group of eskers on Fig. 3 is found within the tunnel channels. They were also reported by Kozlowski et al. (2003) but were not mapped by Farrand (1982). More detailed mapping (Fisher and Taylor, 2002) around Albion (A on Fig. 3) found small eskers, o0.5 km in length, 50 m wide, 10 m high, and not visible on Fig. 3, that were restricted to the younger set of tunnel channels cross-cutting the uplands. Therefore

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the distribution of eskers shown on Fig. 3 should be considered a minimum. The presence of eskers within the tunnel channels indicates that they post-date formation of the tunnel channels. Elsewhere, particularly in the northeastern quadrant of Fig. 3, many of the eskers show only a weak correspondence with tunnel channels. They are mapped outside of tunnel channels, oblique to them and across the tunnel channel interfluves. Brennand and Shaw (1994) observed small eskers on the tunnel channel interfluves, and larger eskers concentrated within tunnel channels of southern Ontario. 6.3. Drumlins and landform associations

Fig. 5. Relationship between tunnel channels (white) and surficial traces of buried bedrock valleys. Where the two map areas overlap, there is a close correspondence between them. Buried bedrock valleys elsewhere show a strong relationship with valleys on the DEM. Inset figure adapted from Rieck and Winters (1976).

In the southwest part of the study area, the upland surfaces host drumlins of the Union City drumlin swarm (Fig. 6). In the area marked C (Fig. 6) the drumlins are small, stubby and have the highest density within the study area (Fig. 7A). They decrease in number to the northeast and east. There is a landform transition to the southwest of hummocks on the upland surfaces in the Albion area, to streamlined forms and drumlins on upland surfaces in the Tekonsha area (Fig. 8). The bestdeveloped drumlins are in the centre of the swarm (just south of Union City—Figs. 6, 7B). The areal extent of uplands and density of drumlins decrease in a southwestward direction towards the Sturgis Moraine. Well logs and shallow excavations (house foundations) into

Fig. 6. The Union City drumlin swarm is found on the upland blocks between tunnel channels. Note that the proximal end of the drumlin swarm corresponds with the bedrock transition from sandstone to shale. Note the wide lowland areas between the distal end of the drumlin swarm and the Sturgis Moraine. Tunnel channels ending at the Sturgis Moraine head large outwash fans. The large white C is the climax fan burying drumlins.

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Fig. 7. Examples of drumlins enlarged from the DEM: (A) Higher density of drumlins is associated with incipient tunnel channels. (B) More fully developed drumlins are larger and individual forms are more clearly delineated.

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set oriented north–south are on the distal ends of the uplands. These two sets of tunnel channels make up the reticulate network of lowlands. On the northeastern side of the drumlin swarm the lowland areas occupies a larger area and its pattern is more complex. Between the drumlins and the Sturgis Moraine the lowland area increases in area. Other glacial landforms younger than the drumlins show up particularly well on the Tekonsha upland. The Climax fan, Dixon Hill kame and Stony Point Hill fan (C, DH, and S, respectively Fig. 1E) described previously by Fisher and Taylor (2002) and a concentration of boulders mapped by Leverett and Taylor (1915) are ice marginal landforms. Further northeast on the Kalamzoo upland, Fisher and Taylor (2002) also described minor recessional ridges approximately parallel to the uplands, similar to minor ridges described from the Des Moines Lobe in Minnesota (Patterson, 1997). Other than the Climax and Stony Point Hill fans, no other large fans are observed on the DEM. 6.4. Glaciofluvial deposits

Fig. 8. Down flow progression of subglacial bedforms from hummocks on the Kalamazoo upland to drumlins on the Tekonsha and surrounding uplands.

