A geomorphic history based on topographic evidence
The Big Sioux River-Rock River drainage divide area is located in southeastern South Dakota, the Minnesota southwest corner, and the Iowa northwest corner. Rock River is a south and south-southwest oriented tributary to the south-oriented Big Sioux River. Topographic map evidence suggests the Rock River valley eroded headward first to capture south- and southeast-oriented flood flow. Headward erosion of Big Sioux River tributary valleys and the Big Sioux River valley next captured the southeast-oriented flood flow, while headward erosion of the Minnesota River-Redwood River valley to the east and north captured southwest-oriented flood flow to the newly eroded Rock River valley.
- The purpose of this essay is to use topographic map interpretation methods to explore Big Sioux River-Rock River drainage divide area landform origins in South Dakota, Minnesota, and Iowa, 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-Rock River drainage divide area landform evidence in South Dakota, Minnesota and Iowa 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-Rock River drainage divide area location map
Figure 1: Big Sioux River-Rock River drainage divide area location map. National Geographic Society map digitally presented using National Geographic Society TOPO software.
Figure 1 provides a location map for the Big Sioux River-Rock River drainage divide area. The western half of figure 1 is located in southeast South Dakota. The figure 1 northeast quadrant is located in Minnesota and the southeast quadrant is located in Iowa. The Big Sioux River flows south from the figure 1 edge to Brookings, Flandreau, Dell Rapids, and Sioux Falls, South Dakota before turning east to flow to Brandon, South Dakota. From Brandon the Big Sioux River flows south along the South Dakota-Iowa border to the towns of Canton and Hudson, South Dakota and then to the figure 1 south edge. South of the figure 1 map area the Big Sioux River joins the southeast-oriented Missouri River. Rock River is not labeled in figure 1, but is the south and southwest oriented tributary that joins the Big Sioux River near Hudson, South Dakota. The Rock River originates in southwest Minnesota near Holland (which is located northeast from Pipestone) and flows south to Luverne, Minnesota and then to Rock Rapids, Doon, and Rock Valley, Iowa before joining the Big Sioux River near Hudson. Other rivers important to understanding the Big Sioux River-Rock River drainage divide origin include the James River and the Minnesota River and its tributary, the Redwood River. The James River is located in the figure 1 southwest quadrant and flows in a southeast direction from the figure 1 west center edge to Yankton, South Dakota and just south of the figure 1 map area joins the Missouri River. The Minnesota River is the southeast-oriented river located in the figure 1 northeast corner. Figure 1 does not show topography, but if it did it would show the south oriented Big Sioux River and Rock River drainage basins are located on an escarpment-bounded upland surface. That upland surface is the Prairie Coteau upland and it is bounded in the northeast by a northeast-facing escarpment immediately southwest of Marshall, Minnesota. The escarpment is the southwest wall of a southeast-oriented lowland region, now drained by the southeast-oriented Minnesota River, which is interpreted here to have originated as an immense southeast-oriented ice-walled and bedrock-floored valley sliced into the surface of a rapidly melting thick ice sheet. The Prairie Coteau is bounded to the west by a west-facing escarpment, which is located east of the James River. The west-facing escarpment is interpreted here to have originated as the east wall of an immense south oriented ice-walled and bedrock-floored valley sliced into the same rapidly melting ice sheet. Today the south and southeast oriented James River flows on the floor of that former south-oriented ice-walled and bedrock-floored valley.
