A geomorphic history based on topographic map evidence

The James River-Big Sioux River drainage divide area investigated here is located south of Huron and Brookings and north of Mitchell and Sioux Falls, South Dakota, USA. The James River flows south in a broad lowland interpreted here to have originated as the floor of a large south-oriented ice-walled and bedrock-floored valley sliced into the surface of a rapidly melting thick ice sheet. A west-facing escarpment separates the James River lowland from the Prairie Coteau upland to the east and is interpreted to be what remains of that ice-walled and bedrock-floored valley’s east wall. The Prairie Coteau upland surface is interpreted to be where an ice sheet remnant was isolated by headward erosion of large ice-walled and bedrock-floored valleys. The Big Sioux River valley is interpreted to have originated as a much smaller and shallower ice-walled and bedrock-floored valley. The south-oriented Vermillion River headwaters drain the west-facing escarpment slope and provide evidence related to how south-oriented melt water floods sliced the immense James River lowland ice-walled and bedrock-floored valley headward.

• The purpose of this essay is to use topographic map interpretation methods to explore James River-Big Sioux River drainage divide area landform origins south of Huron and Brookings and north of Mitchell and Sioux Falls, South Dakota, USA. Map interpretation methods can be used to unravel many geomorphic events leading up to formation of present-day drainage routes and development of other landform features. While each detailed topographic map feature provides detailed evidence to be explained, the solution must be consistent with explanations for adjacent area map evidence as well as solutions to big picture map evidence puzzles. I invite readers to improve upon my solutions and/or to propose alternate solutions that better explain evidence and are also consistent with adjacent map area and big picture evidence. Readers may do so either by making comments here or by writing and publishing their own essays and then by leaving a link to those essays in a comment here.

• This essay is also exploring a new geomorphology paradigm in which erosional landforms are interpreted as evidence left by immense glacial melt water floods. Implied in that interpretation is the immense floods were derived from a thick North American ice sheet that created a deep “hole” in the North American continent and also melted fast. The previously unexplored paradigm being tested in this and other Missouri River drainage basin landform origins research project essays is a thick North American ice sheet, comparable in thickness to the Antarctic ice sheet, occupied the North American region usually recognized to have been glaciated, and through its weight and erosive actions created a deep North American “hole”. The southwestern rim of that deep “hole” is today preserved in the high Rocky Mountains. The ice sheet through its weight and deep erosion (and perhaps deposition along major south-oriented melt water flow routes) caused significant crustal warping and tectonic change, through its action of melting fast produced immense floods that flowed across the continent, and through its action of melting fast systematically opened up space in the ice sheet created “hole” so headward erosion of newly developed north-oriented drainage systems captured immense south-oriented melt water floods and diverted immense melt water floods north into space the ice sheet had once occupied.

• If this previously unexplored paradigm is correct the geographic region explored by this essay should contain evidence of immense floods that were captured by headward erosion of new valley systems so as to cause the floods to flow in a different direction. Ability of this previously unexplored paradigm to explain James River-Big Sioux River drainage divide area landform evidence south of Huron and Brookings and north of Mitchell and Sioux Falls, South Dakota will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm. This essay is included in the Missouri River drainage basin landform origins research project essay collection.

James River-Big Sioux River drainage divide area location map

Figure 1: James River-Big Sioux River drainage divide area location map (select and click on maps to enlarge). National Geographic Society map digitally presented using National Geographic Society TOPO software. 

Figure 1 provides a location map for the James River-Big Sioux River drainage divide area south of Huron and Brookings and north of Mitchell and Sioux Falls, South Dakota. South Dakota is the state occupying much of the figure 1 map area, including the northwest and center areas. The state in the figure 1 northeast quadrant is Minnesota. Iowa is the state in the figure 1 southeast corner and Nebraska is the state south of South Dakota and located in the figure 1 southwest corner. The Missouri River flows southeast from the figure 1 west center edge area to the South Dakota-Nebraska border and then serves as the South Dakota-Nebraska border until it flows south of the figure 1 map area near Elk Point, South Dakota. The Big Sioux River flows south in eastern South Dakota from the figure 1 north edge through Watertown to Brookings and Sioux Falls before joining the Missouri River near the figure 1 south edge (and Elk Point, South Dakota). The James River is located west of the Big Sioux River and flows south and southeast from the figure 1 north edge to Huron and Mitchell before joining the Missouri River near Yankton, South Dakota. Between the James River and the Big Sioux River is an unnamed river (on figure 1) flowing south to join the Missouri River at Vermillion, South Dakota. That unnamed river (in figure 1) is the Vermillion River and it has several south and south-southeast tributaries. The James River-Big Sioux River drainage divide area investigated in this essay is located south of highway 14, which extends from Huron to Brookings, South Dakota and north of Interstate highway 90, which extends from Mitchell to Sioux Falls, South Dakota. After looking at a somewhat more detailed location map this essay begins with maps illustrating the drainage divide area just south of highway 14 between Huron and Brookings. Next the essay illustrates Vermillion River tributaries and finally the essay concludes by looking at maps illustrating the drainage divide area north of Interstate highway 90 between Mitchell and Sioux Falls.

