A geomorphic history based on topographic map evidence

The Big Sioux River-Minnesota River drainage divide area discussed here is located in Brookings County, South Dakota and Lincoln and Lyon Counties, Minnesota, USA. Major landforms include the Prairie Coteau upland surface and the northeast-facing Prairie Coteau escarpment. The Prairie Coteau escarpment is interpreted here to be what remains of an immense southeast-oriented ice-walled and bedrock-floored valley sliced into the surface of a rapidly melting thick North American ice sheet. The Prairie Coteau upland surface is interpreted to be where a detached ice sheet remnant was located. The south-oriented Big Sioux River drainage basin is interpreted to be where headward erosion of a much smaller and shallower ice-walled and bedrock-floored valley eroded headward into the detached Prairie Coteau ice sheet remnant.

Preface:

• 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 Brookings County, South Dakota and Lincoln and Lyon Counties, 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 Brookings County, South Dakota and Lincoln and Lyon Counties, 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 Big Sioux River-Minnesota River drainage divide area location map for this essay. The eastern half of figure 1 is the southwest corner of the state of Minnesota. The figure 1 western half shows an area in eastern South Dakota. The Minnesota River flows in a southeast direction from the figure 1 north center edge to the figure 1 east center edge. From the figure 1 map area the Minnesota River continues to flow in a southeast direction to Mankato, Minnesota, and then makes an abrupt turn to flow in a northeast direction to join the southeast-oriented Mississippi River at Saint Paul, Minnesota. The Big Sioux River flows in a generally south-oriented direction in eastern South Dakota from the figure 1 north edge through Watertown to Estelline, Brookings, and Dell Rapids, South Dakota, before flowing to the figure 1 south edge. South of figure 1 the Big Sioux River flows in a south-oriented direction to join the southeast-oriented Missouri River, which eventually joins the Mississippi River at Saint Louis, Missouri. Also in figure 1 are headwaters of the southeast-oriented Des Moines River, which eventually flows to the Mississippi River. The Big Sioux River-Minnesota River drainage divide area illustrated and discussed in this essay is located in Brookings County, South Dakota and Lincoln and Lyon Counties, Minnesota and encompasses the region between Estelline and Brookings eastward to Marshall, Minnesota. Essays describing other Big Sioux River drainage divide areas can be found under Big Sioux River on the sidebar category list. Note how the Big Sioux River has relatively short tributaries and is flowing south in what appears to be a relatively narrow drainage basin compared to the southeast-oriented Minnesota River. Also note regions with small lakes on either side of the Big Sioux River drainage basin. The Big Sioux River flows on the Prairie Coteau escarpment bounded upland surface, which is an area underlain by thick glacial moraines. This essay illustrates and discusses Prairie Coteau upland surface evidence located east of the Big Sioux River. The southeast-oriented Minnesota River flows in a broad southeast-oriented lowland at the northeast-oriented Prairie Coteau escarpment base. The northeast-facing escarpment slope is seen in figures 5 and 6 below. The west-facing Prairie Coteau escarpment is located near the figure 1 west edge and is the eastern boundary for the large south-oriented James River lowland.

