A geomorphic history based on topographic map evidence

The James River-Big Sioux River drainage divide area south of Redfield and Watertown and north of Huron and Brookings is located in eastern South Dakota, USA. The James River is located in a broad south-oriented lowland and flows south near Redfield and Huron to reach the southeast-oriented Missouri River. The Big Sioux River is located on the escarpment-bounded Prairie Coteau upland and flows south near Watertown and Brookings to also reach the southeast-oriented Missouri River. The James River lowland is interpreted to be what remains of a huge south-oriented ice-walled and bedrock-floored valley sliced into the surface of a rapidly melting thick North American ice sheet. The Prairie Coteau upland is interpreted to be where an ice sheet remnant between two huge ice-walled and bedrock-floored valleys was located, with the James River lowland being the western boundary. The Big Sioux River valley is interpreted to have originated as much smaller and shallower ice-walled and bedrock-floored valley that was sliced into the Prairie Coteau ice sheet remnant.

• 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 Redfield and Watertown and north of Huron and Brookings, 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 Redfield and Watertown and north of Huron and Brookings, 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 Redfield and Watertown and north of Huron and Brookings, South Dakota. The state occupying most of the figure 1 map area (including the northwest quadrant) is South Dakota. East of South Dakota in the figure 1 northeast corner is Minnesota. South of Minnesota in the figure 1 southeast corner is Iowa. And south of South Dakota and in the figure 1 southwest corner is Nebraska. The Missouri River flows in a southeast direction from the figure 1 west edge to the South Dakota-Nebraska border and is the South Dakota-Nebraska border until Sioux City, Iowa, where the southeast-oriented Missouri River flows south of the figure 1 map area. The James River flows south and south-southeast from the figure 1 north center edge near Redfield to Huron and Mitchell before continuing in a south-southeast direction to join the Missouri River near Yankton, South Dakota. The Big Sioux River is located east of the James River (near the South-Dakota-Minnesota border) and flows south from Watertown, South Dakota to Brookings and Sioux Falls and then to join the Missouri River near Sioux City, Iowa. In the figure 1 northeast corner the Minnesota River flows in a southeast direction through Montevideo and Granite Falls, Minnesota and northeast-oriented streams are Minnesota River tributaries. Headwaters of the southeast-oriented Des Moines River and tributaries are located along the figure 1 east edge in southern Minnesota. The Minnesota River and Des Moines River are Mississippi River tributaries. Redfield and Watertown, South Dakota are located along the figure 1 north center edge. Huron is located south-southeast from Redfield and is located near the James River. Brookings is located south-southeast from Watertown and is located near the Big Sioux River. Landforms illustrated and discussed in this essay (after the more detailed location map in figure 2) are located between the James River valley and Big Sioux River valley in the region south of highway 212 between Redfield and Watertown and north of highway 14 between Huron and Brookings. Topographic maps begin with a view of the Big Sioux River valley near Watertown and proceed west along the highway 212 route to the James River valley to just east of Redfield. Next the maps begin with the James River valley near Huron and proceed east to the Big Sioux River valley near Brookings.

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 somewhat more detailed location map for the James River-Big Sioux River drainage divide area south of Redfield and Huron and north of Huron and Brookings. All figure 2 areas are in South Dakota. Spink, Clark, Codington, Hamlin, Deuel, Beadle, Kingsbury, and Brookings are South Dakota county names. The red shaded area in Codington County is land included in the Sisseton-Wahpeton Indian Reservation. Redfield is located in Spink County along the figure 2 west edge and the James River flows south-southwest from the figure 2 north edge almost to Redfield and then jogs east before turning to flow south-southeast into Beadle County. Huron is located in Beadle County and the James River flows just east of Huron before flowing south-southeast to the figure 2 south edge. Watertown is in Codington County and the Big Sioux River flows south from the figure 2 north edge to Watertown and then south through eastern Hamlin County into Brookings County, where it flows west of the city of Brookings before flowing to the figure 2 south edge. Note how areas in the figure 2 east half are marked with numerous lake basins and away from the Big Sioux River and along the figure 2 east edge there is a lack of integrated drainage routes. Contrast the figure 2 east half with the figure 2 west half where there are few lakes and a much better developed south-oriented drainage network. Most of the figure 2 east half is located on the Prairie Coteau upland, which is a region of low hills and numerous closed depressions, many of which contain lake basins. The south-oriented Big Sioux River is the only major drainage route located on the Prairie Coteau upland surface. The Prairie Coteau upland surface is bounded by escarpments both on the west and the northeast. The west-facing escarpment is a north-south oriented landform dividing the figure 2 east and west halves. West of the escarpment the land is 100-200 meters lower than on the Prairie Coteau upland surface, the landscape is much smoother, and there is a well-developed south-oriented drainage network. The area west of the west-facing escarpment is here referred as the James River lowland (and has been named in some other essays as the Midcontinent Trench) and is interpreted to have been formed when a huge ice-walled and bedrock-floored valley was sliced into a rapidly melting thick North American ice sheet. The west-facing Prairie Coteau escarpment is interpreted to be what remains of the Midcontinent Trench’s ice-walled and bedrock-floored valley’s east wall. The Midcontinent Trench west wall is the east-facing Missouri Escarpment, which is located west of the James River valley and is not seen in this essay. The Prairie Coteau is interpreted here to be where a detached ice sheet remnant melted and deposited whatever debris it contained. The Big Sioux River valley is interpreted here to have originated as a much smaller and shallower ice-walled and bedrock-floored valley sliced into the Prairie Coteau ice sheet remnant.