James River-Big Sioux River drainage divide area detailed location map

Figure 2: James River-Big Sioux River drainage divide area detailed location map. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 2 provides a more detailed location map of the James River-Big Sioux River drainage divide area south of Huron and Brookings and north of Mitchell and Sioux Falls. Beadle, Kingsbury, Brookings, Sanborn, Miner, Lake, Moody, Davison, Hanson, McCook, and Minnehaha are South Dakota county names. Huron is located in the figure 2 northwest corner, Brookings is located in the figure 2 northeast corner, Mitchell is located in the figure 2 southwest corner, and Sioux Falls is located in the figure 2 southeast corner. The James River flows south in the figure 2 west half from Huron to near Mitchell and then south to the figure 2 south edge. The Big Sioux River flows south in the figure 2 east half from Brookings to Sioux Falls and then to the figure 2 south edge. The East and West Forks of the Vermillion River flow south in McCook County. Note areas in the figure 2 east half, which are west of the Big Sioux River, where there are numerous lake basins. These areas are primarily found in eastern Kingsbury County, southwest Brookings County, Lake County, western Moody County, and western Minnehaha County. These areas with numerous lakes are located on the Prairie Coteau upland surface, which is a region underlain with thick glacial moraines. The Prairie Coteau upland surface is bounded on the west by a west-facing escarpment, which slopes down into the south-oriented James River lowland, which is 100-200 meters lower in elevation than the Prairie Coteau upland surface elevation to the east. East of the figure 2 map area and the south-oriented Big Sioux River drainage basin is another region with numerous lakes and then a northeast-facing escarpment slope, leading down to the southeast-oriented Minnesota River lowland. The Minnesota River lowland surface is also 100-200 meters lower in elevation than Prairie Coteau upland surface elevations. West of the figure 2 map area and west of the south-oriented James River drainage basin is an east-facing escarpment (the Missouri Escarpment), and at that escarpment’s crest is the Missouri Coteau, which is 100 to 200 meters higher in elevation than the James River lowland elevation. The south-oriented James River lowland is here interpreted to have originated as an immense south-oriented ice-walled and bedrock-floored valley carved into the surface of a rapidly melting thick North American sheet. The southeast-oriented Minnesota River lowland is also interpreted here to have been a large southeast-oriented ice-walled and bedrock-floored valley carved into the surface of that rapidly melting thick ice sheet.The Big Sioux River drainage basin is interpreted here to have formed as a smaller and shallower ice-walled and bedrock-floored valley located between the two much large and deep ice-walled and bedrock-floored valleys. The Prairie Coteau upland surface represents where an ice sheet remnant between the two large ice-walled and bedrock-floored valleys remained, melted, and deposited whatever debris it contained.