Big Sioux River-Rock River drainage divide area detailed location map
Figure 2: Big Sioux River-Rock 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-Rock River drainage divide area. Lake Moody, Minnehaha, Turner, and Lincoln are South Dakota county names. Pipestone, Murray, Rock, and Nobles are Minnesota county names. Lyon and Osceola are Iowa County names. The Big Sioux River flows south in Moody County to Sioux Falls in Minnehaha County and then east before flowing south along the Lincoln-Lyon County border to the figure 2 south edge. The Rock River originates near Holland (in northeast Pipestone County) and flows south through eastern Rock County into Lyon County and then turns to flow in a southwest direction and joins the Big Sioux River just south of the figure 2 south edge. Topographic maps illustrated and discussed below begin just south of the figure 2 map area and show where the Rock River flows to the Big Sioux River and then proceed north along the Big Sioux River-Rock River drainage divide. Mud Creek is an important Rock River tributary and Beaver Creek and Split Rock Creek are important Big Sioux River tributaries illustrated and discussed below. The Redwood River originates north of Holland, and while not shown in figure 2, is a northeast-oriented Minnesota River tributary important to this discussion. Note areas in figure 2 west of the Big Sioux River where there are numerous lakes. The numerous lakes suggest the region is underlain by thick glacial moraines. As described in the figure 1 discussion the Big Sioux River and Rock River flow south on the escarpment bounded Prairie Coteau upland surface. The Prairie Coteau upland surface is interpreted here to be where a decaying thick ice sheet remnant was located after headward erosion of immense south and southeast oriented ice-walled and bedrock-floored valleys on either side detached and isolated that ice sheet remnant. Melt water floods picked up and transported much of the ice sheet transported and contained debris located in the adjacent James River and Minnesota River lowland regions, but ice sheet transported and contained debris was not washed away from much of the Prairie Coteau upland surface region. Instead, the ice sheet transported and contained debris simply settled and was deposited in place as the ice sheet slowly melted. The exception to this was located in the Big Sioux River-Rock River drainage basin areas, where much smaller and shallower ice-walled and bedrock-floored valleys eroded headward into the Prairie Coteau ice sheet remnant. Much of the melt water flood flow responsible for erosion of these valleys came from ice sheet surface areas beyond the present day Prairie Coteau region, suggesting headward erosion of the deeper and larger James River lowland and Minnesota River lowland ice-walled and bedrock-floored valleys captured and beheaded flood flow routes to the developing Big Sioux and Rock River valleys.
Big Sioux-Rock River drainage divide area north of Hudson, South Dakota
Figure 3: Big Sioux River-Rock River drainage divide area north of Hudson, South Dakota. United States Geological Survey map digitally presented using National Geographic Society TOPO software.
Figure 3 illustrates the Big Sioux River-Rock River drainage divide area north of Hudson, South Dakota. Hudson is located near the figure 3 south center edge. The Big Sioux River flows in a southeast and south-southeast direction from the figure 3 northwest corner to Hudson and then continues south of the figure 3 map area to join the southeast-oriented Missouri River. Rock Valley, Iowa is the town located near the figure 3 east center edge. The Rock River flows in a southwest direction from near Rock Valley to join the Big Sioux River just south of the figure 3 map area. Big Sioux and Rock River tributaries in figure 3 provide important information about the history of the present day river valleys. First is Pattee Creek, which flows in a south-southeast direction from Lake Lakota (near the figure 3 west edge) to the figure 3 south boundary and which joins the Big Sioux River south of figure 3. The size of the Pattee Creek valley suggests significant water flowed along that route, which is adjacent to and parallel to the much large Big Sioux River valley. Also, just north of Lake Lakota are shallow through valleys notched into the ridge separating the Pattee Creek valley from the Big Sioux River valley. Those high level through valleys provide evidence that water once flowed south from the present day Big Sioux River drainage basin (upstream from the Lake Lakota area) to the Pattee Creek valley. In addition, note how Pattee Creek headwaters upstream from Lake Lakota are northeast-oriented (west of figure 3 the headwaters are southeast-oriented before turning to flow northeast). This evidence suggests the Big Sioux River and Pattee Creek valleys were initiated as channels in a southeast or south-southeast oriented anastomosing channel complex, which suggests they were eroded during a large south-southeast oriented flood flow event. A look at Rock River tributaries supports this interpretation. North of the Rock River is a south-southeast oriented tributary with a south-southeast oriented tributary (in the figure 3 northeast quadrant). In the figure 3 southeast quadrant is a northwest-oriented tributary. The northwest-oriented tributary valley was probably eroded by a reversal of southeast-oriented flood flow on the northwest end of a beheaded flood flow channel (beheaded by headward erosion of the deep Rock River valley).