Big Sioux River valley near Watertown, South Dakota

Figure 3: Big Sioux River valley near Watertown, South Dakota. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 3 illustrates the Big Sioux River valley near Watertown, South Dakota. The Big Sioux River flows in a southeast direction from the figure 3 north edge (just east of Lake Kampeska) to Watertown and the figure 3 east center area and then turns to flow south to the figure 3 south edge. Willow Creek is a southwest-oriented Big Sioux River tributary coming  from the figure 3 northeast corner area and joining the Big Sioux River just south of Watertown. Other than the Big Sioux River and Willow Creek the figure 3 map area is devoid of an integrated drainage pattern. Instead there are several closed basins, with lakes and swamps located in several of those basins. Between the closed basins are low hills. This landscape suggests the figure 3 map area is underlain by glacial moraines and the various closed basins may represent areas where buried and/or partially buried ice masses melted. The figure 3 landscape is typical of landscape found on the Prairie Coteau upland surface, which is interpreted here to be where an ice sheet remnant. The ice sheet remnant was located between two large and deep ice-walled and bedrock-floored valleys was isolated when the ice-walled and bedrock-floored valleys converged north of the figure 3 map area (see James River-Wild Rice River drainage divide area essay found under James River on the sidebar category list). The detached Prairie Coteau ice sheet remnant subsequently melted and deposited whatever debris it contained. The south oriented Big Sioux River valley may have been initiated as a south oriented supra glacial melt water river flowing on the Prairie Coteau ice sheet remnant’s surface and which in time sliced a narrow and relatively shallow ice-walled and bedrock-floored valley into the decaying ice sheet remnant. Probably there is no way to determine how thick the Prairie Coteau ice sheet remnant was at the time it was detached from the rapidly melting thick North American ice sheet. The Prairie Coteau ice sheet remnant was detached by headward erosion of a huge southeast-oriented ice-walled and bedrock-floored valley along the present day Minnesota River lowland alignment and by headward erosion of an equally large south oriented ice-walled and bedrock-floored valley along the present day James River lowland alignment. The two giant ice-walled and bedrock-floored valleys intersected north of Watertown, where the present day Prairie Coteau east and west-facing escarpments converge. By intersecting the two large ice-walled and bedrock-floored valleys completely isolated the Prairie Coteau ice sheet remnant and perhaps beheaded south- and southwest-oriented melt water flow routes that had been moving melt water floods to the smaller and shallower south-oriented ice-walled and bedrock-floored valley on the Big Sioux River alignment.

