A geomorphic history based on topographic map evidence
The Big Sioux River-Minnesota River drainage divide area is located in eastern South Dakota and western Minnesota, USA. The Big Sioux River flows south on the Prairie Coteau upland and eventually joins the Missouri River at the South Dakota’s southeast corner. The Minnesota River originates along the South Dakota-Minnesota border and flows southeast across southern Minnesota before turning northeast to join the southeast-oriented Mississippi River. The southeast-oriented lowland in which the Minnesota River flows is interpreted to have formed as an ice-walled and bedrock-floored valley sliced by melt water floods into the surface of a rapidly melting thick ice sheet. The northeast-facing Prairie Coteau escarpment, which marks the Minnesota River lowland southwest boundary, is interpreted to be what remains of the ice-walled and bedrock-floored valley’s southwest wall. The Big Sioux River valley is interpreted to have formed as a smaller, but similar, ice-walled and bedrock-floored valley carved into the detached ice sheet located on what is now the Prairie Coteau upland. Glacial moraines found on the Prairie Coteau upland are interpreted to be debris contained within the detached ice sheet and which was deposited as the detached ice sheet remnant melted.
- The purpose of this essay is to use topographic map interpretation methods to explore Big Sioux River-Minnesota River drainage divide area landform origins in eastern South Dakota and western Minnesota, 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 Big Sioux River-Minnesota River drainage divide area landform evidence in eastern South Dakota and western Minnesota 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.
Big Sioux River-Minnesota River drainage divide area location map
Figure 1: Big Sioux River-Minnesota 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 for the Big Sioux River-Minnesota River drainage divide area in eastern South Dakota and western Minnesota. Minnesota is the state contained within the green boundary line. South Dakota is located west of Minnesota and is located near the figure 1 west edge. Iowa is the state located south of Minnesota and along the figure 1 south edge. The state indicated by the yellow boundary along the east edge (north half) is Wisconsin. The Big Sioux River originates northwest of Summit, South Dakota (located in the figure 1 northwest corner area) and flows south through Watertown, Brookings, and Sioux Falls, South Dakota and then to the figure 1 south edge. South of figure 1 the Big Sioux River joins the Missouri River. Note the lack of long tributaries from either side to the Big Sioux River. Reasons for that lack of long tributaries are apparent when looking at detailed topographic maps below. The Minnesota River originates at Big Stone Lake located on the South Dakota-Minnesota border near the figure 1 northwest corner. A Minnesota River tributary originates near Claire City, South Dakota, which is located on the figure 1 north edge northwest of Browns Valley. From Big Stone Lake the Minnesota River flows in a southeast direction to Mankato, Minnesota and then makes an abrupt turn to flow northeast to St. Paul, Minnesota and to join the southeast oriented Mississippi River. Note the northwest-oriented tributary, which joins the Minnesota River at Mankato. Why would the Minnesota River flow southeast and then abruptly turn to flow northeast to join another southeast oriented river? To understand the answer to that question imagine eastern South Dakota and Minnesota at a time when the entire region was covered with a rapidly melting thick North American ice sheet. Huge melt water rivers were forming on the ice sheet surface and flowing in a generally south and southeast oriented direction toward the Mississippi River valley, which was developing into a major south-oriented melt water flood flow route. One such huge melt water river carved an ice-walled and bedrock-floored valley into the decaying ice sheet surface headward into Minnesota along the present day Mississippi River course. Another large southeast-oriented melt water river carved a similar southeast-oriented valley headward from the Mississippi River valley in eastern Iowa into southern Minnesota and then northwest along the present-day southeast-oriented Minnesota River course. For reasons not apparent from topographic map evidence presented in this essay the Mississippi River valley was eroded deeper than the Minnesota River valley (perhaps more melt water was moving down the Mississippi River valley than was moving in the Minnesota River valley or perhaps the Mississippi River valley was bedrock-floored while the Minnesota River valley was still ice-floored and at a much higher elevation). Whatever the reason tributary melt water rivers also carved valleys into the ice sheet surface. Probably a combination of such tributary valleys created a link between the southeast- and south-oriented Mississippi River ice-walled valley and the southeast-oriented Minnesota River ice-walled valley and that link opened up a steeper gradient northeast oriented flood flow route that captured the southeast-oriented Minnesota River melt water river. That capture beheaded the southeast-oriented flood flow route extending southeast from Mankato and flood waters on the northwest end of that beheaded flood flow route reversed flow direction to flow northwest to the newly eroded northeast-oriented Minnesota River valley.