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 for the Big Sioux River-Minnesota River drainage divide area in Brookings County, South Dakota and Lincoln and Lyon Counties, Minnesota. Hamlin, Deuel, and Brookings are South Dakota county names. Lac Qui Parle, Yellow Medicine, Lincoln, and Lyon are Minnesota county names. The Big Sioux River flows south from Watertown (in the figure 2 northwest corner) through eastern Hamlin County to Brookings County and then southeast to the figure 2 south center edge. The southeast-oriented Minnesota River is located in the figure 2 northeast corner and serves as the northeast border for Yellow Medicine County. Estelline, South Dakota is located in the Hamlin County southeast corner and Brookings, South Dakota is located in Brookings County. Parallel northeast-oriented streams located in Lyon County, northeast Lincoln County, western Yellow Medicine County, and northeast-oriented Deuel County are flowing down the northeast-oriented Prairie Coteau escarpment slope and the escarpment slope location can be determined from these streams. The figure 2 region northeast of the escarpment slope is the southeast-oriented Minnesota River lowland, which is generally 100-200 meters lower in elevation than areas to the southwest of the northeast-facing escarpment. Figures below provide topographic maps illustrating the Big Sioux River valley near Estelline in southeast Hamlin County (figure 3) and near Brookings (figure 10), the northeast-facing Prairie Coteau escarpment along the Lincoln-Lyon County border (figure 5) and near Marshall, Minnesota (figure 6), and of Prairie Coteau upland surface areas near Lake Hendricks in northeast Brookings County (figures 4, 7, and 9) and Lake Shaokatan in western Lincoln County (figures 7 and 8). The large southeast-oriented Minnesota River lowland region is interpreted here to have originated as the floor of an immense southeast-oriented ice-walled and bedrock-floored valley sliced into the surface of a rapidly melting thick North American ice sheet. The Prairie Coteau upland surface is interpreted here to be where, what became a detached ice sheet remnant slowly melted and deposited whatever debris it contained. The Big Sioux River drainage basin is interpreted here to have originated as a much smaller and shallower (compared to the Minnesota River lowland) ice-walled and bedrock-floored valley carved in the ice sheet remnant surface.

Big Sioux River valley south of Estelline, South Dakota

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

Figure 3 illustrates the Big Sioux River valley south of Estelline, South Dakota and the region east of that valley. Estelline, South Dakota is located in the figure 3 northwest corner. The Big Sioux River flows west of Estelline and is southeast and south oriented in the figure 3 west half. Peg Munky Run is the Big Sioux River tributary flowing west in Grange township (along the figure 3 north edge) and then flowing south to join the Big Sioux River in the figure 3 northwest quadrant. South of Peg Munky Run is west and south oriented North Deer Creek, which in the figure 3 south half becomes southwest oriented and flows to the figure 3 south edge (and south of the figure 3 map area joins the south oriented Big Sioux River). Other southwest- and south-oriented drainage in the figure 3 southeast quadrant also flows to the Big Sioux River. Toronto, South Dakota is located in the figure 3 northeast corner. The southeast-oriented stream at Toronto flows eventually to the Minnesota River (and will be seen further in figure 4 below). The northwest-southeast oriented Big Sioux River-Minnesota River drainage divide is located in the figure 3 northeast corner (and Toronto is located on the divide). The figure 3 map area appears to be well-drained and to have been eroded by running water. The Big Sioux River drainage basin was eroded by south- and southwest-oriented flood flow with tributary valleys being eroded in sequence from the south to the north. The flood water source cannot be determined from figure 3 evidence alone, but was somewhere to the north or northeast. Figure 3a below illustrates the northeast-facing Prairie Coteau escarpment relationship to Toronto, South Dakota, and demonstrates the flood water source occurred before that escarpment existed. Figure 3a also illustrates areas of hummocky topography and small lakes between the south-oriented Big Sioux River valley and the northeast-facing escarpment slope. Those hummocky areas with small lakes provide evidence of glacial moraines on the Prairie Coteau upland surface, which is located south and west of the escarpment crest. The northeast-facing escarpment is interpreted here to be what remains of the southwest wall of a large southeast-oriented ice-walled and bedrock-floored valley. The Prairie Coteau upland surface is interpreted here to be where what became a detached ice sheet remnant was once located. The Big Sioux River drainage basin is interpreted here to have developed as a smaller and shallower ice-walled and bedrock-floored valley eroded into the detached ice sheet remnant.