Prairie Coteau upland surface near Clark, South Dakota

Figure 4: Prairie Coteau upland surface near Clark, South Dakota. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 4 illustrates the Prairie Coteau upland surface west of the figure 3 map area and includes overlap areas with figure 3. As can be seen the figure 4 map area shows no integrated drainage pattern, but instead many low hills and small closed basins, some of which contain lakes or swamps. This region, like much of the area seen in figure 3, is typical of a region where a stagnant ice sheet has melted and deposited whatever debris it contained. As previously explained the Prairie Coteau upland surface is interpreted here to have been covered with a detached thick ice sheet remnant, which was detached by headward erosion of huge ice-walled and bedrock-floored valleys on either side. How thick the ice sheet was is difficult to determine. The “thick ice sheet that melted fast” geomorphology paradigm being tested by this Missouri River drainage basin research project essay series suggests the thick ice sheet originally formed on a topographic surface now preserved only in the highest level Rocky Mountain erosion surfaces (if the original topographic surface is preserved at all). That original topographic surface has since been greatly warped by ice sheet weight, with down warping under the ice sheet and up warping elsewhere. In addition the thick ice sheet deeply eroded the region underneath it, meaning the thick ice sheet, like the Antarctic Ice Sheet today, had roots located in a deep “hole”. Being located in a “hole” meant the ice sheet had roots that extended well below the ice sheet rim elevation, although the ice sheet also rose significantly above the ice sheet rim elevation as well. If the present day Antarctic Ice Sheet is used as a model, the thick North American ice sheet may have had roots extending as much as 2000 meters below the ice sheet rim elevation and at its peak may have risen more than 3000 meters above the ice sheet rim elevation. When ice sheet melting began, the melting first affected areas above the ice sheet rim elevation (and probably along the ice sheet southern margin). The immense south and southeast oriented ice walled and bedrock-floored valleys would not have been sliced into the ice sheet surface until very late during the ice sheet melting history. By that time ice sheet melting would have progressed below the ice sheet rim elevation and melt water floods were eroding northeast-oriented valleys headward from space in the deep “hole” being opened up as the ice sheet melted. These northeast-oriented valleys captured south oriented melt water floods and diverted the flood water into space being up in the ice sheet’s deep “hole”. Once in the deep “hole” the flood water flowed in large south oriented ice-walled and bedrock-floored valleys to reach southern outlets where south oriented valleys were being developed to drain those melt water floods south to the Gulf of Mexico. It is possible when first detached by headward erosion of the south and southeast oriented ice-walled and bedrock-floored valleys the Prairie Coteau ice sheet remnant was as much (or more) than 1000 meters thick, although in time that thickness decreased until the ice sheet remnant had completely melted.

Prairie Coteau west-facing escarpment west of Clark, South Dakota

Figure 5: Prairie Coteau west-facing escarpment west of Clark, South Dakota. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 5 illustrates the Prairie Coteau west-facing escarpment located west of Clark, South Dakota and is located west of the figure 4 map area (and includes overlap areas with figure 4). The Prairie Coteau upland surface is seen along the figure 5 east edge area. West of the poorly drained area with small lakes and no integrated drainage pattern is a well-drained west-facing slope, which is the Prairie Coteau west-facing escarpment. Elevations at the escarpment crest in this region are approximately 120 meters higher than at the escarpment base. Note the contrast between landscape features found on the Prairie Coteau upland surface near the figure 5 east edge and the landscape features found on the James River lowland floor near the figure 5 west edge. Unlike the Prairie Coteau upland surface region the James River lowland floor area is much smoother in appearance, lacks numerous closed basins, and has an identifiable drainage system. What we are seeing in figure 5 is what remains of the east-wall area of what originated as an immense south-oriented ice-walled and bedrock-floored valley that was sliced into the surface of a rapidly melting thick ice sheet. The James River lowland surface represents the bedrock floor of that immense south-oriented valley. Ice sheet contained debris from the figure 5 west half was largely washed away by large south-oriented melt water floods responsible for carving the ice-walled and bedrock-floored valley (flood waters also brought in debris from further north and northwest so there is evidence of glacially transported debris on the James River lowland floor). The Prairie Coteau upland surface is where the ice-walled valley’s ice wall once stood. It is possible, when first formed, this ice wall stood more than 1000 meters high, although as melting continued the ice wall height decreased until all that was left was what is seen today. Debris contained within the ice wall was not washed away by melt water floods, but instead was deposited in place as the detached ice sheet remnant melted. Note how lowland streams (in the figure 5 west half) away from the escarpment face are south-oriented and streams flowing down the escarpment face are generally west-oriented. The west-oriented stream valleys were probably eroded by melt water from the ice wall, while the south-oriented lowland stream valleys were probably eroded by south-oriented melt water floods originating further to the north.