Big Sioux River-Rock River drainage divide area east of Canton, South Dakota
Figure 4: Big Sioux River-Rock River drainage divide area east of Canton, South Dakota. United States Geological Survey map digitally presented using National Geographic Society TOPO software.
Figure 4 illustrates the Big Sioux River-Rock River drainage divide area north of the figure 3 map area and includes overlap areas with figure 3. Canton, South Dakota is the town located in the figure 4 southwest quadrant. The south-southwest oriented Rock River is located in the figure 4 southeast corner. The south-southeast oriented Rock River tributary flowing through Alvord, Iowa in the figure 4 east half is Mud Creek (and will be seen again in figure 5 below). The Big Sioux River flows in a south-oriented direction in the figure 4 west half. The region immediately east of the Big Sioux River is generally 30-40 meters higher than regions west of the Big Sioux River. Southeast-oriented Big Sioux River tributaries and the northwest-southeast oriented erosional residual (located northeast of Canton) provide evidence the area west of the Big Sioux River was eroded by southeast-oriented flood water moving to what was probably at that time the newly eroded Big Sioux River valley. Inwood is the Iowa town located east of Canton (next to the word Richland) and is located on a relatively low relief upland erosion surface. That upland erosion surface has been eroded by headward erosion of south-southeast oriented Rock River tributaries, suggesting the erosion surface was formed by southeast and south-southeast oriented flood water prior to headward erosion of the Big Sioux River valley. In other words, the figure 4 evidence provides a sequence of drainage history events. First, southeast and/or south-southeast oriented flood water flowed across the entire figure 4 map area. Headward erosion of the deep southwest-oriented Rock River valley next captured the south-southeast oriented flood flow and diverted the water to the southwest and probably to what was then the actively eroding deep Big Sioux River valley head. Headward erosion of the deep Big Sioux River valley next captured the south-southeast oriented flood water and diverted the water south and in the process beheaded flood flow to Rock River tributaries. Southeast-oriented flood water continued to flow into the Big Sioux River valley from the northwest, but no longer flowed across the upland surface east of the Big Sioux River valley. The continued flood flow lowered the surface west of the Big Sioux River, while the erosion surface east of the Big Sioux River remained untouched.
Beaver Creek-Mud Creek drainage divide southwest of Luverne, Minnesota
Figure 5: Beaver Creek-Mud Creek drainage divide southwest of Luverne, Minnesota. United States Geological Survey map digitally presented using National Geographic Society TOPO software.
Figure 5 illustrates the Beaver Creek-Mud Creek drainage divide area north of the figure 4 map area and there is a gap between figures 4 and 5. Luverne, Minnesota is the town located in the figure 5 northeast corner. The Rock River flows south along the figure 5 east edge from Luverne to the figure 5 south edge. Beaver Creek is the southwest-oriented stream east of Luverne, which flows through the small towns of Beaver Creek, Manley, and Valley Springs. South of Beaver Creek, in the figure 5 south center area is the town of Hills. Just east of Hills is south oriented Mud Creek, which was seen in figure 4 and which is a Rock River tributary. Note how Beaver Creek tributaries from the north are southwest-oriented and evidence of the south-southeast oriented flood flow is not present. Instead Beaver Creek appears to mark the southern boundary of a region where southwest-oriented flood flow beheaded south-oriented flood flow to the Mud Creek valley and other south-oriented flood flow routes to the south-oriented Rock River tributaries. The drainage history that can interpreted from the figure 5 map evidence begins with south-oriented flood flow across the figure 5 map area. Headward erosion of the Rock River and Mud Creek valleys captured much of this flood flow and channeled it into the newly eroded and evolving Rock River valley system. Headward erosion of the deep Big Sioux River valley to the east of the figure 5 map area next began to capture the south-oriented flood flow. The southwest-oriented Beaver Creek valley eroded to the northeast to behead south-oriented flood flow to Rock River tributaries and to divert the flood waters to the deeper Big Sioux River valley. A close look at the Beaver Creek-Mud Creek drainage divide reveals shallow through valleys, which provide evidence of south- and southeast-oriented anastomosing channels that were beheaded by headward erosion of the deep Beaver Creek valley. Figure 6 below provides a detailed map of these shallow through valleys in the area where the railroad between Manley and Hills crosses the Beaver Creek-Mud Creek drainage divide.