Big Sioux River-Minnesota River drainage divide area detailed location map
Figure 2: Big Sioux River-Minnesota 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 of the Big Sioux River-Minnesota River drainage divide area. Grant, Codington, Hamlin, and Deuel are names of South Dakota counties. Swift, Lac Qui Parle, Chippewa, Yellow Medicine, Lincoln, and Lyon are names of Minnesota counties. The Big Sioux River originates in western Grant County in the figure 2 northwest corner and flows to Watertown and then south through eastern Hamlin County and then to the figure 2 south edge. The Minnesota River originates at Big Stone Lake (on the figure 2 north edge at the South Dakota-Minnesota border) and flows southeast to the figure 2 east edge (the Minnesota River serve as the southeast-oriented county line in Minnesota). Figure 2 does not show topography, but if it did it would show the Big Sioux River flowing south on an upland surface (the Prairie Coteau) and the southeast-oriented Minnesota River flowing in a broad lowland. Between the two rivers is northeast-facing escarpment (the northeast-facing Prairie Coteau escarpment), the location of which can be identified on figure 2 by numerous northeast-oriented Minnesota River tributaries originating at the northeast edge of a northwest-southeast oriented band of small lakes (extending from Grant County through Deuel County to Lincoln County). The band of small lakes is located in a hummocky topography area at the escarpment crest and probably represents an area of unusually thick glacial moraine deposits. The elevation difference between the Big Sioux River and the Minnesota River valleys is probably related to events which occurred during the rapid melt of a thick ice sheet. The entire figure 2 map area was covered by that ice sheet and during the rapid melt down an immense southeast-oriented melt water river sliced a broad and deep ice-walled and bedrock-floored valley into the thick ice sheet surface following the present day Minnesota River alignment. A somewhat narrower and shallower ice-walled and bedrock-floored valley was also sliced by a south-oriented melt water river into the ice sheet surface along what is today the Big Sioux River alignment. Headward erosion of the large southeast-oriented ice-walled and bedrock-floored valley on the Minnesota River alignment beheaded melt water flow to the south-oriented valley on the Big Sioux River alignment, which limited the ability of the south-oriented valley to erode as deep or to develop as broad a valley. The band of lakes (or the thick glacial moraine band) located between the two valleys today (and at the crest of the northeast-facing Prairie Coteau escarpment) probably reflects a drainage divide between supra glacial melt water drainage basins. Debris contained in the melting ice sheet in this drainage divide area was not washed away by south-oriented melt water floods, but instead simply was deposited in place as the ice sheet melted.
Big Sioux River-Minnesota River drainage divide area near Summit, South Dakota
Figure 3: Big Sioux River-Minnesota River drainage divide area near Summit, South Dakota. United States Geological Survey map digitally presented using National Geographic Society TOPO software.
Figure 3 illustrates the Big Sioux River-Minnesota River drainage divide area near Summit, South Dakota and also near the Big Sioux River headwaters. Ortley, South Dakota is located in the figure 3 northwest quadrant. The Big Sioux River flows south in the figure 3 edge area (west of Ortley) to the figure 3 southwest corner, where the Big Sioux River turns to flow in a southeast direction. Summit, South Dakota is located near the figure 3 major highway junction. A Big Sioux River tributary originates south of Summit and flows northwest before turning to flow southwest to join the south-oriented Big Sioux River. East of Summit is a northwest-southeast band of small lakes and hummocky topography, which forms the drainage divide between the Big Sioux River drainage basin and the Minnesota River drainage basin to the northeast. Northeast of the drainage divide are numerous east-northeast-oriented streams, which flow down a significant slope. That slope is the northeast-facing Prairie Coteau escarpment. The upland west and southwest of that escarpment is the Prairie Coteau upland. The Prairie Coteau upland is generally characterized by numerous lake basins and other features typical of glacial moraine regions. Further to the west is a west-facing escarpment and then the south-oriented James River lowland. The ridge of hummocky topography with small lakes located east and northeast of the Big Sioux River valley generally is higher than the Prairie Coteau area west of the Big Sioux River, although the area west of the Big Sioux River generally has larger lake basins. The Big Sioux River headwaters area does not show evidence of a large valley, although further south the Big Sioux River valley becomes more obvious (see figure 10 below). Probably what has happened here is the lowland northeast of the northeast-facing Prairie Coteau escarpment was carved by immense southeast-oriented melt water rivers as they sliced a huge southeast-oriented canyon into the surface of a rapidly melting thick ice sheet. The Big Sioux River valley may be located on the alignment of a much smaller and shallower south-oriented ice-walled and bedrock-floored valley that was also sliced into the thick ice sheet surface. The smaller south-oriented valley never developed into the size (or achieved the depth) of the much larger southeast-oriented ice-walled and bedrock-floored valley to the east (or the large south-oriented James River lowland to the west), perhaps because headward erosion of the southeast-oriented ice-walled and bedrock-floored valley beheaded south-oriented melt water flood flow to the south-oriented valley. In any case, the Prairie Coteau area was not eroded as deep as areas east of the northeast-facing Prairie Coteau escarpment (and areas west of the Prairie Coteau west-facing escarpment) and, with the exception of the much smaller ice-walled and bedrock-floored Big Sioux River valley area, melt water floods did not remove much of the ice sheet contained debris. When the ice sheet remnants melted the ice sheet contained debris simply was deposited in place.