Figure 3a: Northeast-facing Prairie Coteau escarpment northeast from Toronto (located in southwest corner). United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Minnesota River tributaries near Lake Hendricks

Figure 4: Minnesota River tributaries near Lake Hendricks. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 4 illustrates the Prairie Coteau upland surface area and headwaters of Minnesota River tributaries east of the figure map area and includes overlap areas with figure 3. Toronto, South Dakota is located in the figure 4 northwest corner. While the figure 4 map area does have a drainage system there are also many poorly drained areas with lakes and swamps. This figure 4 landscape is typical of many Prairie Coteau upland surface areas and provides evidence the region is underlain by thick glacial moraines. As previously mentioned this area was where the detached Prairie Coteau ice sheet remnant once stood. The northeast-oriented streams generally have not eroded deep valleys into the Prairie Coteau upland surface, which suggests the streams probably developed as the ice sheet remnant occupying the figure 4 map area melted. Lakes and other low spots (now filled with swamps) probably represent where final blocks of ice were once buried or partially buried in ice deposited debris. As those final ice masses melted they left depressions, which have now been filled with water to form lakes and swamps. In the figure 4 south center area at the Lake Hendricks southwest end is a “Y”-shaped pattern of somewhat deeper valleys. Figure 7 below provides a more detailed map of those deeper valleys and also illustrates them south of the figure 4 map area. Based on what can be seen in figure 4 the valley pattern suggests the valleys were eroded by south-oriented flow. However, today the western valley is drained by a southeast-oriented stream, which at the “Y” turns to flow northeast into Lake Hendricks. The Lake Hendricks outlet (named the Lac Qui Parle River) is located near Hendricks, Minnesota and flows northeast to the figure 4 northeast corner (and then to the northeast-facing escarpment slope). The south-oriented valley south of the figure 4 map area leads to Deer Creek, which flows in a southwest direction to the south-oriented Big Sioux River. Probably what has happened here headward erosion of the ice-walled and bedrock-floored Big Sioux River valley and tributary valleys reached into the figure 4 map area before headward erosion of the deep and large Minnesota River ice-walled and bedrock-floored valley beheaded those supra glacial melt water flood flow routes. Ice still remained in the present day Lake Hendricks basin and elsewhere in the figure 4 map area. Melt water from the north and east of the figure 4 map area initially flowed south to the ice-walled and bedrock-floored Big Sioux River valley, but headward erosion of the deep and large southeast-oriented Minnesota River ice-walled and bedrock-floored valley beheaded and reversed those flood flow routes.

Northeast-facing Prairie Coteau escarpment

Figure 5: Northeast-facing Prairie Coteau escarpment. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 5 illustrates the northeast-facing Prairie Coteau escarpment slope east of the figure 4 map area and includes overlap areas with figure 4. The Lac Qui Parle River is located in the figure 5 northwest quadrant. The southwest margin of the broad southeast-oriented Minnesota River lowland is located in the figure 5 northeast corner. The Prairie Coteau upland surface is located in the figure 5 southwest quadrant. Note how the northeast-facing escarpment is drained. Drainage routes are all northeast-oriented and have not eroded deep valleys into the escarpment slope or into the Prairie Coteau upland surface at the escarpment crest. This evidence suggests the northeast-oriented streams originated after the lowland area to the northeast and the escarpment face had been eroded by immense southeast-oriented floods. As already described those immense southeast-oriented floods sliced a gigantic southeast-oriented ice-walled and bedrock-floored valley into the surface of a rapidly melting thick North American ice sheet. The northeast-facing escarpment slope is what remains of the ice-walled and bedrock-floored valley’s southwest wall. At the time the large southeast-oriented ice-walled and bedrock-floored valley was first eroded the figure 5 southwest quadrant was covered by an ice sheet remnant. That ice sheet remnant subsequently melted and melt water from ice near the escarpment crest flowed down the escarpment slope and also created the present day northeast-oriented streams at the escarpment crest. However, prior to headward erosion of the southeast-oriented Minnesota River lowland ice-walled and bedrock-floored valley melt water was probably flowing in a southwest direction over the ice sheet surface to what was then an initial south-oriented Big Sioux River ice-walled and bedrock-floored valley. That southwest-oriented melt water movement probably eroded southwest-oriented ice-walled and ice-floored valleys into the ice sheet surface. Headward erosion of the large southeast-oriented Minnesota River lowland valley beheaded southwest-oriented flow routes to the early Big Sioux River valley and in the process limited the ability of the Big Sioux River ice-walled and bedrock-floored to ever erode as large or as deep of an ice-walled and bedrock-floored valley as the Minnesota River lowland to the northeast. However, the southwest-oriented ice-walled and ice-floored valleys remained on the detached Prairie Coteau ice sheet remnant surface and channeled melt water flow as the ice sheet remnant melted (and probably explain the northeast-southwest orientation of many figure 4 and 5 Prairie Coteau upland surface streams).