Detailed map of Beaver Creek-Mud Creek drainage divide area
Figure 6: Detailed map of Beaver Creek-Mud Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.
Figure 6 provides a detailed map of the Beaver Creek-Mud Creek drainage divide area seen in less detail in figure 5 above. Beaver Creek flows in a southwest direction across the figure 6 northwest corner and continues in a southwest direction to join the south-oriented Big Sioux River. Note northwest-oriented tributaries to the southwest-oriented Beaver Creek. Mud Creek is located along the figure 6 east edge (south two-thirds) and has several southeast-oriented tributaries. South of the figure 6 map area Mud Creek flows in a south and south-southeast oriented direction to eventually join the Rock River (see figure 4). The northeast-southwest oriented Beaver Creek-Mud Creek drainage divide extends from the figure 6 northeast corner to the figure 6 south center area. A close look at that drainage divide reveals a number of shallow notches or saddles in what appears to be a high level ridge. The Great Northern Railroad has taken advantage of one of those notches where it crosses the drainage divide in the figure 6 south center area. Many of the notches or saddles provide links between heads of northwest-oriented Beaver Creek tributaries and southeast-oriented Mud Creek tributaries. The notches or saddles are evidence of a southeast-oriented anastomosing channel complex that once moved a large southeast-oriented flood across the present day drainage divide. Initially flood waters moved in a southeast direction across the entire figure 6 map area and probably were moving to what was then the newly eroded south-oriented Rock River valley located east of the figure 6 map area (see figure 5). Headward erosion of the deep Mud Creek valley next captured the southeast-oriented flood water and diverted the water more directly to the south. Southeast-oriented Mud Creek tributary valleys were eroded by southeast-oriented flood flow moving into the newly eroded Mud Creek valley. Next headward erosion of the southwest-oriented Beaver Creek captured the southeast-oriented flood flow. Flood waters on the northwest ends of the beheaded flood flow channels reversed flow direction and flowed northwest to the newly eroded and deeper Beaver Creek valley. Because the Beaver Creek valley eroded headward it beheaded flood flow routes one channel at a time. And, because channels were anastomosing (or interconnected), reversed flood flow in one channel could capture southeast-oriented flood flow from adjacent channels. Capture of this yet to be beheaded flood flow provided the water volumes needed to erode the northwest-oriented valleys.
Beaver Creek-Rock River drainage divide north of Luverne, Minnesota
Figure 7: Beaver Creek-Rock River drainage divide north of Luverne, Minnesota. United States Geological Survey map digitally presented using National Geographic Society TOPO software.