the drumlins on the upland block southwest of Tekonsha indicate that they consist of horizontal beds of various textured diamictons overlying gravels (Fisher and Taylor, 1999) and were interpreted by Fisher and Taylor (2002) as erosional remnant drumlins, resulting from erosion by subglacial meltwater (cf. Shaw and Sharpe, 1987). Such drumlins are also referred to as Beverley’s (Shaw, 1996). The northeast edge of the drumlin swarm is coincident with a change in bedrock from sandstone to shale, with the drumlins underlain by shale (Fig. 6). Associated with the drumlins on the upland blocks are the tunnel channels and broad lowland areas that give the landscape its dissected appearance (Figs. 6 and 8). In general the proximal (northeast) side of the uplands are indented with reentrants and have a more streamlined geometry. As described above, most of the reentrants are the proximal end of the most recent set of tunnel channels, some of which do not cut across the entire upland surface. Southwest of Tekonsha and on the far western upland surface, the older tunnel channel

Outwash in the lowlands has been interpreted by previous researchers (Leverett and Taylor, 1915; Ekblaw and Athy, 1925; Zumberge, 1960) however, rarely was the sediment described. Near the Sturgis Moraine, outwash making up fans was deposited from the retreating Huron-Erie Lobe (Brown, 1999). This meltwater flowed northwestward into southern Michigan building up the Brighton Fan south of the Sturgis Moraine and deposited outwash on the northeast (proximal) side of the Sturgis Moraine (Fig. 6). The large lowland areas oriented northwest–southeast parallel to the Sturgis Moraine may be former tunnel channels associated with the Lake Michigan Lobe. Kehew et al. (1999, 2005) present arguments for interactions between the Lake Michigan and Saginaw Lobes describing palimpsest fans and tunnel channels. Much of the Sturgis Moraine is a glaciofluvial deposit (Fig. 9A) with large fans extending from it. The youngest set of tunnel channels ends at the Sturgis Moraine (Figs. 3 and 7). Our general observations from various lowlands in the southern half of Fig. 3 revealed that smaller calibre and better-sorted gravel is found in the older set of tunnel channels compared with the younger set. Crossbedded sand and gravel (Fig. 9B) is consistent with an outwash interpretation. In most of these older tunnel channels associated with the lowest elevations, the water table is near the surface and aggregate mining involves draglines in ponds; and sedimentary exposures do not exist. Conversations with gravel pit operators indicates gravel thickness often greater than 35 m. Fisher and Taylor (2002) described the gravel found in the upland surfaces and from within the youngest tunnel channel

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Fig. 9. (A) Vertical exposure of headwall on the distal side of the Sturgis Moraine. (B) Sets of cross-bedded sand and pebble gravel a gravel pit within outwash in the Nottawa Valley, 20 km west of Union City (Fig. 7). (C) Poorly sorted cobble and boulder gravel from half-way pit, east of Albion, MI.

Fig. 10. (A) Location of GPR transect on a gravel hummock within a tunnel channel at the Rockwell pit. Flow direction is indicated by white arrow, view is to the southwest. Blue lines are only a few of the GPR lines, additional lines are parallel and perpendicular to these. Bedding in the gravel was not obvious from the outcrop. (B,C) Ground penetrating radar (GPR) cubes that illustrate semi-continuous horizontal and inclined reflectors. (D) Traced reflectors in (C) that are interpreted to show 2-D gravel bedforms migrating from the northeast to the southwest. The offset or sag structure within the ellipse is syndepositional, and likely indicates a slump or local erosion within the foresets during bedform migration.

set, and it can be characterized by a larger calibre with poorer sorting (Fig. 9C). Because much of the sediment within tunnel channels and lowland areas is rarely exposed, the GPR survey

was intended to provide 3D sedimentologic and stratigraphic information on gravel buried under till from within a tunnel channel on an upland surface (Fig. 10). High quality data, with events at up to 250 ns (depth of

