A geomorphic history based on topographic map evidence

The James River-Big Sioux River drainage divide area south of Mitchell and Sioux Falls is located in the southeast corner of South Dakota, USA. Major drainage routes present include the south-oriented James, Vermillion, and Big Sioux Rivers, all of which flow to the southeast-oriented Missouri River, which forms the drainage divide area south boundary. The drainage divide area includes the southern end of the Prairie Coteau upland surface and also an isolated northwest-southeast oriented upland area known as Turkey Ridge. River valleys and adjacent lowlands are interpreted to have formed as ice-walled and bedrock-floored valleys carved into a rapidly melting thick North American ice sheet. Upland regions are interpreted to represent ice sheet remnant areas located between the ice-walled and bedrock-floored valleys, where the ice sheet remnants melted and deposited whatever debris they contained.

Preface:

• 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 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 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 Mitchell and Sioux Falls, South Dakota. Minnesota is the state in the figure 1 northeast corner. Iowa is south of Minnesota and is located in the figure 1 southeast corner. Nebraska is located in the figure 1 southwest area and South Dakota is state with the largest area shown in figure 1 and is located in the figure 1 northwest region. The Missouri River flows southeast from the figure 1 west edge to the South Dakota-Nebraska border and then flows along the South Dakota-Nebraska border to Yankton and Vermillion, South Dakota before reaching Sioux City,, Iowa and flowing southeast along the Nebraska-Iowa border. The Big Sioux River flows south in eastern South Dakota to near Brookings and Sioux Falls before flowing along the South Dakota-Iowa border to join the Missouri River near Sioux City, Iowa. The James River is located west of the Big Sioux River and flows south-southeast from the figure 1 north edge to Huron and Mitchell before turning to flow southeast to join the Missouri River near Yankton, South Dakota. Between the southeast-oriented James River and the south-oriented Big Sioux River is an unnamed river, which joins the Missouri River at Vermillion, South Dakota. That unnamed (in figure 1) river is the Vermillion River. The James River-Big Sioux drainage divide area illustrated and discussed in this essay is located south of Interstate highway 90, which extends from Mitchell to Sioux Falls and north of the Missouri River. Essays describing nearby drainage divides include the James River-Big Sioux River drainage divide area south of Huron and Brookings and north of Mitchell and Sioux Falls essay and the James River-Big Sioux River drainage divide area south of Redfield and Watertown and north of Huron and Brookings essay, both of which can be found under Big Sioux River or James River on the sidebar category list. This essay begins with a somewhat more detailed location map. Next topographic maps illustrate the James River-Big Sioux River drainage divide area immediately south of Interstate highway 90, extending from Mitchell to Sioux Falls. The essay continues with maps illustrating the northwest-southeast oriented Turkey Ridge upland area, which is located south of Freeman and north of Yankton, South Dakota. The essay concludes with maps illustrating locations where the James River, Vermillion River, and Big Sioux River valleys enter the much larger Missouri River valley.

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 Mitchell and Sioux Falls, South Dakota. Davison, Hanson, McCook, Minnehaha, Hutchinson, Turner, Lincoln, Bon Homme, Yankton, Clay, and Union are South Dakota county names. Mitchell is located in the figure 2 northwest corner in Davison County. Sioux Falls is located in the figure 2 northeast quadrant in Minnehaha County. The Missouri River flows generally east and southeast along the figure 2 south edge and is seen in the figure 2 west half (near the south edge). The James River flows southeast and south from near Mitchell in the figure 2 northwest corner to join the Missouri River near Yankton, South Dakota (near the figure 2 south edge). The Big Sioux River flows south in eastern Minnehaha County (east of Sioux Falls) and then along the eastern borders of Lincoln and Union Counties to join the Missouri River just south of the figure 2 map area. The Vermillion River headwaters are located in McCook County and the Vermillion River flows south-southeast through eastern Turner County into Clay County and joins the Missouri River near Vermillion (just south of the figure 2 south edge). Note areas with many small lakes in McCook and western Minnehaha Counties. Those areas are located on the Prairie Coteau upland surface, which is a region covered with thick glacial moraines, probably representing deposits left by a stagnant ice sheet that gradually melted and left whatever debris it contained. The Prairie Coteau upland surface is interpreted here to be where a thick ice sheet remnant, isolated by headward erosion of large ice-walled and bedrock-floored valleys which had been sliced into a rapidly melting thick ice sheet’s surface, melted and deposited whatever debris it contained. The present day James River flows south and southeast in what was the large ice-walled and bedrock-floored valley that bounded the Prairie Coteau ice sheet remnant on the west. The eastern ice-walled and bedrock-floored valley was a broad southeast-oriented valley located along the alignment of the present day Minnesota River, located east and north of the figure 2 map area. The western and eastern ice-walled and bedrock-floored valleys intersected in southern North Dakota and completely detached the Prairie Coteau ice sheet remnant. Melt water floods flowing south on the Prairie Coteau ice sheet remnant surface also eroded smaller and shallower ice-walled and bedrock-floored valleys headward from the Missouri River valley (which probably at that time had a north wall of ice). Headward erosion of south-oriented ice-walled and bedrock-floored valleys, such as the Big Sioux River valley and the Vermillion River valley, deeply eroded the Prairie Coteau ice sheet remnant southern margin, although at least some ice sheet remnants lingered on.