Prairie Coteau-Big Stone Lake area east of Summit, South Dakota
Figure 4: Prairie Coteau-Big Stone Lake area east of Summit, South Dakota. United States Geological Survey map digitally presented using National Geographic Society TOPO software.
Figure 4 illustrates the northeast-facing Prairie Coteau escarpment to Big Stone Lake area located east and north of the figure 3 map area. Wilnot, South Dakota is the town located in the figure 4 northwest quadrant. The southeast-oriented stream flowing from the Wilnot area to the figure 4 southeast corner is the North Fork Whetstone River, which is a Minnesota River tributary. Note how northeast-oriented drainage routes from the northeast-facing escarpment slope (located along the figure 4 west edge) flow to the southeast-oriented North Fork valley. The figure 4 evidence suggests drainage down the escarpment slope developed independently from drainage routes on the lowland floor. The Big Stone Lake valley, where the Minnesota River originates, is southeast-oriented and roughly parallels the North Fork Whetstone River valley, providing evidence that multiple southeast-oriented flood flow channels developed on the lowland floor. These multiple channels probably were components of a large-scale southeast-oriented anastomosing channel complex that developed late during the thick ice sheet rapid melt down and reflect melt water that originated from sources further to the north and northwest. The northeast-oriented drainage routes originating on the northeast-facing escarpment slope probably were developed by melt water from decaying ice sheet remnants located at the escarpment crest. The southeast-oriented ice-walled and bedrock-floored Minnesota River lowland valley to the east and northeast and the south-oriented ice-walled and bedrock-floored James River lowland valley to the west (see James River-Big Sioux River drainage divide area essays found under either James River or Big Sioux River on sidebar category list) detached and isolated a section of the rapidly decaying thick ice sheet. By chopping the thick ice sheet up into isolated sections headward erosion of these and other ice-walled and bedrock-floored valleys greatly hastened the ice sheet melting process. The isolated thick ice sheet section located on the present day Prairie Coteau upland was further chopped up by headward erosion of the smaller and shallower ice-walled and bedrock-floored Big Sioux River valley. However, much of the isolated Prairie Coteau ice sheet section remained in place and was not affected (at least late during the ice sheet melt down history) by the immense south-oriented melt water floods. These ice sheet sections simply melted and deposited whatever debris they contained as the melting took place. Melt water from these Prairie Coteau ice sheet remnants that did not flow south in the Big Sioux River valley flowed into the adjacent southeast-oriented Minnesota River lowland to the east (and into James River lowland to the west) and eroded the present day drainage routes on the Prairie Coteau escarpment slopes.
Mud Creek-Minnesota River drainage divide area near Punished Woman Lake
Figure 5: Mud Creek-Minnesota River drainage divide area near Punished Woman Lake. United States Geological Survey map digitally presented using National Geographic Society TOPO software.