Northeast-facing Prairie Coteau escarpment southwest of Marshall, Minnesota

Figure 6: Northeast-facing Prairie Coteau escarpment southwest of Marshall, Minnesota. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 6 illustrates the northeast-facing Prairie Coteau escarpment southwest of Marshall, Minnesota and south and east of the figure 5 map area (and includes overlap areas with figure 5). Marshall is located in the figure 6 northeast corner. The northeast-oriented stream flowing from the figure 6 south center edge to Marshall is the Redwood River,  which northeast of the figure 6 map area eventually flows to the southeast-oriented Minnesota River. Marshall is located at the southwest edge of the southeast-oriented Minnesota River lowland, which is interpreted here to have been formed by headward erosion of a large southeast-oriented ice-walled and bedrock-floored valley into a rapidly melting thick North American ice sheet. The northeast-facing  escarpment seen in figure 6 is what remains of that ice-walled and bedrock-floored valley’s southwest wall. The Prairie Coteau upland surface located in the figure 6 southwest third is where the ice wall once stood. Note  how today much of the Prairie Coteau upland surface is poorly drained, even though it is located at the crest of the northeast-facing escarpment (elevations on the Prairie Coteau upland surface in figure 6 are approximately 150 meters higher than elevations in Marshall). Poor drainage on the Prairie Coteau upland surface is because ice transported debris in the ice wall was not washed away by the immense southeast-oriented floods responsible for the large southeast-oriented ice-walled and bedrock-floored valley. Instead ice transported debris contained in the ice wall was simply deposited in place as the ice wall melted. This deposition was uneven, with debris sliding into low spots on the ice sheet surface, and then exposing new ice areas to sun light and more rapid melting. Melting of final ice masses left depressions where those ice masses had been, and many of those depressions lack outlets and are lakes and swamps. Note how other than the Redwood River most drainage routes on the northeast-facing escarpment originate at the escarpment crest. The northeast-oriented Redwood River does flow across a significant Prairie Coteau upland surface area and is linked by a through valley to the south and southwest oriented Rock River, which joins the Big Sioux River south of the region illustrated and discussed in this essay. This linkage suggests there may have been a southwest and south oriented ice-walled and ice-floored valley along the present day Redwood River-Rock River alignment before headward erosion of the large southeast-oriented Minnesota River ice-walled and bedrock-floored valley beheaded the southwest and south oriented flow. An “abandoned” southwest and south oriented ice-walled and ice-floored valley could have channeled melt water flow as the ice wall melted. However, the fact the Redwood River has not cut a deeper valley into the Prairie Coteau upland surface argues against any major melt water flood flow moving to the northeast-facing escarpment slope.