James River valley southeast of Mitchell, South Dakota

Figure 3: James River valley southeast of Mitchell, South Dakota. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 3 illustrates the James River valley south and east of Mitchell, South Dakota. Mitchell is the city located in the figure 3 northwest corner. The James River flows from the figure 3 northwest corner to the figure 3 south center edge and has eroded its valley into a relatively flat topographic surface, which in this essay is referred as the James River lowland. The James River lowland is interpreted here to have been eroded as an immense south-oriented ice-walled and bedrock-floored valley sliced into the surface of a rapidly melting thick North American ice sheet. This immense south-oriented ice-walled and bedrock-floored valley is in this essay named the Midcontinent Trench and the James River lowland is the Midcontinent Trench floor. Today the James River lowland is bounded by a west-facing escarpment in the east and an east-facing escarpment in the west. These escarpments are much better seen in northern South Dakota, although some evidence of the west-facing escarpment is seen in the figures below. At the escarpment crests on both sides are hummocky upland surface areas with many small closed depressions, some of which are filled with water. The upland surface east of the James River is named in this essay as the Prairie Coteau upland surface and the upland surface west of the James River lowland is referred to here as the Missouri Coteau. The Prairie Coteau and Missouri Coteau represent where the ice walls bounding the Midcontinent Trench were once located. The west-facing Prairie Coteau escarpment and the east-facing Missouri Escarpment represent all that remains of the Midcontinent Trench ice walls. The Coteau areas east and west of the escarpment crests represent locations where ice sheet remnants, which formed the Midcontinent Trench ice walls, slowly melted and deposited whatever debris they contained. The present day narrower James River valley and its various tributary valleys were eroded into the Midcontinent Trench floor very late during the thick ice sheet’s melt down history, probably by the last major melt water flood flow event to flow south on the ice-walled and bedrock-floored Midcontinent Trench floor. Prior to that time there were many previous large-scale south-oriented melt water flood flow events. Earlier flood flow events may have deposited flood transported debris on the low gradient Midcontinent Trench floor, and such deposits may have contributed to the flat surface seen today.

Wolf Creek-West Fork Vermillion River drainage divide area

Figure 4: Wolf Creek-West Fork Vermillion River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 4 illustrates the Wolf Creek-West Fork Vermillion River drainage divide area east of the figure 3 map area and includes a narrow overlap area with figure 3. Wolf Creek flows south-southeast in the figure 4 west half. The West Fork Vermillion River flows south-southeast in the figure 4 east half. Note the gradual rise to the east, which is most obvious in the figure 4 east half. That gentle rise is the base of the west-facing Prairie Coteau escarpment, more of which will be seen in figure 5 below. As previously mentioned the escarpment is what remains of the east wall of the ice-walled and bedrock-floored Midcontinent Trench. The figure 4 map area is near the south end the north-south oriented escarpment, which begins at the North Dakota-South Dakota border and which is much steeper further to the north. Also, in the figure 4 map area Wolf Creek and the West Fork Vermillion River are flowing roughly parallel to the escarpment’s orientation, as opposed to flowing down the escarpment slope. Further north (see James River-Big Sioux River drainage divide area south of Redfield and Watertown and north of Huron and Brookings essay) streams draining the escarpment slope flow down the escarpment slope, not parallel to the escarpment orientation. A possible reason for this difference is further north drainage routes on the escarpment slope were formed by melt water from the ice wall after the north-south oriented escarpment wall had been eroded by south-oriented floods while in figure 4 map area the south-oriented streams were in process of eroding a series of south-southeast oriented ice-walled and bedrock-floored valleys into Midcontinent Trench eastern ice wall. Small lakes in the figure 4 map area suggest the presence of glacial deposits, which suggests the eastern ice wall was not completely removed. Figure 4a provides a detailed map of the Wolf Creek-Plum Creek drainage divide area north of Emery (located near the figure 4 west center edge). Plum Creek in figure 4 is the unnamed southwest-oriented stream immediately west of Emery. Note the southwest-oriented anastomosing channel complex north of Emery, which headward erosion of the southeast-oriented Wolf Creek valley has beheaded. Also note numerous water filled depressions, which combined with the anastomosing channel complex provide evidence the figure 4a landscape originated during a major south-oriented flood event when ice was still present in the region. The flood may have deposited debris around remnant ice masses and/or debris may already have been deposited around the remnant ice masses, but in either case remnant ice masses melted and left depressions.