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12 m), were obtained. Below 7 m some portions of the cube reflections are not as strong, probably due to signal attenuation caused by incomplete removal of the surface till. Reflections in the upper 7 m of the cube generally correspond with the depth of gravel present in the exposure (Fig. 10A). Inclined reflections predominate along the northeast–southwest face (Fig. 10C). Interpreted in 3-D, two packages of inclined reflections (ranging in dip from 51 to 321 and 3–5 m thick) are separated with the lower one onlapping an undulating, continuous to semi-continuous reflection. Within the cube perpendicular to the inclined reflections, the reflections are discontinuous, horizontal and slightly wavy (Figs. 10B, C). The upper 3 m package, consists of a semi-continuous, sub-horizontal reflection pattern that truncates the upper inclined reflection package. The ellipse in Fig. 10D outlines a fault-like, or sag reflection pattern. A subglacial environment for the gravels comprising the hummock is indicated by overlying till with S1 values of 0.5 and dominated by Canadian Shield lithologies that drape the Kalamazoo upland and were interpreted to be stagnation deposits (Fisher and Taylor, 2002; Fisher et al. 2003). Additional evidence for a subglacial environment for deposition of the gravel was outlined by Fisher and Taylor (2002): (1) lodgment and subglacial, melt-out till beneath the gravel, and a supraglacial melt-out till or flow till above the gravel, and (2) clast lithologies within the till (both underlying and overlying till) and gravel show a consistent upsection transition from locally derived clasts to fartraveled erratic clasts from the Canadian Shield. If the gravel were deposited proglacially, in front of an advancing glacier, then such a sequential change in lithology would not develop. In addition, the gravel would be overlain by lodgment and subglacial, melt-out till, not a supraglacial till. Based on the dip angle, scale and sediment facies, the reflection packages are interpreted to represent a migrating glaciofluvial bedform. The inclined reflections are foreset beds while the sub-horizontal reflections that separate the foresets represent the erosional surface over which the bedform is migrating. Only the foresets are being preserved, the sub-horizontal reflections are a disconformity. The main reflection at about 7 m depth appears to increase in elevation to the southwest, suggesting that the bedform is infilling a depression. A similar interpretation was made from one 2-D, GPR transect on an adjacent hummock (Fisher et al., 2003). The fault-like, or sag reflection pattern may indicate a slump or local erosion within the foresets during bedform migration. In the SW corner of Fig. 10C, the bedding below 7 m appears to be dipping to the west and northwest, and the more continuous, albeit dimmer (indicating attenuation of the signal) reflections, suggest

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large-scale sand bedforms. With the 3-D internal view of the hummock, the bedform appears to have a twodimensional geometry characteristic of planar rather than trough cross-bedding. Other subglacial bedforms, such as straight-crested, or slightly linguoid gravel dunes up to 10 m high were deposited within the tunnel channels of the Trenton drumlin field in southern Ontario (Shaw and Gorrell, 1991). These bedforms are interpreted to be subglacial because they are in tunnel channels, eroded into drumlins, associated with adjacent eskers and lie below the level of a subsequent proglacial lake (Shaw and Gorrell, 1991). The dunes consist of unconformable, complex sets of foresets recording bedform migration with changing crest lines. Huggenberger et al. (1998) observed a similar GPR stratigraphy within coarsegravel bedforms in the Altai area of Siberia deposited from immense outburst floods (cf. Baker et al., 1993) and the hummocky morphology observed in the field area is similar to the scabland topography of the Channeled Scablands (Sjogren et al., 2002). Other large gravel dunes deposited subaerially from jo¨kulhlaups commonly contain boulders and well-defined crossbedding (Baker, 1973; Carling, 1996). We therefore conclude that the hummocks are large-scale, subglacial sand and gravel dunes that developed within the younger tunnel channel set, associated with large subglacial metlwater events.

7. Discussion The lowland valleys described above are interpreted as tunnel channels, because of their anabraching, rectilinear pattern, convex-up long profiles, inter-channel divides, and host eskers. Not all of the channels host eskers or have a convex-up long profile, but the anastomosing network pattern of the channels strongly suggests that they are indeed tunnel channels. One important difference seen in this study area is the relationship of drumlins cross-cutting an older channel set. The origin of the older channels may be as simple as an older set of tunnel channels from the same, or an older glaciation with a different ice surface profile, or may reflect a polygentic origin because of their coincidence with deeper bedrock valleys (Fig. 5). The landform association of tunnel channels and drumlins that progress down ice to a moraine incised by tunnel channels with large outwash fans is a well-documented association in the American Midwest (cf. Blewett and Winters, 1995; Patterson, 1997; Clayton et al., 1999; Johnson, 1999). A similar landform relationship exists in the Finger Lakes area of New York State. A band of drumlins with tunnel channels transitions to a set of tunnel channels ending at the waterlain Valley Heads