Big Sioux River-Minnesota River drainage divide area near Lake Shaokatan

Figure 7: Big Sioux River-Minnesota River drainage divide area near Lake Shaokatan. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 7 illustrates the Big Sioux River-Minnesota River drainage divide area near Lake Shaokaton and is located west of the figure 6 map area and south of the figure 4 map area (and includes overlap areas with figure 4). Lake Hendricks (see figure 4) is located along the figure 7 north edge in the figure 7 northwest quadrant. The north-south South Dakota-Minnesota state border and Brookings County-Lincoln County line extends south from Lake Hendricks. The west and southwest oriented drainage system seen in the figure 7 southwest quadrant is the headwaters of southwest-oriented Deer Creek, which flows to the south-oriented Big Sioux River (see figure 10 below). The “Y” oriented southeast and northeast oriented Lake Hendricks inlet discussed in figure 4 is located west and south of Lake Hendricks. South of the “Y” is a north-south oriented through valley, which is drained at its south end by headwaters of southwest-oriented Deer Creek (figure 9 below provides a detailed map of the through valley). The through valley provides evidence water once flowed between the present day Minnesota River drainage basin and the present day Big Sioux River drainage basin. Lake Shaokatan is located south and east of Lake Hendricks in the figure 7 southeast quadrant and is also linked by a through valley with west and southwest oriented Deer Creek headwaters (a detailed map of that through valley is provided in figure 8 below). The northeast-oriented Yellow Medicine River is the Lake Shaokatan outlet and flows to the figure 7 northeast corner and then to the northeast-facing escarpment slope seen in figure 5. The figure 7 through valleys provide evidence that large volumes of water flowed in multiple channels in a southwest direction across the figure 7 map area. The water came from somewhere, and today the region north and east of figure 7 is approximately 150 meters lower than the figure 7 map area, meaning present day topography did not exist at the time the through valleys were eroded. An interpretation that the water came from melting ice in the figure 7 map area could be made, but such an interpretation would require melt water to have also flowed down the northeast-facing escarpment and to have eroded comparable valleys on that escarpment slope. Such valleys as seen in figures 5 and 6 do not exist. Figures 8 and 9 provide detailed maps of two of the through valleys and discuss possible southwest-oriented water sources.

Through valley west of Lake Shaokatan

Figure 8: Through valley west of Lake Shaokatan. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 8 provides a detailed map of the through valley system at the southwest end of Lake Shaokatan seen in less detail in figure 9 above. One of the most obvious figure 8 features is a large through valley, which extends in southeast direction from the figure 8 northwest corner and then turns to extend in a northeast direction to Lake Shaokatan in the figure 8 northeast corner. Note how the northeast end of the valley is drained by a northeast-oriented stream flowing into Lake Shaokatan, which as seen in figure 7 drains to the northeast-oriented Yellow Medicine River and eventually to the Minnesota River. The north and northwest oriented valley segments are drained by a northwest-oriented stream, which as seen in figure 8 flows to southwest-oriented Deer Creek and eventually to the south oriented Big Sioux River. Today the Big Sioux River-Minnesota River drainage divide crosses this through valley. Yet, the valley is large and deep enough that it provides evidence that significant amounts of water once flowed across what is now a major drainage divide. Which way did the water flow and how did the drainage divide originate? The answers to these questions are not obvious and require visualizing the region at a time when ice still covered the region and before headward erosion of the southeast-oriented Minnesota River ice-walled and bedrock-floored valley beheaded south and southwest oriented melt water flood flow routes across the figure 7 (and 8) map region. At that time an anastomosing complex of ice-walled and ice-floored valleys was probably carved into the ice sheet surface. The figure 8 valleys originated as south-oriented channels in that south-oriented ice-walled and ice-floored anastomosing channel complex. Next headward erosion of the Big Sioux River and its tributary Deer Creek ice-walled and bedrock-floored valleys beheaded south-oriented melt water flood flow in the west half of figure 8. Flow on the beheaded channel reversed flow direction to flow northwest to the newly eroded and deeper Deer Creek valley and in doing so captured southwest-oriented coming from northeast of Lake Shaokatan. Headward erosion of the large and deep southeast-oriented Minnesota River ice-walled and bedrock-floored valley next beheaded the southwest-oriented melt water flood flow, which caused a reversal of flow direction on the northeast end of the beheaded flow route to create the northeast-oriented Yellow Medicine River route. By this time all valleys were bedrock-floored and the final flow reversal created the present day drainage divide.