Figure 7 illustrates the Beaver Creek-Rock Creek drainage divide area north of Luverne, Minnesota and includes overlap areas with figure 5. Luverne is the town located along the figure 7 south center edge. North of Luverne is Blue Mound State Park, and the Blue Mound State Park area is shown in greater detail in figure 8 below. The Rock River flows south in the area east of the north-south red highway through Battle Plain Township and east of Blue Mound State Park to Luverne and to the figure 7 south edge. Campepadan Creek is the southwest-oriented Pock River tributary located in Vienna Township. Elk Creek is the southwest-oriented Rock River tributary in the figure 7 southeast corner. Beaver Creek flows south in the region west of the north-south red highway and has its headwaters in the Denver Township area. Note in figure 7 that Beaver Creek is much more of a south-oriented stream than it is in the figure 5 map area (where Beaver Creek is southwest-oriented). Beaver Creek in the figure 7 map area has several southeast-oriented tributaries providing evidence the Beaver Creek valley eroded headward to capture southeast-oriented flood flow. Rock River also has southeast-oriented tributaries, although some of these are south-oriented in the figure 7 north half. One of the most interesting Rock River tributaries is Mound Creek, which originates in Denver Township near the Beaver Creek headwaters and then flows south to the Blue Mound State Park area. At Blue Mound State Park Mound Creek turns to flow in an east-direction through a water gap cut across the north end of the Mound (from which Blue Mound State Park gets its name) and then joins the south-oriented Rock River. The Mound stands 60-70 meters higher than the surrounding region and quarries located along the Mound’s south side (see figure 8) suggest the Mound is composed of some type of resistant bedrock. The water gap suggests the Mound Creek route was established at a time when the Mound was buried in less erosion resistant material, which has since been removed with Mound Creek maintaining its previously established route and simply eroding its valley across the more resistant bedrock material. If so, flood waters may have removed a significant thickness of material from the figure 7 surface, although it possible the region was ice-covered.
Detailed map of Beaver Creek-Rock River drainage divide area near Blue Mound State Park, Minnesota
Figure 8: Detailed map of Beaver Creek-Rock River drainage divide near Blue Mound State Park, Minnesota. United States Geological Survey map digitally presented using National Geographic Society TOPO software.
Figure 8 provides a detailed map of the Beaver Creek-Rock River drainage divide area near Blue Mound State Park seen in less detail in the figure 7 map area above. The south-oriented Rock River meanders back and forth along the figure 8 east edge. Mound Creek flows in a south-southeast oriented direction from the figure 8 north edge to Blue Mound State Park and turns to flow east through the water gap to the Rock River. Beaver Creek flows in a south-southwest direction in the figure 8 northwest corner and Little Beaver Creek is the south-southwest oriented tributary immediately to the southeast. Figure 8 reveals numerous through valleys across present day drainage divides. South of the The Mound is a southeast-oriented Rock River tributary, which is linked by a through valley with headwaters of a northwest-oriented tributary to a southwest-oriented Beaver Creek tributary. A northeast-southwest oriented through valley in Mound Township (in the figure 8 center) provides evidence water once flowed south from the Mound Creek headwaters area to a southwest-oriented Beaver Creek tributary valley. Through valleys further to the northwest provide evidence that headward erosion of the Little Beaver Creek and Beaver Creek valleys beheaded east and southeast oriented flood flow to the Mound Creek valley. While reconstruction of figure 8 flood flow directions and drainage history is more complex than the figure 5 reconstructions, the evidence of through valleys across the drainage divides is present. Headward erosion of what was then probably a deep and the south-oriented Rock River valley first captured the flood waters. Flood waters moving to this newly eroded valley began to erode southeast-oriented tributary valleys around the resistant rock masses encountered as the regional topography was lowered. Headward erosion of southwest-oriented Beaver Creek tributary valleys and then the Beaver Creek valley next captured the flood flow, although headward erosion of the Mound Creek valley then captured and beheaded a southwest-oriented flood flow channel to the Beaver Creek valley. Headward erosion of the Mound Creek valley may have been delayed by having to erode the water gap, but it occurred while anastomosing flood flow channels still were moving south-oriented flood waters across the figure 8 map area.
Split Rock Creek-Rock River drainage divide area near Pipestone, Minnesota
Figure 9: Split Rock Creek-Rock River drainage divide area near Pipestone, Minnesota. United States Geological Survey map digitally presented using National Geographic Society TOPO software.