Figure 5 illustrates the Big Sioux River-Minnesota River drainage divide area in the Mud Creek and Punished Woman Lake area and is located south of the figure 3 map area. The Big Sioux River flows south in the figure 5 west half. Mud Creek originates at Punished Woman Lake in the figure 5 east center and flows southwest to join the south-oriented Big Sioux River south of the figure 5 map area. An unnamed stream flows south and northeast to the west edge of Antelope Valley and then south-southeast along the west edge of Antelope Valley to Punished Woman Lake. Just west of headwaters of that unnamed stream are headwaters of southwest-oriented Soo Creek, which is located north of Mud Creek and which also flows to the south-oriented Big Sioux River. Northeast-oriented drainage along the figure 5 east edge is located along the crest of the northeast-facing Prairie Coteau escarpment and is flowing to the Minnesota River lowland and eventually to the southeast-oriented Minnesota River. The ridge between the Big Sioux River valley and headwaters of northeast-oriented Minnesota River tributaries suggests that area was probably the location of an ice sheet remnant between the deep southeast-oriented ice-walled and bedrock-floored Minnesota River lowland valley to the northeast and the south-oriented ice-walled and bedrock-floored Big Sioux River valley to the west. Generally debris contained in that ice sheet remnant was not washed away by melt water flood erosion, but was deposited in place. The lakes probably represent kettle lakes where buried or partially buried ice masses melted and left depression that are now filled with water. Antelope Valley is not really a valley, but is an area where the surface suggests a somewhat different origin than the more hummocky regions on either side. A similar unnamed area is located in the figure 5 southeast quadrant. Based on topographic map evidence alone I can only speculate on the origin of those flatter areas. Such speculation leads me to suggest the flatter area might be locations where there were lakes on the ice sheet remnant surface, which filled with melt water transported debris. If so the lakes existed late during decay of the ice sheet remnant and may have had bedrock floors, although ice walls would have surrounded them. Field inspection of the region may reveal a different origin for those flatter areas. West of the Big Sioux River valley, along the figure 5 west edge is what appears to be another hummocky area with small lakes, suggesting it too was the location of another ice sheet between the south-oriented Big Sioux River ice-walled and bedrock-floored valley and the much larger and deeper south-oriented ice-walled and bedrock-floored James River lowland valley to the west.
Prairie Coteau escarpment southwest of Milbank, South Dakota
Figure 6: Prairie Coteau escarpment southwest of Milbank, South Dakota. United States Geological Survey map digitally presented using National Geographic Society TOPO software.
Figure 6 illustrates the northeast-facing Prairie Coteau escarpment slope southwest of Milbank, South Dakota and is south of figure 4 and east of figure 5 (and includes overlap areas with figure 5). Milbank is the town in the figure 6 north center. The east-oriented stream along the figure 6 north edge and flowing through Milbank is the South Fork of the Whetstone River, which turns northeast at Milbank to join the southeast-oriented North Fork (seen in figure 4) and then to flow in a northeast direction to the southeast-oriented Minnesota River. The named east-oriented stream (south of Milbank) is the North Fork Yellow Bank River, which near the figure 6 east edge turns to flow north before turning to flow southeast and to eventually join the southeast-oriented Minnesota River (see figure 7 below). Figure 6 evidence shows drainage routes on the escarpment slope to generally be northeast-oriented, which means the water is flowing directly down the escarpment slope. Only when the streams reach the lowland at the escarpment base do the streams turn to flow southeast. This evidence suggests drainage routes on the escarpment slope were developed by melt water flowing from melting ice located at the escarpment crest and were developed after the escarpment had been developed. The escarpment is here interpreted to have been formed as the southwest wall of a huge southeast-oriented ice-walled and bedrock-floored valley sliced into a rapidly melting thick ice sheet surface. At the time the escarpment was eroded flood waters were moving in a southeast direction along the present day escarpment slope. However, melting of the ice wall located at the escarpment crest occurred after the escarpment slope had been developed. At that later time southeast-oriented flood waters were confined to lower areas in the southeast-oriented Minnesota River lowland and were probably eroding the present day Minnesota River valley and its various tributary valleys. Evidence that the northeast-oriented drainage moving down the escarpment slope flowed northeast to those lower southeast-oriented valley areas supports that interpretation.
Minnesota River valley down stream from Big Stone Lake
Figure 7: Minnesota River valley down stream from Big Stone Lake. United States Geological Survey map digitally presented using National Geographic Society TOPO software.