James River lowland southeast of Huron

Figure 3: James River lowland southeast of Huron. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 3 illustrates the James River lowland south and east of Huron, South Dakota. Huron is the city in the figure 3 northwest corner. The James River flows south-southeast, and south from Huron to the figure 3 south edge. Note how level the region surrounding the James River valley is, and for that matter how level the entire figure 3 map area is (compare it to figures 4 and 5 below). This very low gradient landscape is on the floor of what originated as a large south-oriented ice-walled and bedrock-floored valley sliced by an immense south-oriented supra glacial melt water river into the surface of a rapidly melting thick North American ice sheet. That south-oriented ice-walled and bedrock-floored valley is here named the Midcontinent Trench. Figure 4 below illustrates where the Midcontinent Trench’s east wall was located, the west wall was located west of the figure 3 map area. All figure 3 drainage routes are James River tributaries and most are south-oriented, although the South Fork of Pearl Creek is a northwest-oriented tributary to southwest-oriented Pearl Creek (see Pearl Creek township southeast of Huron). The present day figure 3 drainage routes were probably established very late during the thick ice sheet’s rapid melt down, in fact they may have been established after the rapid melt down halted. However, the present day drainage routes do reflect a major south-oriented melt water flood event. The northwest-oriented South Fork Pearl Creek valley segment originated as a southeast-oriented channel in what was probably a large-scale south-oriented anastomosing channel complex encompassing the entire Midcontinent Trench floor (from the west wall to the east wall). Headward erosion of the deep southwest-oriented Pearl Creek channel beheaded southeast-oriented flood flow in what was then the southeast-oriented flood flow channel. Flood waters on the northwest end of the beheaded flood flow channel reversed flow direction to flow northwest to the deeper southwest-oriented Pearl Creek valley. The deeper Pearl Creek valley probably eroded headward when the present day James River valley eroded north and captured flood flow from the anastomising channel complex channels. At the time this final major flood event occurred there was probably still ice along the Midcontinent Trench walls (both to the east and west). Small hills in the figure 3 south center edge area may be dunes developed on flood deposited sandy soils.

West-facing Prairie Coteau escarpment and Prairie Coteau upland surface

Figure 4: West-facing Prairie Coteau escarpment and Prairie Coteau upland surface. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 4 illustrates the west-facing Prairie Coteau escarpment and the Prairie Coteau upland surface east of the figure 3 map area and includes overlap areas with figure 3. Note how there is a steady rise in elevation from the figure 4 west edge area to an upland region with many low hills and closed depressions, many of which contain lakes. The rise in elevation is the west-facing Prairie Coteau escarpment and is what remains of the Midcontinent Trench’s east wall. The upland region east of the escarpment crest is the Prairie Coteau upland surface and is where the Midcontinent Trench’s east ice wall once stood. Further north the escarpment is oriented in a north-south direction, although in figure 4 the orientation is south-southeast, suggesting the Midcontinent Trench width may be increasing to the south. Also, further north drainage routes on the west-facing escarpment are west-oriented (see James River-Big Sioux River drainage divide area south of Redfield and Watertown and north of Huron and Brookings essay which can found under James River or Big Sioux River on the sidebar category list). In figure 4 drainage routes on the escarpment slope are south and south-southeast oriented. The south-southeast oriented stream linked by a through valley to Lake Thompson and flowing to the figure 4 south center edge is the East Fork Vermillion River and in figure 4 is flowing along the escarpment crest. The south-southeast oriented stream to the west (flowing through Grafton Township) is the West Fork Vermillion River and is flowing along the escarpment face (as opposed to flowing down the escarpment slope). Figures 6 and 7 below illustrate the East and West Forks Vermillion River in more detail. Further west there are other south-oriented streams flowing along the escarpment face as opposed to flowing down the escarpment slope. This remarkable difference in escarpment face drainage orientation from regions further north suggests that in this figure 4 map area the south-southeast oriented West and East Fork Vermillion River valleys (and other south-oriented valleys further west) were eroded by the south-oriented melt water floods that carved the large south-oriented Midcontinent Trench valley into the thick ice surface. Further north, the west-oriented drainage routes flowing straight down the escarpment slope were probably eroded by melt water from the melting ice wall. The hummocky Prairie Coteau upland surface was probably created by deposition of glacially transported debris that was unevenly deposited as the ice wall melted. Many of the lake basins are probably kettles, where buried and partially buried ice masses melted, leaving depressions that are now filled with water.