Through valley south of Lake Hendricks

Figure 9: Through valley south of Lake Hendricks. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 9 provides a detailed a map of the through valley located south and east of Lake Hendricks and seen in less detail in figures 4 and 7 above. As previously discussed in figure 4 the stream in the southeast-oriented valley in the figure 9 northwest quadrant makes a U-turn upon entering the north-south through valley and flows north into Lake Hendricks. The Lake Hendricks outlet as shown in figures 4 and 5 is the northeast-oriented Lac Qui Parle River, which eventually reaches the southeast-oriented Minnesota River. The south end of the through valley seen in figure 9 is drained by south and southeast oriented Deer Creek, which eventually drains to the south-oriented Big Sioux River. Today the Big Sioux River-Minnesota River drainage divide crosses the north-south through valley, yet the through valley is evidence large quantities of water once flowed across the present day drainage divide. Which way did the water flow and how did the present day drainage divide develop? Again, to answer the questions it is necessary to visualize the region when ice still covered the figure 7 and 9 map areas and before the ice-walled and bedrock-floored Minnesota River and Big Sioux River valleys eroded headward into the region. At that time immense supra glacial melt water floods carved anastomosing complexes of south-oriented channels into the decaying ice sheet surface and many of the figure 9 larger valleys probably had their origins then. Headward erosion of the Big Sioux River and Deer Creek ice-walled and bedrock-floored valleys next captured south-oriented flow, including flow along the present day southeast-oriented valley (in the figure 9 northwest quadrant) and flow moving in a southwest direction from northeast of Lake Hendricks. Headward erosion of the deep and large southeast-oriented Minnesota River ice-walled and bedrock-floored valley next beheaded the southwest-oriented flood flow channel through the Lake Hendricks area. Flood waters on the northeast end of that beheaded flood flow route reversed flow direction to flow in a northeast direction and to create the present day Lac Qui Parle River route. The reversed flood flow also captured southeast-oriented flood flow from the valley in the figure 9 northwest quadrant and in the process created the present day drainage divide.

Area northeast of Big Sioux River near Brookings, South Dakota

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

Figure 10 illustrates the Deer Creek and Big Sioux River valleys near Brookings, South Dakota and south and west of the figure 7 map area and includes overlap areas with figure 7. The Big Sioux River is located in the figure 10 southwest corner and is flowing in a south-southeast direction. Deer Creek flows south in the figure 10 northeast corner and then turns to flow southwest and joins the Big Sioux River south of Brookings and also south of the figure 10 map area. Unlike the Prairie Coteau areas further to the east this figure 10 map area does not have any obvious lakes and swamps and appears to be well-drained. Note how much of the figure 10 map area is drained by southwest-oriented Big Sioux River tributaries. As seen in figures above these southwest-oriented tributary valleys received significant melt water flood flow from areas north and east of the present day Big Sioux River-Minnesota River drainage divide. That flood water was received at a time when a decaying ice sheet covered much of the region and before headward erosion of the southeast-oriented Minnesota River ice-walled and bedrock-floored valley beheaded and reversed the southwest-oriented flood flow. This area was probably eroded by south- and southwest-oriented melt water floods when the Big Sioux River ice-walled and bedrock-floored valley eroded headward into the region and the Big Sioux River tributary ice-walled and and bedrock-floored valleys eroded headward into the adjacent regions. Flood water probably hastened ice sheet melting in this figure 10 map area and also removed much of the ice sheet contained debris from this region (at least compared with the region further east). The northwest-oriented stream in the figure 10 southeast corner is Medary Creek, which turns to flow in a southwest direction to join the Big Sioux River south of the figure 10 map area. The northwest-oriented Medary Creek valley segment probably originated as a southeast-oriented melt water flood flow channel that was beheaded when headward erosion of the south-oriented Big Sioux River and its southwest-oriented tributary Medary Creek ice-walled and bedrock-floored valley beheaded the southeast-oriented flood flow route.

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.