Figure 9 illustrates the Split Rock Creek-Rock River drainage divide area south of Pipestone, Minnesota and is north of the figure 7 map area (and there is a gap between figure 7 and figure 9). Split Rock Creek flows in a south-southwest direction from near Pipestone to Ihlen and Split Rock Creek State Park and then to the figure 9 southwest corner. Rock River flows generally south in the eastern quarter of figure 9 and flows just west of Edgeerton in the figure 9 southeast corner. An unnamed south-southeast oriented Rock River tributary flows from near Hatfield to join the Rock River in the figure 9 southeast quadrant. A south-southeast oriented tributary flows from Trosky in the figure 9 south center to east and southeast oriented Poplar Creek along the figure 9 south edge, which then flows to the south-oriented Rock River. Note how Rock River tributaries are frequently southeast or south-southeast oriented and/or have at least south-southeast oriented valley segments. Also note how headwaters of Split Rock Creek tributaries frequently are northwest oriented, even if the tributaries turn to flow in a southwest direction before joining south-southwest oriented Spit Rock Creek. A close look at the Split Rock Creek-Rock River drainage divide again reveals shallow through valleys providing evidence of a former south-southeast-oriented anastomosing channel complex that moved large quantities of flood water cross the present day drainage divide before headward erosion of the south-southwest-oriented Split Rock Creek valley. Valley systems in figure 9 were eroded in sequence beginning with the Rock River valley in the east. The south-southeast oriented tributary valley near Hatfield was eroded next and probably the Poplar Creek valley was eroded about the same time. Headward erosion of the Split Rock Creek valley was next. It is possible and in fact probable that all valleys were eroded at approximately the same time, with the eastern valleys eroding headward slightly in advance of the western valleys. Flood waters were from melting ice and it is possible the Big Sioux River-Rock River drainage divide region was ice-covered at the time headward erosion of the various valleys began. However, unlike many other regions in the Big Sioux River drainage basin topographic maps of the Big Sioux River-Rock River drainage divide area so far have not provided evidence that decaying ice sheet remnants were still present when the valleys eroded headward.
Redwood River-Rock River through valley near Holland, Minnesota
Figure 10: Redwood River-Rock River through valley near Holland, Minnesota. United States Geological Survey map digitally presented using National Geographic Society TOPO software.
Figure 10 illustrates the Redwood River-Rock River drainage divide area and through valley near Holland, Minnesota. Ruthton, Minnesota is the town located where the red highway and railroad cross the figure 10 north edge. Holland is located on the red highway and railroad in the northeast corner of the figure 10 southwest quadrant. The Rock River originates in the northeast-southwest-oriented through valley located to the northeast of Holland and flows south from Holland to the figure 10 south edge. The through valley is used by both the railroad and the highway and is drained at its southwest end by the Rock River. Note how the northeast end of that through valley is drained by a northeast-oriented tributary to the southeast and northeast oriented “River” in the figure 10 northeast corner. That “River” is the Redwood River, which from the figure 10 map area turns to flow north and northeast to Marshall, Minnesota (see figure 1) and then east to join the southeast-oriented Minnesota River. Headward erosion of the Redwood River valley appears to have beheaded south-oriented flow to the Rock River valley (and also reversed flow on the northeast end of the beheaded flow route). Unlike previous maps in this essay the east half of the figure 10 map area includes evidence that ice sheet remnants may have been present as the melt water floods eroded the Rock River valley. Lakes and swamps and the lack of a good drainage suggest ice sheet remnants may have melted and deposited debris in those regions, which were not eroded by melt water floods. If so, it is possible and even probable that melt water floods played a significant role in hastening the melting of ice sheet remnants in the Big Sioux River-Rock River drainage divide area and in also eroding ice sheet deposits. Figure 10 evidence also suggests Rock River once received significant melt water flow from the area north and east of he figure 10 map area. While figure 10 evidence is not sufficient to determine where that melt water flow came from. it is possible some of the melt water flow came from ice sheet areas north and east of the present day northeast-facing Prairie Coteau upland escarpment (see Big Sioux River-Minnesota River drainage divide area in Brookings County, South Dakota and Lincoln and Lyon Counties, Minnesota essay found under Big Sioux River on sidebar category list). If so, it may be possible to develop a relationship between the time the northeast-facing Prairie Coteau was eroded and the time the Big Sioux River valley was eroded (and the time of other Prairie Coteau landscape formation events occurred).
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.