Big Sioux River valley and Prairie Coteau upland surface southeast of Brookings

Figure 5: Big Sioux River valley and Prairie Coteau upland surface southeast of Brookings. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 5 illustrates the Big Sioux River valley and the Prairie Coteau upland surface south and east of Brookings, South Dakota. Brookings is the city in the figure 5 northeast corner. The Big Sioux River is the southeast oriented stream immediately southwest of Brookings. The Big Sioux River has a number of tributaries, primarily in the figure 5 east half. The figure 5 west half lacks an integrated drainage pattern, which is consistent with the interpretation the landscape was formed by deposition of ice sheet contained debris as the ice sheet slowly melted. The Big Sioux River valley and drainage area in the figure 5 east half suggests a somewhat different origin. The Big Sioux River drainage basin is interpreted to have been formed by a south-oriented ice-walled and ice-floored valley, which later became an ice-walled and bedrock-floored valley, although not as deep or as large as the Midcontinent Trench to the west or the southeast oriented ice-walled and bedrock-floored Minnesota River lowland valley to the northeast. Big Sioux River tributaries suggest the presence of other south-oriented ice-walled and ice-floored valleys in the figure 5 map area as well. Perhaps most interesting is Battle Creek, which flows southeast from Lake Badus (near northeast corner of figure 5 southwest quadrant) to near the figure 5 south center edge, and then which flows in a northeast direction to Lake Campbell (south-southwest from Brookings). The Lake Campbell outlet flows northeast and southeast to the southeast oriented Big Sioux River. The southeast- and northeast-orientation of Battle Creek suggests the northeast-oriented valley segment originated as a southwest-oriented ice-walled and ice-floored valley moving melt water southwest to join a southeast-oriented ice-walled and ice-floored valley, with water continuing south in a south-oriented ice-walled and bedrock-floored valley (south of the figure 5 map area there is evidence of a south-oriented drainage route-see figure 10). Southwest-oriented flow in the present day northeast-oriented Battle Creek valley segment was reversed to flow northeast when headward erosion of the deeper south-oriented Big Sioux River ice-walled and ice-walled or bedrock-floored valley beheaded the southwest-oriented ice-walled and ice-floored valley. Flood waters on the northeast end of the beheaded flow route reversed flow direction to flow northeast to the deeper south-oriented Big Sioux River valley. The reversed flow also captured the southeast-oriented ice-walled and ice-floored valley flow, resulting in the present day Battle Creek route.

West Fork-East Fork Vermillion River drainage divide area north

Figure 6: West Fork-East Fork Vermillion River drainage divide area north. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 6 illustrates the West and East Forks of the Vermillion River and the west-facing Prairie Coteau escarpment slope in more detail than seen in figure 4 above. The Prairie Coteau upland surface is located near the figure 6 east edge. West of the Prairie Coteau upland surface is the west-facing Prairie Coteau escarpment slope, which continues west of the figure 6 map area. Elevations at the escarpment crest are more than 100 meters higher than elevations at the escarpment base, although the elevation difference in figure 6 is slightly less. The East Fork Vermillion River flows in a south-southeast direction in the figure 6 east half near the escarpment crest (near the north-south county line). The West Fork Vermillion River flows in a south-southeast direction in the figure 6 west half and is flowing along the escarpment face rather than flowing down the escarpment slope. Other figure 6 drainage also shows evidence of flowing along the escarpment face (at least for a distance) rather than flowing down the escarpment slope. As previously mentioned, further north drainage routes flow down the escarpment slope so these drainage orientations suggest a different origin. Where drainage routes are down the escarpment face they are interpreted to have originated after the escarpment had been formed and most probably represent valleys eroded by melt water from the slowly melting Midcontinent Trench ice wall. The south-southeast oriented valleys along the escarpment face are interpreted to have formed by south-southeast oriented melt water floods at the time the escarpment face was being eroded. Why would southern Midcontinent Trench valley margins be eroded by south-southeast oriented melt water flood waters when valley margins further north were not? To the south of this James River-Big Sioux River drainage divide area north of Mitchell and Sioux Falls is the southeast-oriented Missouri River valley (see figure 1). The present day Big Sioux River, Vermillion River, and the James River all flow to the southeast-oriented Missouri River valley, providing strong evidence the Missouri River valley existed before the Big Sioux River, Vermillion River, or James River ice-walled and bedrock-floored valleys. However, the south-oriented Midcontinent Trench ice-walled and bedrock-floored valley probably was initiated before the southeast-oriented Missouri River valley existed. But once formed, the southeast-oriented Missouri River valley, which at that time probably had a north or northeast ice wall, provided a deep valley from which south-oriented ice-walled and ice-floored valleys could erode headward to capture south-oriented melt water.