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moraine and large gravel outwash fans (Mullins and Hinchey, 1989). 7.1. Incompletely filled lowlands and buried bedrock valleys That the tunnel channels can be recognized today implies that they were incompletely filled with sediment upon deglaciation. From this we can assume that the lowlands were considerably deeper at deglaciation, but it is unknown what the relative amounts of outwash and subglacial, glaciofluvial sediments are. The dune bedforms described above, and glaciofluvial sediment described from a nearby pit (Fisher et al., 2003) were both capped by till and record deposition from subglacial meltwater. Additional geophysical investigations of the tunnel channel fills are needed to determine the volume and character of the sediments associated with the tunnel channel flow. Rare outcrops in the field area (e.g., Fig. 9B), exhibit outwash typical of braided stream environments (cf., Smith, 1985). Outwash systems in proglacial settings are usually considered to be aggradational, which does not explain incision of the valley hosting the sediment. This is especially evident on the proximal side of the Sturgis Moraine (Fig. 6) where outwash from the Saginaw and Huron-Erie Lobes was deposited (Brown, 1999). The Erie Lobe meltwater appears to have trimmed the southwestern side of the upland blocks proximal to the Sturgis Moraine. The relative amount of glaciofluvial sediment from tunnel channel and outwash environments is unknown in the field area, but was insufficient to fill the valleys upon deglaciation. There appears to be some correspondence of tunnel channels overlying buried bedrock valleys (Fig. 5). Previous workers identified the relationship of rivers and lakes (lowlands) with buried bedrock valleys, and also noted that new river courses cut across high-relief uplands instead of staying within lowlands (Rieck and Winters, 1980; Winters and Rieck, 1982). These authors did not have a reasonable explanation for these observations other than recognizing that it was not postglacial landscape development, but was either the influence of a bedrock-controlled Pleistocene paleosurface or erosional affects of the last glaciation (Rieck and Winters, 1980). When the origin of the lowlands are viewed as tunnel channels, then such a distribution of lowlands cross-cutting uplands and palimpsest topography with bedrock valleys is more simply explained. During glaciation, meltwater flow would be affected by the distribution of buried bedrock valleys (Fig. 5), or tunnel channels dating from the previous glaciation(s) with a similar ice flow direction. Although speculative at this point, with a recognized need for more stratigraphic control on the channel fills, the tunnel channel/bedrock relationship may have begun with the initial glaciation

of southern Michigan. Episodic development of a tunnel channel during numerous glaciations has been thoroughly documented by Piotrowski (1994) and we propose that similar processes were operating in southern Michigan. It is now recognized that tunnel channels crosscut the uplands with rivers and lakes inheriting this topography upon deglaciation. With this realization that tunnel channels crosscut the uplands we now focus on the origin of the uplands and suggest that they are not recessional moraines as originally suggested by Leverett and Taylor (1915). 7.2. Recessional moraines or upland ridges? Ever since the original mapping by Leverett and Taylor (1915), the series of uplands, many with transverse orientations to ice flow out of Saginaw Bay, have been interpreted as recessional moraines (e.g., Figs. 1, 3). This deglacial paradigm was recently questioned using sedimentological and geomorphological arguments. Fisher and Taylor (2002) observed that the uplands in the Kalamazoo ‘Moraine’ are composed of boulder gravel interbedded between sub- and supraglacial till deposits, and crosscut by tunnel channels with fans absent on the distal side of the uplands. The landform transition from boulder gravel hummocks draped with till on the Kalamazoo upland, to drumlins proximal, on top of, and distal to the Tekonsha upland records a subglacial rather than an ice-marginal landscape (Fisher and Taylor, 2002; Sjogren, et al., 2002). Therefore, the Tekonsha upland itself must be stratigraphically older than the drumlins, tunnel channels, and minor ice marginal landforms (kames and fans) that it hosts. Using these landform relationships we briefly discuss the evidence for other ice margins of the Saginaw Lobe in the study area. The Sturgis Moraine is a significant landform composed primarily of gravel as indicated by road-cut exposures and active gravel pits (Fig. 9A), and it has large outwash fans on its distal side. The moraine is not streamlined and consequently is interpreted not to have been overridden. It is these characteristics that we look for on the DEM for mapping recessional moraines of the Saginaw Lobe. As described above, from our analysis of the DEM in Fig. 3, no large fans were observed at the ends of the tunnel channels up ice of the Sturgis Moraine. The tunnel channels are observed to be crosscutting the uplands previously mapped as moraines (Figs. 1, 3) and without fans at their distal ends such as is characteristic of the Sturgis Moraine and other moraines in the Midwest (e.g., Mooers 1989; Patterson, 1997; Clayton et al., 1999; Johnson, 1999) we do not interpret the unnamed, Tekonsha, Kalamazoo and possibly even the Charlotte uplands (Fig. 1C) to be moraines. There is no doubt that significant outwash deposits are associated

