A geomorphic history based on topographic evidence

The Rock River-Des Moines River drainage divide area is located in southwest Minnesota, USA. The Rock River flows south to the south-oriented Big Sioux River. The Des Moines River flows in a southeast direction along the crest of the northeast-facing Prairie Coteau escarpment and eventually joins the Mississippi River. The northeast-facing escarpment is interpreted here to be what remains of an immense southeast-oriented ice-walled and bedrock-floored valley sliced by melt water floods into the surface of a thick North American ice sheet. The Prairie Coteau upland surface is interpreted here to be where the southeast ice wall once stood. The Rock River, Des Moines River, and other major Rock-River-Des Moines River drainage divide area drainage routes are interpreted to have been initiated as smaller and shallower ice-walled and bedrock-floored valleys were sliced into the decaying ice sheet surface.

Preface:

• The purpose of this essay is to use topographic map interpretation methods to explore Rock River-Des Moines River drainage divide area landform origins in southwest 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 Rock River-Des Moines River drainage divide area landform evidence in southwest 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.

Rock River-Des Moines River drainage divide area location map

Figure 1: Rock River-Des Moines River drainage divide area location map.

• Figure 1 provides a location map for the Rock River-Des Moines River drainage divide area. The state in the western quarter of figure 1 is South Dakota. East of South Dakota in the north is Minnesota and south of Minnesota is the state of Iowa. The Des Moines River flows through Currie, Windom, and Jackson, Minnesota and then in a south-southeast direction to Estherville and Emmetsburg, Iowa before reaching the figure 1 south edge (near southeast corner). The Des Moines River continues to flow south-southeast and southeast and eventually joins the Mississippi River. The Big Sioux River flows south in eastern South Dakota from north of Brookings to Sioux Falls and then east to Brandon, South Dakota. From Brandon the Big Sioux River flows south along the South Dakota-Iowa border to Canton and Hudson, South Dakota, before reaching the figure 1 south edge. South of figure 1 the Big Sioux River joins the southeast-oriented Missouri River, which eventually flows to the Mississippi River. The Rock River is not named in figure 1, but is the south- and southwest-oriented tributary joining the Big Sioux River near Hudson, South Dakota. The Rock River originates near Holland, Minnesota and flows south to Luverne, Minnesota and then to Rock Rapids, Doon, and Rock Valley, Iowa before joining the Big Sioux River. Also important to this essay is the Redwood River, which originates near Ruthton, Minnesota (northeast from Holland) and flows southeast and then north and northeast to Marshall, Minnesota. From Marshall the Redwood River flows in an east direction to join the southeast-oriented Minnesota River, which flows to New Ulm and Mankato in the figure 1 northeast corner. Figure 1 does not show topography, but if it did it would show the southeast-oriented Des Moines River headwaters flow along the crest of a northeast-facing escarpment. The escarpment provides the northeast boundary of the Prairie Coteau upland surface, which is drained by the Big Sioux and Rock Rivers (and Des Moines River headwaters). The escarpment is interpreted here to be what remains of the southwest wall of what was once 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 where what became a detached ice sheet remnant melted and deposited whatever ice-transported debris it contained.