West Fork-East Fork Vermillion River drainage divide area south

Figure 7: West Fork-East Fork Vermillion River drainage divide area south. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 7 illustrates the West Fork Vermillion River-East Fork Vermillion River drainage divide area south and slightly east of the figure 6 map area. The East Fork Vermillion River is again located along the crest of the west-facing Prairie Coteau escarpment and is located near the figure 7 east edge. The West Fork Vermillion River is continuing to flow south and south-southeast along the escarpment face and is flowing through Howard in the figure 7 west half. Between the West and East Forks is the south-oriented Little Vermillion River, which is also flowing along the escarpment face, rather than down the escarpment slope. As mentioned in the figure 6 discussion the south-oriented Vermillion River tributaries are interpreted here to have eroded headward from the southeast-oriented Missouri River valley at the time immense south-oriented floods were eroding the south-oriented Midcontinent Trench bedrock-floored valley. Probably events recorded by this figure 7 drainage evidence developed late during the thick ice sheet melt down history (and also during the Midcontinent Trench history), although not before the immense south-oriented melt water floods ended. The hundreds of Missouri River drainage basin landform origins research project essays are making a case that the thick ice sheet formed on a topographic surface now preserved in the highest level Rocky Mountain erosion surfaces (if the surface is preserved at all). Crustal warping has since significantly altered elevations of that surface, with down warping under the thick ice sheet and up warping elsewhere on the continent. Also, deep glacial erosion, combined with crustal down warping, created a deep “hole” in which the ice sheet was located. A modern-day model would be the Antarctic Ice Sheet and it is possible the thick North American ice sheet had “roots” extending 2000 meters or more below the ice sheet rim elevation, with the ice sheet at peak development standing 3000 meters or more above the ice sheet rim elevation. Melting of that immense ice sheet took time, even though once it started the melting was fast. This figure 7 location would have been located well below the ice sheet rim elevation and there was a long ice sheet melting history before ice-walled and ice-floored valleys finally eroded deep enough to reach the South Dakota bedrock surface seen in figure 7.

James River lowland northeast of Mitchell, South Dakota

Figure 8: James River lowland northeast of Mitchell, South Dakota. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 8 illustrates the James River lowland region north and east of Mitchell, South Dakota and is located south and west from figure 7 (and the figure 8 northeast corner includes overlap areas with the figure 7 southwest corner). Mitchell is the city located in the figure 8 southwest corner. The James River flows southeast, south, and south-southeast in the figure 8 west half and is located just of Mitchell. The rise in elevation seen near the figure 8 east edge is the lower reaches of the west-facing Prairie Coteau escarpment slope. The south-southeast oriented West Fork Vermillion River is located in the figure 8 northeast quadrant. The James River lowland surface is remarkably level and it is possible melt water floods deposited sediments in this region. The present day James River valley probably was eroded late in the ice-walled and bedrock-floored Midcontinent Trench (or James River lowland) history and is probably related to the final large south-oriented melt water flood flow event to significantly alter figure 8 landscape features. Prior to that final flood flow event there were probably many other flood flow events, some of which could have temporarily ponded water in the Midcontinent Trench valley, which would have resulted in deposition of sediments flood waters were carrying. Also, flood events probably carried ice blocks into the figure 8 map region and if water flow rates decreased for any reason the ice blocks may have become stranded and then had sediment deposited around them. Further, late during the rapid ice sheet melt down history the immense south-oriented floods (moving south in large ice-walled and bedrock-floored valleys like the Midcontinent Trench) were captured and diverted north by headward erosion of north-oriented ice-walled and bedrock-floored valleys. This diversion of immense south-oriented melt water floods from the Gulf of Mexico to Hudson Bay and other northern points probably altered ocean currents and triggered a major Northern Hemisphere cooling event. That cooling event probably froze north-oriented melt water floods on the floors of the newly carved ice-walled and bedrock-floored valleys and created a wet based thin ice sheet with thick ice sheet remnants embedded in it. That subsequent ice sheet and its melt water did not significantly alter the underlying landscape as the thick ice sheet and its melt water had done, although the subsequent ice sheet did make minor landscape alterations. Probably some of the figure 8 landscape features originated as a result of the subsequent  ice sheet and its melt water.