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with the recession of the Saginaw Lobe. It is possible that some fans were buried or eroded in a proglacial setting, and that some of the tunnel channels were later occupied during deglaciation and small fans developed. The Climax and Stony Point Hill fans and Dixon Hill kame (Fig. 1E) all developed along an ice margin, but only on top of the Tekonsha upland. Small fans, not evident on Fig. 3 likely also developed during deglaciation where tunnel channels of different orientations intersect. It is likely then, that when the tunnel channels were active in the study area, the ice margin was at the Sturgis Moraine, or possibly even further south. Numerous lineaments, primarily ridges at least an order of magnitude smaller than the upland ridges, are orientated perpendicular to the drumlins and tunnel channels, and may record ice recession in the area (Fisher and Taylor, 2002). The presence of either a supraglacial melt-out till or debris and till flows with S1 values of 0.5 was interpreted as a stagnation deposits but could also represent deposition at a continuously receding ice margin. Significantly more detailed sedimentological investigations of this largely unstudied landscape are required to further elucidate the style of deglaciation. 7.3. Regional scale subglacial meltwater flow Elements of the geometry, duration and spatial characteristics of the subglacial meltwater activity that sculpted the landscape in south-central Michigan, can be inferred from the sediments and geomorphology. In this paper we have documented a reticulate pattern of tunnel channels, with a crosscutting relationship with drumlins. Here we observed that drumlins crosscut tunnel channels, implying that part of the tunnel channel network is older. Such a relationship should not be surprising if one tries to envision what would happen in the field area during a subsequent glaciation, the older tunnel channels valley walls would serve as escarpments, obstacles to ice or water flow. The stratigraphic work by Fisher and Taylor (2002) documented that boulder-gravel sheets extensive over a 415 km width on the Kalamazoo upland is subglacial in origin, and is cross-cut by tunnel channels. This was interpreted as a sheetflow collapsing into channelized flows. This evidence for a sheetflow is unique because there is a sedimentary record for it rather than just an unconformity with scattered boulders (cf. Munro and Shaw, 1997). In this paper, the landscape is examined further to the west and southwest of the earlier study, and more tunnel channels are mapped, and erosional remnant drumlins described. Down flow of the hummocky boulder gravel in the Kalamazoo uplands, to the southwest, and on a negative slope, the erosional remnant drumlins with many boulders on their surface can be considered an unconformity. In this reconstruction, flow acceleration on a negative slope is consistent