James River valley east and southeast of Redfield, South Dakota

Figure 6: James River valley east and southeast of Redfield, South Dakota. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 6 illustrates the James River valley east and southeast of Redfield, South Dakota and is located immediately east of the figure 5 map area (although there is no overlap with figure 5). Redfield is located just west of the figure 6 map area (on the red highway 212). The James River flows southeast from the figure 6 northwest corner and turns to flow south before reaching the figure 6 south edge. Joining the James River at the Glendale Colony (near the figure 6 south center) is south and south-southwest oriented Timber Creek, which flows from the figure 6 north edge. North of Frankfort is south and south-southwest oriented Dry Run, which joins the southeast-oriented James River in the figure 6 northwest quadrant. An unnamed stream in the figure 6 southeast quadrant flows southwest and northwest to join south-southwest oriented Timber Creek. Also in the figure 6 east half is evidence of a north-south oriented through valley linking the unnamed stream in the figure 6 southeast quadrant with an unnamed south- and west-oriented stream in the figure 6 northeast quadrant. South of the James River in the figure 6 west half is evidence of a southeast-oriented anastomosing channel complex. The present day figure 6 drainage pattern probably reflects melt water flood events occurring very late during the ice sheet melt down history and almost certainly do not reflect melt water flood events responsible for slicing the ice-walled and bedrock-floored valley into the thick ice sheet surface. Southeast and northwest oriented valleys and the southeast-oriented anastomosing channel complex provide evidence of southeast-oriented flood flow moving into and across he figure 6 map area. At the same time the multiple south-oriented valleys provide evidence of south-oriented flood flow flowing into and across the region. Based on figure 6 map evidence alone it is possible this figure 6 map area was where a southeast-oriented melt water flood met a south-oriented melt water flood. If so, the south-oriented melt water flood was probably coming from further north in the ice-walled and bedrock-floored James River lowland valley (also named the Midcontinent Trench in some other essays) and the southeast-oriented flood waters may have been coming from ice-marginal melt water flood waters that had breached the ice sheet’s detached southwest margin and was flowing to the lower elevation James River lowland (or Midcontinent Trench) floor.

James River valley east and northeast of Huron, South Dakota

Figure 7: James River valley east and northeast of Huron, South Dakota. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 7 illustrates the James River valley east and northeast of Huron, South Dakota and is located south of the figure 6 map area (there is a significant gap between figure 6 and figure 7). Figure 7 is the first of four figures (7, 8, 9, and 10) illustrating landform evidence along a west-to-east oriented line extending east from Huron along highway 14 between Huron and Brookings. The figure 7 map area is located on the James River lowland or Midcontinent Trench floor and like figure 6 has well-developed south oriented drainage routes and a lack of hummocky topography and numerous lakes like those seen in the Prairie Coteau upland region. The James River is the large south-southeast and south-southwest oriented river in the figure 7 west half that flows along the east edge of Huron. The south- and southwest-oriented stream flowing from the figure 7 north center edge to join the James River is Shue Creek, which north of figure 7 is primarily south-oriented. Drainage in the figure 7 east half is primarily south-oriented, probably reflecting the movement of south-oriented floods during final melt water flood flow events on the James River lowland floor. As already mentioned final melt water flood flow events in this region occurred late during the ice sheet melt down history and may have occurred subsequent to a halt in the thick ice sheet rapid melt down. The rapid melt down ended when ice sheet melting progressed to the point where the immense melt water floods, which had been moving south to the Gulf of Mexico, were captured and diverted north to Hudson Bay and other northern points. This diversion of giant melt water floods from the Gulf of Mexico to northern locations significantly changed ocean circulation patterns and resulted in a major Northern Hemisphere cooling event. That cooling event halted the thick ice sheet rapid melt down and probably froze north-oriented melt water flood waters on the former ice sheet floor (located in the large ice-walled and bedrock-floored valleys between isolated thick ice sheet remnants). The result may have been a wet based thin ice sheet with thick ice sheet remnants embedded in it. Melting of that resulting ice sheet did not produce the immense floods melting of the thick ice sheet had produced, although melt water from that resulting ice sheet almost certainly moved across the figure 7 map area.

West-facing Prairie Coteau escarpment west of De Smet, South Dakota

Figure 8: West-facing Prairie Coteau escarpment west of De Smet, South Dakota. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 8 illustrates the west-facing Prairie Coteau escarpment west of De Smet, South Dakota and is located east of the figure 7 map area (and includes overlap areas with figure 7). The Prairie Coteau upland surface with its hummocky topography and numerous small lakes can be seen in the figure 8 northwest corner area. The west-facing Prairie Coteau escarpment is located immediately west of the Prairie Coteau upland surface area and has a south-southeast orientation in the figure 8 map area. De Smet, South Dakota (in the figure 8 southeast corner area) is located on the escarpment crest. Elevations along the figure 8 west edge average 100-120 meters lower than elevations along the escarpment crest. Note south-oriented drainage routes in the figure 8 west half and extending right up the west-facing escarpment base. The escarpment slope is drained by southwest and west-oriented streams, suggesting those stream valleys developed after the escarpment slope had been eroded. Probably the source of water responsible for eroding valleys on the escarpment slope was melting ice on the Prairie Coteau upland surface. This figure 8 map area was once the location of the east wall of the immense south-oriented Midcontinent Trench (or James River lowland) ice-walled and bedrock-floored valley. The west-facing escarpment is all that remains of what was probably a deep canyon wall that may at one time have been hundreds of meters (or more) high, although as time passed the wall became lower and lower until the present day escarpment is all that remains. Melting after the cooling event that halted the ice sheet rapid melt down was probably slow and generally did not produce large melt water floods. As the canyon ice wall slowly melted, whatever debris it contained probably accumulated unevenly on the remaining ice sheet surface and eventually was deposited to form the present day Prairie Coteau upland surface. Lake basins probably represent where final ice remnants melted, leaving depressions, some of which are now filled with water. Low hills probably represent where debris, which had accumulated on the melting ice sheet surface, slid into low spots where ice sheet melting had lowered the ice sheet surface elevation.