Figure 4a: Wolf Creek Plum Creek drainage divide area north of Emery (Plum Creek is located in the southwest-oriented valley west of Emery). United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

East Fork Vermillion River valley and Prairie Coteau upland surface

Figure 5: East Fork Vermillion River valley and Prairie Coteau upland surface. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 5 illustrates the East Fork Vermillion River and the Prairie Coteau upland surface east of the figure 4 map area and includes overlap areas with figure 4. The East Fork Vermillion River flows south in the figure 5 west half through East Vermillion Lake to the figure 5 south edge. East of the East Fork Vermillion is the Prairie Coteau upland surface, which is characterized by hummocky topography and numerous closed basins, some of which are filled with water. West of the East Fork Vermillion River valley is a ridge, approximately the same height as the Prairie Coteau upland surface elevation and then a gradual slope down to the west. The slope to the west (best seen in the figure 5 southwest corner area) is the top of the west-facing Prairie Coteau escarpment. The south-oriented East Vermillion River valley has been eroded along the escarpment crest. The Prairie Coteau upland surface is where the thick ice sheet remnant (that had been isolated by headward erosion of the south-oriented Midcontinent Trench and the southeast-oriented Minnesota River lowland ice-walled and bedrock-floored valley) once stood. The south margin of this ice sheet remnant was eroded by smaller and shallower ice-walled and bedrock-floored valleys (such as the East Fork Vermillion River valley), although areas of undisturbed ice remained between those smaller and shallower ice-walled and bedrock-floored valleys. One such area of undisturbed ice was located in the figure 5 map area east of the East Fork Vermillion River valley. Ice in this area melted slowly and deposited whatever debris it contained. Melt water floods did not remove the ice sheet transported and contained debris, but instead the debris remained where it had been deposited. What evidence is there that the south-oriented East Fork Vermillion River valley was eroded by south-oriented melt water floods? Note how west of East Vermillion River there is a north-south through valley linking various east and southeast-oriented East Fork Vermillion River tributaries (see Greenland and Spring Valley townships). That parallel valley provides evidence of multiples valleys such as those found in a south-oriented anastomosing channel complex, which provides evidence of a large south-oriented flood event. Figure 5a below illustrates the north end of that through valley. Note how Battle Creek flows south-southeast in a segment of that through valley. Also note the parallel south-southeast oriented (unnamed) valley between Battle Creek and East Vermillion Lake in the figure 5a southeast quadrant. These multiple south-oriented valleys are best explained in the context of flood eroded anastomosing channels.

Figure 5a: Through valley located west of East Vermillion Lake. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Big Sioux River valley in the Sioux Falls, South Dakota area

Figure 6: Big Sioux River valley in the Sioux Falls, South Dakota area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 6 illustrates Sioux Falls, South Dakota and the Big Sioux River valley east and south of the figure 5 map area and there is a small gap between figures 5 and 6. Elevations in the figure 6 map area are generally lower than the Prairie Coteau upland surface elevations in figure 5. Like in figure 5, the figure 6 elevations tend to decrease toward the south and of course toward the Big Sioux River valley. Unlike Prairie Coteau upland surface areas seen in figure 5, the figure 6 landscape appears to be much better drained and appears to be lacking the numerous lakes found in figure 5. More detailed topographic maps do show evidence of hummocky topography typical of glaciated regions, but the figure 6 map does not, which suggests the glacial deposits are probably thinner in the figure 6 map area. If so, the thinner glacial deposits probably are related to more intense melt water flood erosion in the figure 6 map area than in the figure 5 map area to the west and north. This greater melt water flood erosion is probably due to headward erosion of Big Sioux River ice-walled and bedrock-floored valley into the south margin of the isolated Prairie Coteau ice mass. It is possible the headward erosion of the Big Sioux River ice-walled and bedrock-floored valley began before the Prairie Coteau ice mass was detached and isolated from the main ice sheet mass. If so, it is possible headward erosion of the southeast-oriented Minnesota River lowland ice-walled and bedrock-floored valley beheaded south-oriented supra glacial melt water flood flow rivers leading to the south-oriented Big Sioux River ice-walled and bedrock-floored valley. Without large melt water flood flow rivers from further north on the thick ice sheet surface, the Big Sioux River ice-walled and bedrock-floored valley was not able to erode as deep or as broad a valley as the much larger and deeper Midcontinent Trench ice-walled and bedrock-floored valley to the west or the much larger and deeper southeast-oriented ice-walled and bedrock-floored Minnesota River lowland valley to the east. However, there was sufficient melt water flood flow moving to (and in) the south-oriented Big Sioux River ice-walled and bedrock-floored valley to erode into the Prairie Coteau ice mass south end and remove at least some of the glacially transported and included debris from that ice mass south end area.