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with the transition from a sheetflow capable of eroding, transporting and depositing boulders, to a purely erosional meltwater sheetflow that eroded the substrate, leaving behind streamlined residual drumlins. The youngest set of tunnel channels incised into the boulder gravel indicates they developed after the sheetflow that deposited the Kalamazoo upland boulder gravel. Gravel hummocks on the Kalamazoo uplands and within tunnel channels explored using GPR (Fisher et al., 2003) and this paper, reveal the presence of large gravel bedforms deposited in a subglacial environment, similar to those observed in southern Ontario (Shaw and Gorrell, 1991). Comparable deposits have been described from modern jo¨kulhlaups from Iceland (Russell et al., 2001), from large subaerial jo¨kulhlaup deposits in the Altai Mountains, Siberia (Carling, 1996), and the Channeled Scablands (Baker, 1973). The initiation of tunnel channel flows and sheetflows, and their relative timing is an important question. It is unknown whether subglacial sheetflows could develop from tunnel channel flows as the discharge increased, or if sheetflows only result from drainage of subglacial lakes (e.g., as modeled by Flowers et al., 2004). Catania and Paola (2001) modeled subglacial water flow in a pressurized flume with a non-cohesive bed, and when discharge was increased, there was greater lateral pressure variability that led to increased braiding and channel development. By analogy, beneath an ice sheet, this would result in increasing the number of tunnel channels and presumably a decreasing effective bed pressure, which if it continued, may have led to flotation of parts of the glacier (cf. Piotrowski and Tulaczyk, 1999). Flowers et al. (2004) used a sheetflow to explain the Grıˆ msvo¨tn jo¨kulhlaup and Roberts et al. (2000) described from the same flood, the near-instantaneous rise to peak subglacial water pressure that resulted in debris-charged, supercooled water exiting the glacier in supraglacial positions during the meltwater event. The mechanisms of a downflow transition from a sheetflow to conduit flows is unknown. The nature of the bed beneath Skeidara´rjo¨kull after the 1996 jo¨kulhlaup is also unknown. It appears that the initial flow exited the glacier as a sheetflow from hydrofractures at the ice margin in englacial positions, and also as large conduit flows in preexisting drainage ways (Roberts et al., 2000). These observations are consistent with the modeling by Flowers et al. (2004) that sheetflows must have existed to explain the hydrograph. The relationship of tunnel channels crosscutting drumlins in Michigan suggests that sheetflows do not abruptly stop, but may evolve into channelized flows. This would especially be true if the location of the sheetflow coincided with the location of a subglacial lake as it began to drain. The very wide lowlands between the drumlins and the Sturgis Moraine (Fig. 6) may record the transition from a sheet to conduit flow.

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Fig. 11. Model for development of the most recent set of tunnel channels: (A) Existing landforms where the thick black lines are the most recent tunnel channel set, and dashed and thin black lines are older channel sets. (B) Subglacial meltwater likely followed preexisting conduits when subglacial meltwater discharge increased, assumed to be from release of water from a reservoir further up ice. (C) Continued increase in discharge led to formation of new channels (cf. Catania and Paola, 2001), and the laterally extensive (415 km width) of boulder gravel between basal and supraglacial till in the area (Fisher and Taylor, 2002) suggests a sheetflow developed (large arrow). At this time, some of the new tunnel channels likely acted as channels within the broader flow. (D) Collapse of sheetflow into the new and older tunnel channels. Eskers within some of these channels record subsequent flow. (E) Modern topography with reticulate pattern of tunnel channels and distribution of subglacial landforms: H—hummocks; SH—streamlined hummocks; D—drumlins, see also Fig. 8 for a plan view of the landform transition.