Rock River-Des Moines River drainage divide area detailed location map

• Figure 2 provides a slightly more detailed map of the Rock-River-Des Moines River drainage divide area. Lincoln, Lyon, Redwood, Brown, Pipestone, Murray, Cottonwood, Rock, Nobles, and Jackson are Minnesota county names. The west to east oriented Minnesota-Iowa state line is located at the southern boundaries of Rock, Nobles, and Jackson Counties. The north-south South Dakota-Minnesota state line is located west of Lincoln, Pipestone, and Rock Counties. The Rock River originates near Holland in northeast Pipestone County and flows south in eastern Pipestone and Rock Counties. The Des Moines River originates in south central Lyon County and flows in a southeast direction to the Murray County southeast corner before making a jog to the northeast in Cottonwood County. In Cottonwood County the Des Moines River again makes an abrupt turn to flow in a south-southeast direction to Windom and then through Jackson County. The southeast-oriented Minnesota River is located the figure 2 northeast corner. The Redwood River originates near Ruthton in the Pipestone County northeast corner and flows in a southeast direction into the Murray County northwest corner before turning to flow in a northeast direction to Marshall, Minnesota in Lyon County. From Marshall the Redwood River flows north of the figure 2 map area to eventually reach the southeast-oriented Minnesota River. Another river of importance is the Little Sioux River, which originates in southern Jackson County. The Little Sioux River flows south from Jackson County into Iowa and eventually turns to flow in southwest direction to reach the Missouri River. In other words the Rock River-Des Moines River drainage divide area is located almost entirely Murray County and northern Nobles County, with a small area in Pipestone County. To the north is the Big Sioux River-Minnesota River drainage divide area in Brookings County, South Dakota Lincoln, and Lyon Counties, Minnesota and to the south is the Rock River-Little Sioux River drainage divide area (essays describing these drainage divide areas can be found under Big Sioux River on the sidebar category list). This essay begins with the Redwood River-Rock River-Beaver Creek drainage divide area near the Lincoln, Lyon, Pipestone, and Murray County corner. Next the essay illustrates the Redwood River valley northeast of the drainage divide to show how the drainage divide is related to the northeast-facing Prairie Coteau escarpment. The essay then follows the escarpment in a southeast direction and concludes by showing the escarpment’s relationship to the Des Moines River headwaters area.

Redwood River-Rock River-Beaver Creek drainage divide area

Figure 3: Redwood River-Rock River-Beaver Creek drainage divide area.

• Figure 3 illustrates the Redwood River-Rock River-Beaver Creek drainage divide area in northeast Pipestone County and northwest Murray County. The Rock River is located near the figure 3 west edge in the southern half of figure 3. Holland, Minnesota is the small town near the Rock River headwaters (located along the west edge). The East Branch of the Rock River is located further east in the figure 3 southwest quadrant and flows south and southwest to join the Rock River south of the figure 3 map area. The Rock River flows south and southwest from the figure 3 map area to eventually join the south-oriented Big Sioux River (see Big Sioux River-Rock River drainage divide area essay). The red highway and railroad extend in a northeast and north direction from Holland via a through valley (see figure 4 below) to Ruthton, which is located near the figure 3 north edge. The northeast-oriented stream at Ruthton is the Redwood River, which north of figure 3 turns to flow southeast back into the figure 3 map area and in the figure 3 north center flows southeast and then north to the figure 3 north edge. From the figure 3 map area the Redwood River flows in a northeast direction to Marshall, Minnesota (see figure 5 below) and eventually joins the southeast-oriented Minnesota River, which eventually joins the Mississippi River. The third major drainage system seen in figure 3 is Beaver Creek, located in west half of the figure 3 southeast quadrant. Beaver is a Des Moines River tributary and the Des Moines River flows in a southeast direction to join the Mississippi River. The figure 3 map area is located on the Prairie Coteau upland surface and note the hummocky topography and small lakes and other evidence of poor drainage. Figure 3 landscape features are typical of regions covered by glacial deposits and figure 3 drainage routes originated as melt water flow routes. However, links between the different drainage systems suggests the drainage routes were initiated at a time when ice was still present and were perhaps initiated as ice-walled and ice-floored valleys on the ice sheet surface. If so melt water was flowing south from north of the figure 3 map area to what must have been south-oriented ice-walled and ice-floored (or bedrock-floored) valleys crossing the figure 3 map area and extending south to the ice sheet margin. As the ice sheet melted, headward erosion of the large southeast-oriented Minnesota River lowland ice-walled and bedrock-floored valley beheaded and reversed melt water flow on the north end of what is now the Redwood River-Rock River and Redwood River-Beaver Creek valley routes. The reversal of flow created the north and northeast oriented Redwood River valley and created the Redwood River-Rock River and Beaver Creek drainage divide.

Detailed map of Redwood River-Rock River drainage divide

Figure 4: Detailed map of Redwood River-Rock River drainage divide.