Turkey Ridge south of Freeman, South Dakota

Figure 7: Turkey Ridge south of Freeman, South Dakota. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 7 illustrates the Turkey Ridge upland region south of Freeman, South Dakota and south of the figure 4 map area (there is a gap between figure 4 and figure 7). Freeman is located near the red north-south oriented highway just north of the figure 7 map area. The James River flows southeast in the figure 7 southwest quadrant. The south and southeast oriented stream in the figure 7 northeast corner is the Vermillion River (the East and West Forks join just north of the figure 7 map area). Turkey Ridge as seen here is a northwest-southeast oriented streamlined erosional residual standing approximately 100 meters above the surrounding lowland regions. Turkey Ridge on a very small and limited scale resembles the much larger Prairie Coteau upland surface and is probably an erosional residual between what were two anastomosing ice-walled and bedrock-floored valleys. To the west was the south-oriented Midcontinent Trench (or James River lowland), which probably was reoriented at its south end to become southeast-oriented when headward erosion of the deep southeast-oriented Missouri River valley captured its south-oriented melt water floods. To the east was the Vermillion River ice-walled and bedrock-floored valley and its tributary West Fork Vermillion River tributary valley. Apparently headward erosion of the West Fork Vermillion River ice-walled and bedrock-floored valley eroded headward into the Midcontinent Trench valley north of the figure 7 map area and by doing so captured significant southeast-oriented melt water flood flow moving to the newly eroded and deep southeast-oriented Missouri River valley. The Turkey Ridge erosional residual was located between these two major southeast-oriented flood flow routes and probably remained ice-covered as flood waters eroded the surrounding region. Turkey Creek (see Turkey Valley township and south) drains the south end of Turkey Ridge and provides evidence south-oriented flood waters did flow across Turkey Ridge, although probably before surrounding elevations had been lowered to their present day elevations. Figure 7a below illustrates a through valley eroded across the Turkey Ridge south end, which links the present day southeast-oriented Vermillion River and James River valleys. The through valley in figure 7a was eroded at a time when south-oriented flood water was moving between the present day Vermillion and James River valleys, suggesting the two valleys were channels in a large-scale ice-walled and bedrock-floored anastomosing channel complex.

Figure 7a: Through valley linking Turkey Creek valley with southeast-oriented Vermillion River tributary valleys northeast of Turkey Ridge and the figure 7a map area (see figure 7 above). United States Geological Survey map digitally presented using National Geographic Society TOPO software.   

Detailed map of Turkey Ridge upland surface landscape features

Figure 8: Detailed map of Turkey Ridge upland surface landscape features. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 8 provides a detailed topographic map of the Turkey Creek headwaters in Salem and Spring Valley townships located on top of the Turkey Ridge upland surface seen in less detail in figure 7 above. Southeast-oriented Turkey Creek headwaters are located in the figure 8 southeast quadrant. Other figure 8 areas lack an integrated drainage pattern and are characterized by the type of hummocky topography and small closed depressions typical of glacial moraine topography. This area on top of Turkey Ridge appears similar to Prairie Coteau landscapes and is probably the location where an ice sheet remnant, between the two southeast-oriented ice-walled and bedrock-floored valleys, slowly melted and deposited whatever debris it contained. The presence of this northwest-southeast oriented ice sheet remnant suggests the southeast-oriented James and Vermillion River valleys on either side of the Turkey Ridge erosional residual were eroded late during the thick ice sheet rapid melt down, probably when headward erosion of the deep southeast-oriented Missouri River valley reached in the region south of figure 7 (see figures 9 and 10 below). Prior to headward erosion of the Missouri River valley into the region melt water floods flowing south in what is now the James River lowland were probably flowing in the ice-walled and ice-floored Midcontinent Trench, which had been carved into the thick ice sheet surface. At that time the thick ice sheet southwest margin may have extended further south into Nebraska (and prior to that maybe even into northeast Kansas). The Missouri River valley may have eroded headward into the decaying ice sheet’s southwest margin and probably captured south and southeast oriented supra glacial melt water rivers flowing from the ice sheet surface further to the north. The immense south-oriented melt water river responsible for carving the Midcontinent Trench ice-walled and ice-floored (and later bedrock-floored) valley was one of the largest south-oriented melt water flood rivers captured by the newly eroded southeast-oriented Missouri River valley. Turkey Ridge provides evidence this capture took place at time when the ice sheet margin just north of the newly eroded Missouri River valley was being carved up by headward erosion of southeast-oriented and anastomosing ice-walled and bedrock-floored valleys.