Vermillion River drainage on west-facing Prairie Coteau escarpment

Figure 9: Vermillion River drainage on west-facing Prairie Coteau escarpment. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 9 illustrates the west-facing Prairie Coteau escarpment and the south-oriented Vermillion River drainage basin east of the figure 8 map area and includes overlap areas with figure 8. The Prairie Coteau upland surface is located in the eastern half of figure 9. West of the Prairie Coteau upland surface is a gradual slope to the figure 9 west edge area. That gradual slope is the west-facing Prairie Coteau escarpment, and here in the figure 9 map area is much more gradual than further to the north. The East Fork Vermillion River flows south-southeast and south near the escarpment crest, although the escarpment is much harder to identify in figure 9 than it is in regions further to the north. The Little Vermillion River flows in a southeast direction from the figure 9 north edge (west half) to join the East Fork Vermillion River near Montrose in the figure 9 southeast quadrant (northwest quarter of southeast quadrant). The region between the Little Vermillion and the East Fork Vermillion has some characteristics of the Prairie Coteau upland surface, but tends to be lower in elevation than the Prairie Coteau upland surface east of the East Fork Vermillion River valley. Probably this difference is related to deeper erosion by ice-walled and bedrock-floored valleys eroding north and north-northwest along the present day Vermillion River tributary alignments. These valleys were not as large or as deep as the much larger Midcontinent Tench (or James River lowland) ice-walled and bedrock-floored valley to the west, but they were significant enough to alter the west-facing Prairie Coteau escarpment face and the adjacent Prairie Coteau upland surface. These smaller and shallower ice-walled and bedrock-floored valleys had eroded north from the southeast-oriented Missouri River valley located south of the figure 9 map area. At the time they eroded north the Prairie Coteau upland surface area in the figure 9 east half was where the Midcontintent Trench’s east ice wall stood. Headward erosion of ice-walled and bedrock-floored floored valleys from the south was probably how the Midcontinent Trench valley was being broadened and indicates melt water responsible for carving the Midcontinent Trench (or James River lowland) ice-walled and bedrock-floored valley came from north of the figure 9 map region (and was not derived locally). Had the melt water been derived locally the melt water flow on the melting ice sheet surface would have been flowing west into the deeper south-oriented Midcontinent Trench ice-walled and bedrock-floored valley.

Vermillion River-Big Sioux River drainage divide northeast of Sioux Falls

Figure 10: Vermillion River-Big Sioux River drainage divide northeast of Sioux Falls. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 10 illustrates the East Fork Vermillion River-Big Sioux River drainage divide area east of the figure 9 map area and includes overlap areas with figure 9. Sioux Falls, South Dakota is the city located in the figure 10 southeast corner. Dell Rapids, South Dakota is the community located in the figure 10 northeast corner. The Big Sioux River flows south from Dell Rapids to Sioux Falls in the figure 10 east half. The East Fork Vermillion River flows south in the figure 10 west half. West of the Big Sioux River is Skunk Creek, which flows south-southwest from the figure 10 north edge and then turns to flow southeast to the figure 10 south edge (just west of Sioux Falls). Figure 5 above illustrated how Battle Creek flows southeast and then northeast and the discussion suggested the northeast-oriented Battle Creek valley segment had once been a southwest-oriented valley. Figure 10 illustrates the Skunk Creek valley to which water in that southwest-oriented valley (and the present day southeast-oriented Battle Creek valley) probably once flowed. Reversal of flow to create the northeast-oriented Battle Creek valley and to capture the southeast-oriented Battle Creek headwaters also beheaded south-oriented flow to the Skunk Creek valley seen in figure 10. Between the East Fork Vermillion River valley and the Skunk Creek valley is a Prairie Coteau upland surface similar to the Prairie Coteau upland surface seen further north (e.g. figure 5 and also see James River-Big Sioux River drainage divide area south of Redfield and Watertown and north of Huron and Brookings essay). Headward erosion of the smaller south-oriented ice-walled and bedrock-floored valleys along the present day alignments of the south-oriented Vermillion River tributaries and the Big Sioux River and tributaries apparently was in the process of chopping up the southern end of the thick ice sheet remnant located west of the Big Sioux River ice-walled and bedrock-floored valley and east of the much larger and deeper Midcontinent Trench (or James River lowland) ice-walled and bedrock-floored valley. Again, note how these melt water originated ice-walled and bedrock-floored valleys are south-oriented, suggesting they originated independent of adjacent ice-walled and bedrock-floored valleys, probably by melt water flowing south on the rapidly melting thick ice sheet surface.

Additional information and sources of maps studied

This essay has provided only a sample of the detailed topographic map evidence supporting the flood erosion interpretation. Many additional illustrations could be provided. Readers are encouraged to look at mosaics of detailed topographic maps to see the abundance of available data. Maps used in this study were created and published by the United States Geologic Survey and can be obtained directly from the United States Geological Survey and/or from dealers offering United States Geological Survey maps. Hard copy maps can also be observed at United States Geological Survey map depositories which are located throughout the United States and elsewhere. Illustrations used here were created using National Geographic Society TOPO software and digital map data. TOPO software and map data can be obtained from the National Geographic Society and/or dealers offering National Geographic Society digital map data.