Figure 7 illustrates the Minnesota River valley downstream from Big Stone Lake and is located east of figure 6 and includes overlap areas with figure 6. Big Stone Lake is the lake next to Ortonville in the figure 7 northwest quadrant (along the figure 7 north edge). The Minnesota River flows southeast from Big Stone Lake in a southeast oriented valley through the Big Stone National Wildlife Refuge to the Lac Qui Parle State Wildlife Management Area. The Whetstone River flows northeast and east to join the Minnesota River near Big Stone City in the figure 7 northwest corner. Further south and southeast is the east-northeast and southeast oriented North Fork Yellow Bank River, which joins the north-oriented Yellow Bank River, which joins the southeast oriented Minnesota River in the figure 7 center area. Figure 7 provides evidence of anastomosing channels in the Minnesota River valley area. Note what looks like a southeast oriented valley south of Louisburg in the figure 7 southeast quadrant. Figure 8 below illustrates headwaters of the northwest-oriented stream located in that valley. The valley itself was eroded by southeast-oriented flood water and Louisbug is located on an erosional residual between two anastomosing channels. Headward erosion of a deeper channel along the present day Minnesota River route beheaded southeast-oriented flood flow in the valley south of Louisburg. Flood waters on the northwest end of the beheaded flood flow route then reversed flow direction to flow northwest to the deeper Minnesota River channel and to create the northwest-oriented valley seen today. More anastomosing channels and erosional residuals are located on the Minnesota River valley north side east of Odessa. Closer to the northeast-facing Prairie Coteau escarpment Minnesota River tributaries have southeast- and northwest-oriented segments and/or tributaries, which provide evidence of a southeast-oriented anastomosing channel complex that has since been altered by northeast-oriented melt water flow from the escarpment slope. The later northeast-oriented flow both moved along preexisting southeast-oriented channels and also deposited sediments blocking the preexisting southeast-oriented channels to form the complex drainage system seen today. Figure 8 illustrates the region south and east of figure 7.
Minnesota River valley and tributaries northeast of Madison, Minnesota
Figure 8: Minnesota River valley and tributaries northeast of Madison, Minnesota. United States Geological Survey map digitally presented using National Geographic Society TOPO software.
Figure 8 illustrates the Minnesota River valley and tributaries northeast of Madison, Minnesota and is south and east of the figure 7 map area. The southeast-oriented Minnesota River is located in the figure 8 northeast corner and is flowing southeast from Lac Qui Parle. Note the anastomosing channel complex discussed in the figure 7 description above. The present day northwest-oriented stream, which flows to the southeast-oriented Minnesota River is located north of the words “LAKE SHORE” in the figure 8 north center. Water flows east before turning northwest, although it appears much of the east oriented drainage has been altered by human action to drain the region. The northwest-southeast oriented through valley continues to the southeast and southeast-oriented Emily Creek originates just southeast of the elbow of capture (where the east oriented drainage turns to flow northwest). The northeast-oriented stream in the figure 8 southeast corner is the Lac Qui Parle River, which originates on the northeast-facing Prairie Coteau escarpment slope. Southeast oriented drainage west of Madison flows to the Lac Qui Parle River. Note how drainage routes diverge to the southeast. Again, some of the drainage routes are ditches constructed to drain the region, although the ditches probably follow preexisting flood eroded anastomosing channel locations. Immense southeast-oriented floods responsible for slicing the huge southeast-oriented ice-walled and bedrock-floored valley into the thick ice sheet surface ended when headward erosion of ice-walled and bedrock-floored valley intersected with east and north oriented ice-walled walled and bedrock-floored valleys further to the north and northwest. The east-oriented ice-walled and bedrock-floored valleys provided shorter routes and steeper gradients to sea level via the present day Great Lakes region and the north-oriented ice-walled and bedrock-floored valleys subsequently provide even shorter routes with even steeper gradients to sea level. These shorter routes with steeper gradients captured much of the southeast-oriented melt water flood flow that had been moving through the figure 8 map area and diverted the flood water east and later north. The diversion caused a major flood flow reversal in what is now the north-oriented Red River valley located along the present day North Dakota-Minnesota River boundary. What had been a major south-oriented ice-walled and bedrock-floored valley became a north-oriented flood flow route. That is why today, immediately northwest of Big Stone Lake (in the same northwest-southeast oriented through valley), there are headwaters of north-oriented Red River tributaries. Capture of south-oriented melt water floods, which had been flowing to the Gulf of Mexico, and diversion of the water north to Hudson Bay and other northern locations, triggered a major climate change, which stopped the rapid ice sheet melt down. Following that climate change there were no more immense melt water floods, although ice sheet remnants on the Prairie Coteau upland and elsewhere eventually melted.