Our qualitative model for landscape evolution in south-central Michigan may be summarized as follows. An older set of tunnel channels palimpsest with buried bedrock valleys probably operated as subglacial conduits with unknown discharges, but not necessarily catastrophic, before the development of a sheetflow (Fig. 11B). The origin of the sheetflow is unknown, but the sheets of boulder gravel and erosional-remnant drumlins further downstream, are evidence for one. If the meltwater event began slowly, as seen on generalized jo¨kulhlaups hydrographs (Tweed and Russell, 1999), there could have been an increase in the number of channels before decoupling of the glacier from its bed by a sheetflow developing in response to increasing discharge and glaciohydraulic pressure (Fig. 11C). Alternatively, draining of a subglacial lake where the ice was already decoupled from the bed generated the sheetflow. The boulder-gravel sheets on the Kalamazoo upland, and the Union City drumlins across the Tekonsha upland may have formed at this time (Fig. 11C). It is uncertain what role, if any, the change in bedrock lithology (Fig. 5) might have had on the transition from hummock forms to drumlins. Waning discharge, and evolution of the younger tunnel channels (Fig. 11D) was succeeded by formation of eskers within the tunnel channels. During ice recession, outwash partially filled the tunnel channels. Within the Great Lakes region, other large subglacial meltwater floods have been inferred from landforms upice of the study area in Ontario (#1, 2 on Fig. 1A). Kor and Cowell (1998) documented s-forms and drumlins incised into the Niagara Escarpment on the Bruce Peninsula and in shallow water of Georgian Bay and Lake Huron (#2). Further up-ice, Kor et al. (1991)

described similar landforms with an 80 km flow width centred on the French River (#1). It is unknown whether these landforming events are related in time to each other, or with the landforms described in this paper. Their alignment suggests a relationship, but mapping channels, drumlins and stratigraphy across the floor of Georgian Bay and then across Lake Huron into Michigan is necessary to determine whether all of these landforms are from the same subglacial meltwater event, or if their development is time transgressive. For additional information on other regional-scale meltwater floods in the Great Lakes area, see Munro-Stasiuk et al. (2005). The source of the meltwater remains unresolved but two possibilities are discussed, a local Great Lakes source or a more distal James Bay source (Kor et al., 1991). First, water was either stored in the Saginaw Bay and its Lowlands or within the Huron basin as modeled by Shoemaker (1999). Storage and then release of the water may have been initiated by hydraulic ponding beneath the Saginaw Lobe, driven by the more active, and encircling Michigan and Huron Erie Lobes (Steve Brown person. Comm., 2001; Kehew et al., 2005). The release of stored meltwater was mostly likely initiated when the Saginaw Lobe stood at the Sturgis Moraine, perhaps triggered by the decay of ice-marginal permafrost conditions (cf. Clayton et al., 1999; Cutler et al., 2002). Second, the study area is in the down flow area of more regional events described to the north in Ontario by Kor et al. (1991) and Kor and Cowell (1998). Moreover, the base of the tunnel channels and surface of the drumlins may correspond to the regional unconformity described recently by Sharpe et al. (2004).

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8. Conclusions With the recognition that tunnel channels are common landforms in southern Michigan, this paper reassesses the origin of the glacial landscape associated with the Saginaw Lobe of the Laurentide Ice Sheet. Tunnel channels, many with eskers, are characteristic landforms of the Saginaw Lobe. Two sets of tunnel channels were described that form a reticulate network of lowland valleys. The presence of eskers within the tunnel channels, and drumlins beginning at channel escarpments, illustrates the subglacial origin of the channels. The palimpsest relationship of buried bedrock valleys and tunnel channels suggests a cause and effect relationship between them. Once the tunnel channels become established they likely focused meltwater during subsequent glaciations. Initial focusing of the meltwater may have been controlled by the location of bedrock valleys; alternatively, many of the buried bedrock valleys may be the result of tunnel channel formation. An older set of tunnel channels is cross-cut by drumlins. Well logs indicate that the drumlins are erosional remnants of a more extensive surface, and are explained as erosional forms sculpted by subglacial sheetflows (cf. Shaw and Sharpe, 1987). Up flow of the drumlins, evidence for lower velocity, subglacial meltwater sheetflows is a boulder gravel sequence underlain by subglacial tills and overlain by supraglacial tills or flow tills. The gravel sheet has a hummocky morphology; some hummocks are gravel bedforms that transition to drumlins of the Union City swarm in a down-ice direction. It is suggested that a sheetflow may have evolved from tunnel channel flow as discharge increased. After the boulder-gravel sheets were deposited, and drumlins sculpted, the sheetflow collapsed into a conduit flow and the youngest set of tunnel channels parallel to the drumlins formed. Tunnel channels crosscutting upland surfaces host eskers, and tunnel channels lower in the landscape are incompletely filled with outwash. The ice margin at the time of the tunnel channel flows was likely at the Sturgis Moraine where the tunnel channels end at the heads of large outwash fans. Subglacial glaciofluvial activity created much of the landscape in central Michigan. The Tekonsha and Kalamazoo upland areas associated with the Saginaw Lobe are not ice marginal landforms, instead they record evidence of subglacial process such as drumlin and tunnel channel formation. More detailed mapping is required north of the Kalamazoo uplands to determine if the uplands (moraines on Fig. 1C) are indeed moraines. Outwash fans, kames and small ridges some of which are found on the Tekonsha and Kalamazoo upland ridges record ice recession. Thus the ice marginal features are relatively minor, compared with the scale of the upland ridges, with outwash and supraglacial till