James River and Vermillion River valleys enter the Missouri River valley

Figure 9 illustrates the where the James River and Vermillion River valleys enter the southeast-oriented  Missouri River valley and is located south of figure 7 (and includes overlap areas with figure 7). The Missouri River is located in the figure 9 south half and flows from Gavins Point Dam near the figure 9 west edge in an east and southeast direction to the figure 9 southeast corner. Yankton, South Dakota is the city in the figure 9 west half and Vermillion, South Dakota is the city located in the figure 9 southeast corner area. The James River flows south from the figure 9 north edge to join the Missouri River a short distance downstream from Yankton. The Vermillion River is located in the figure 9 northeast quadrant and flows generally south-southeast to join the Missouri River southeast of the figure 9 map area (see figure 10 below). The south end of Turkey Ridge with the south-oriented Turkey Creek valley seen in figure 8a above can be seen near the figure 9 north center edge. Note how Turkey Ridge separates a broad western lowland in which the James River valley has been eroded from the eastern Vermillion River lowland. The broad western lowland is the south end of the south-oriented Midcontinent Trench and the James River valley has been eroded into the Midcontinent Trench floor. Also note how downstream from the Yankton the Missouri River valley is much larger than it is upstream. Gavins Point Dam is located at the east end of a narrow Missouri River valley segment, which demonstrates that much of the water that eroded the large Missouri River valley downstream from Yankton came south in the south-oriented Midcontinent Trench valley before the present day narrower James River valley was eroded. The remarkable change in the Missouri River valley size is evidence that headward erosion of the southeast-oriented Missouri River valley captured immense melt water floods moving south in the ice-walled and bedrock-floored Midcontinent Trench. The southeast-oriented Vermillion River lowland to the east of Turkey Ridge provides evidence of a southeast-oriented anastomosing complex of ice-walled and bedrock-floored channels at the time headward erosion of the Missouri River valley reached this area. The present day narrow James River valley was eroded during the final major south-oriented melt water flood flow event.

Vermillion River and Big Sioux River valleys enter the Missouri River valley

Figure 10: Vermillion River and Big Sioux River valleys enter the Missouri River valley. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 10 illustrates where the Vermillion River and Big Sioux River valleys enter the large southeast-oriented Missouri River valley and is located southeast of the figure 9 map area (and includes overlap areas with figure 9). The southeast-oriented Missouri River is located in the figure 10 southwest quadrant, with the state of Nebraska being located south of the Missouri River. Vermillion, South Dakota is the city located north of the Missouri River in the figure 10 southwest quadrant and the Vermillion River flows south and southwest around the west edge of Vermillion before turning southeast to enter the Missouri River. The southeast end of Turkey Ridge is located in the figure 10 northwest corner and separates the eastern Vermillion River lowland from the western James River lowland. The Big Sioux River flows south-southwest from the figure 10 northeast corner to enter the Missouri River valley and then to flow southeast to join the Missouri River south of the figure 10 map area. The state of Iowa is located east of the Big Sioux River. Elk Point, South Dakota is the smaller city located near the figure 10 south edge in the figure 10 southeast quadrant. Akron, Iowa is the smaller town located near the figure 10 east center edge. Note how the size of the south-southwest oriented Big Sioux River valley compares with the size of the southeast-oriented Vermillion River valley and the southeast-oriented James River lowland and Missouri River valley. The Big Sioux River valley as previously discussed probably originated as a south-oriented ice-walled and bedrock-floored valley draining what was then a detached and isolated thick ice sheet remnant occupying the area where the Prairie Coteau upland surface is now located. The southeast-oriented Vermillion River lowland obtained some of its water from the southern margin of that detached and isolated ice sheet remnant, however most of its water was obtained from immense melt water floods that flowed south in the large Midcontinent Trench ice-walled and ice-floored (and later bedrock-floored) valley. The southeast-oriented James River lowland and Missouri River valley obtained some of its water from sources upstream on the present day Missouri River route, although most of its water was also obtained from immense melt water floods that flowed south in the Midcontinent Trench ice-walled and bedrock-floored valley.

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.