Hidewood Creek-Lac Qui Parle River drainage divide area
Figure 9: Hidewood Creek-Lac Qui Parle River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.
Figure 9 illustrates the Hidewood Creek-Lac Qui Parle River drainage divide area along the Prairie Coteau northeast facing escarpment crest and is located south and west of the figure 8 map area. Hidewood Creek originates in the Clear Lake area and flows southwest to the figure 9 southwest corner and then to the south-oriented Big Sioux River. Monighan Creek originates at Mud Lake (a short distance northeast of Clear Lake) and flows northeast to join the northeast oriented West Branch of the Lake Qui Parle River, which then flows north-northwest before turning northeast again to flow to the figure 9 northeast corner. South of the West Branch headwaters is northeast, east, and northeast oriented Florida Creek, which east of the figure 9 map area joins the West Branch of the Lac Qui Parle River. Headwaters of the northeast-oriented Lac Qui Parle tributaries have eroded valleys further into the Prairie Coteau surface than seen further to the north, although the valleys do not extend very far into the Prairie Coteau region. Also, the southwest-oriented Hidewood Creek valley provides evidence of tributary valleys to the south-oriented Big Sioux River, although much of the figure 9 map area consists of hummocky topography with water filled depressions. This figure 9 map area is interpreted to have been the location of the southwest ice wall of the huge southeast-oriented Minnesota River lowland ice-walled and bedrock-floored valley. When the southeast-oriented valley was first sliced into the thick ice sheet this ice wall was probably many hundreds of meters high (if not higher), and the decaying ice sheet remnant would have covered all of the figure 9 map area except the northeast corner. Perhaps some of the present day figure 9 drainage routes had already been initiated at that time as narrow ice-walled and ice-floored (or even bedrock-floored) valleys, which continued to serve as drainage routes as the ice sheet remnant melted. Other present day drainage routes probably developed as the ice sheet remnant melted. Debris contained in the ice sheet remnant was deposited in place, and except along major melt water flow routes was not eroded. Lakes are probably kettle lakes and represent locations where blocks of ice buried or partially buried in debris the ice sheet deposited, leaving depressions which are filled with water.
Hidewood Creek valley southwest of Clear Lake, South Dakota
Figure 10: Hidewood Creek valley southwest of Clear Lake, South Dakota. United States Geological Survey map digitally presented using National Geographic Society TOPO software.
Figure 10 illustrates the Big Sioux River-Hidewood Creek drainage divide area southwest of Clear Lake, South Dakota and includes overlap areas with figure 9. The Big Sioux River flows in a south-southeast oriented direction from the figure 10 northwest corner to the figure 10 south edge (just west of Estelline. Hidewood Creek flows southwest from the figure 10 northeast corner (just south of Clear Lake) to join the south-southeast oriented Big Sioux River near the figure 10 south edge (northwest of Estelline). Other than the Dry Lake and Lake Florence area of the figure 10 southwest quadrant area there are no lakes in the figure 10 map area (the lake south of Dry Lake and Stone Bridge in the figure 10 southwest corner is Lake Poinsett, which is much larger than Dry Lake). As seen in figure 9 there are lakes further east along the Big Sioux River-Lac Qui Parle River drainage divide. West of the figure 10 map there are also lake basins, although some of these are larger basins comparable in size to the Dry Lake basin or larger (as opposed to the smaller lake basin seen in figure 9). Why is much of the figure 10 map area devoid of lake basins when on either side are areas with numerous lake basins. Based on topographic map evidence alone any interpretation must be considered subject to revision based on field evidence. However, the figure 10 map area shows the Big Sioux River valley and a Big Sioux River tributary valley and appears to have been water eroded, while areas with lake basins in figure 9 did not appear to be water eroded. Based on this evidence it is possible much of the figure 9 map area may have been on the floor of the smaller and shallower south-oriented Big Sioux River ice-walled and bedrock-floored valley, while Prairie Coteau areas which do not appear water eroded (and which contain abundant lake basins) represent locations where ice sheet remnants remained and deposited whatever debris they contained. If so, the Dry Lake basin, which is larger than lake basins seen in figure 9 and which has a name suggesting it may be shallow, may not be a kettle lake, but may be related to a now dammed former melt water flood flow channel, although the adjacent lake basins suggests consideration of the kettle lake basin origin possibility should not be rejected based solely on the figure 10 evidence.
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.