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minimizing rather than accentuating relief on the landscape. The identification of tunnel channels over broad areas demonstrates the increasingly important role meltwater played in the evolution of landscapes in formerly glaciated areas.

Acknowledgments The Carr Brothers & Sons generously allowed us repeated access to their gravel pits. Reviews by David Sharpe, Jan Piotrowski and Hazen Russell, and editorial handling by Andrew Russell clarified many of our thoughts. Larry Taylor is thanked for his many stimulating discussion about the glacial landscape in southern Michigan and for helping collect the GPR data. References Baker, V.R., 1973. Paleohydrology and sedimentology of Lake Missoula flooding in eastern Washington: Geological Society of America Special Paper 144, 79pp. Baker, V.B., Benito, G., Rudoy, A.N., 1993. Paleohydrology of late Pleistocene superflooding Altay Mountains, Siberia. Science 259, 348–350. Barnett, B.J., Sharpe, D.R., Russell, H.A.J., Brennand, T.A., Gorrell, G., Kenny, F., Pugin, A., 1998. On the origin of the Oak Ridges Moraine. Canadian Journal of Earth Sciences 35, 1152–1167. Beaney, C.L., 2002. Tunnel channels in southeast Alberta, Canada: evidence for catastrophic channelized drainage. Quaternary International 90, 67–74. Beaney, C.L., Hicks, F.E., 2000. Hydraulic modeling of subglacial tunnel channels, south-east Alberta, Canada. Hydrological Processes 14, 2545–2557. Bjo¨rnsson, H., 2002. Subglacial lakes and jo¨kulhlaups in Iceland: Global Planetary Change 35, 255–271. Blewett, W.L., Winters, H.A., 1995. The importance of glaciofluvial features within Michigan’s Port Huron Moraine. Annals of the Association of American Geographers 85, 306–319. Boyd, R., Scott, D.B., Douma, M., 1988. Glacial tunnel valleys and Quaternary history of the outer Scotian shelf. Nature 333, 61–64. Breemer, C.W., Clark, P.U., Haggerty, R., 2002. Modeling the subglacial hydrology of the late Pleistocene Lake Michigan Lobe, Laurentide Ice Sheet. Geological Society of America Bulletin 114, 665–674. Brennand, T.A., Shaw, J., 1994. Tunnel channels and associated landforms, south-central Ontario: their implications for ice-sheet hydrology. Canadian Journal of Earth Sciences 31, 505–522. Brown, S.E., 1999. Part 2—Ice marginal environments of Huron-Erie and Saginaw Lobes, north central Indiana. Guidebook for the 45th Midwest Friends of the Pleistocene Field Conference, Goshen, Indiana, Indiana Geological Survey, 81pp. Carling, P.A., 1996. Morphology, sedimentology and palaeohydraulic significance of large gravel dunes, Altai Mountains, Siberia. Sedimentology 43, 647–664. Catania, G., Paola, C., 2001. Braiding under glass. Geology 29, 259–262. Clayton, L., Attig, J.W., Mickelson, D.M., 1999. Tunnel channels formed in Wisconsin during the last glaciation. In: Mickelson, D.M., Attig, J.W. (Eds.), Glacial Processes Past and Present.

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