Prairie Coteau upland surface east of De Smet, South Dakota

Figure 9: Prairie Coteau upland surface east of De Smet, South Dakota. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 9 illustrates the Prairie Coteau upland surface east of De Smet, South Dakota and east of the figure 8 map area (and also includes overlap areas with figure 8). Unlike the figure 4 Prairie Coteau upland surface area seen above this Prairie Coteau upland surface region does have some identifiable drainage routes. Perhaps the most obvious of these drainage routes extends from the Lake Preston Lakebed (just north of west-to-east oriented red highway in figure 9 southeast quadrant) to Lake Badger in the figure 9 northeast corner. This drainage route is located in what appears to be a well-defined northeast-oriented valley. A close look at figure 9 reveals streams flowing into the Lake Preston Lakebed area and also into Lake Whitewood to the south. Lake Whitewood appears to be connected by a through valley with the Lake Preston Lakebed area. The Lake Preston Lakebed appears to be an intermittent lake area, which is only flooded during wet years. Flow in the figure 9 streams does appear to be significant. Badger Lake may have an outlet in wet years, but that outlet only leads to adjacent lakes, also located in closed basins. The elongate nature of Lake Whitewood and of the Lake Preston Lakebed suggest those lake basins may have originated as some sort drainage routes, which have since become blocked. Probably the drainage routes, if that is what they area, reflect melt water flow routes developed as the detached Prairie Coteau ice sheet mass slowly melted, which took place after rapid melting of the thick ice sheet was halted. For the most part the figure 9 map area landscape features are similar to those in figure 4 and suggest the presence of a stagnant ice sheet, that was slowly melting and depositing whatever debris it contained. Melt water flowing from the region did not significantly alter the landscape and did not erode the deposited debris or erode deep or broad drainage routes.

Big Sioux River valley near Brookings, South Dakota

Figure 10: Big Sioux River valley near Brookings, South Dakota. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 10 completes our eastward journey to the Big Sioux River valley near Brookings, South Dakota and includes overlap areas with figure 9. The east half of Lake Badger can be seen near the figure 10 northwest corner. Brookings is the city located in the figure 10 southeast corner. The Big Sioux River flows south in a fairly well-defined and broad valley in the figure 10 east half and is located west of Brookings. Several south-southwest oriented tributaries flow to the Big Sioux River in the figure 10 east half. West of the Big Sioux River valley the figure 10 map area generally lacks an integrated drainage system and has numerous closed basins and small hills. The landscape west of the Big Sioux River valley in other words is typical of Prairie Coteau upland surface areas seen in previous figures. The Big Sioux River valley in the figure 10 map area is much broader than it was further north in the Watertown area (figure 3). A possible reason is the Big Sioux River valley as previously mentioned may have originated as a smaller and shallower ice-walled and bedrock-floored valley (than the ice-walled and bedrock-floored valleys on either side of the Prairie Coteau). As we progress south along what was this smaller ice-walled and bedrock-floored valley the former valley size broadens and perhaps tributary valleys entered from the northeast. The tributary valleys may have originated before headward erosion of the larger and deeper ice-walled and bedrock-floored valley northeast-oriented Prairie Coteau beheaded southwest-oriented supra glacial melt water flow routes leading to what was then the evolving south-oriented Big Sioux River ice-walled and bedrock-floored valley. Headward erosion of the larger and deeper ice-walled and bedrock-floored valleys on either side of the Prairie Coteau subsequently beheaded all south-oriented supra glacial melt water flow routes to the smaller and shallower Big Sioux River ice-walled and bedrock-floored valley and detached the Prairie Coteau ice sheet remnant, which insured that the Big Sioux River ice-walled and bedrock-floored valley would always remain smaller and shallower than the much larger and deeper ice-walled and bedrock-floored valleys bounding the Prairie Coteau upland region.

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.