• Figure 5 illustrates a detailed map of the Redwood River-Rock River drainage divide area seen in less detail in figure 3 above. Holland, Minnesota is located in the figure 4 southwest corner. Note the through valley extending northeast from Holland, in which the highway and railroad are located. The southwest end of that through valley is drained by Rock River headwaters, which eventually flow to the Big Sioux River and Missouri River. The through valley’s northeast end is drained by a Redwood River tributary, which eventually flows to the Minnesota River. The Redwood River-Rock River drainage divide crosses the valley floor. Existence of the through valley is evidence that water once flowed from the present day Redwood River drainage basin to the present day Rock River drainage basin. The figure 4 map area shows evidence of being covered by glacially deposited materials, which suggests glacial melt water eroded the region. However, the through valley provides evidence at least some of the melt water came from north and northeast of the figure 4 map area. Figure 5 below illustrates the northeast-facing escarpment that is located north and northeast of the figures 3 and 4 map areas today. Southwest-oriented flow in the through valley had to occur before the north-east-oriented Redwood River originated. Instead flow in at least the southwest end of the Redwood River valley had to be in a southwest direction. The logical time for a reversal of flow in the Redwood River valley (from being southwest-oriented to being northeast-oriented) would have been when the northeast-oriented escarpment was formed. So the question becomes, how did the northeast-oriented escarpment in figure 5 originate? What can be learned from the figure 4 map area before looking at the escarpment in figure 5. Lakes in the figure 4 southeast quadrant are probably kettle lakes, formed when buried ice masses melted and left depressions, which have now filled with water. The ice masses would have been buried by ice sheet transported debris that was deposited as the ice sheet melted. Presence of the kettle lakes suggests the figure 4 map area was once covered by an ice sheet, which melted and deposited debris where the ice melted.

Redwood River northeast of Florence, Minnesota

Figure 5: Redwood River northeast of Florence, Minnesota.

• Figure 5 illustrates the Redwood River valley north and slightly east of the figure 3 map area and there is a small gap between figure 3 and figure 5. Florence is the town located in the figure 5 southwest corner area. The Redwood River is the northeast oriented stream flowing from just east of Florence to the figure 5 north edge. Note the northeast-facing escarpment in the figure 5 northeast half. The escarpment crest extends from the figure 5 southeast corner area to the figure 5 northwest corner area.The area southwest of the escarpment crest is similar to the area seen in figure 3 and is characterized by hummocky topography and many small lakes. This region is the Prairie Coteau upland surface and shows evidence of being covered with glacially deposited materials, which were deposited by an ice sheet melting in place. The Cottonwood River is the northeast, southeast, and northeast oriented stream in the figure 5 southeast quadrant and will be discussed in the figure 7 discussion below. In the figure 5 northeast corner is a small section of the lowland at the escarpment base. Lake Marshall is approximately 150 meters lower in elevation than lakes located on the Prairie Coteau upland surface. Note how the escarpment slope is well-drained, and much of the Prairie Coteau upland surface appears to be poorly drained and to be a region of many small closed depressions. Also note that northeast-oriented drainage routes have not eroded deep valleys into the escarpment slope or into the Prairie Coteau upland surface. The lack of deep valleys is evidence the northeast-oriented drainage routes developed after the escarpment slope had been formed. The northeast-facing escarpment is interpreted here as being what remains of the southwest wall an immense southeast-oriented ice-walled and bedrock-floored valley that was sliced by melt water floods into the surface of a rapidly melting thick ice sheet. Prior to headward erosion of the large ice-walled and bedrock-floored valley there may have been smaller southwest-oriented ice-walled and ice-floored valleys moving water across the figure 5 region ice sheet surface to what may have been a south-oriented ice-walled and ice-floored or bedrock-floored Rock River valley. Headward erosion of the larger southeast-oriented ice-walled and bedrock-floored valley captured that southwest-oriented melt water flow and beheaded the flow route. Water on the northeast end of the beheaded flow route then reversed flow direction to flow in a northeast direction to the newly eroded and much deeper southeast-oriented Minnesota River lowland ice-walled and bedrock-floored valley.

Detailed map of Beaver Creek area

• Figure 6 illustrates the Beaver Creek route from its headwaters area to near Lake Shetek where Beaver Creek joins the southeast-oriented Des Moines River and includes overlap areas with figure 3. Beaver Creek flows from the Klinker Slough area (along the figure 6 west edge) in a south direction before turning to meander along a northeast and southeast oriented route before turning to flow north and northeast to Lake Shetek, located in the figure 6 northeast corner. The southeast-oriented Des Moines River is the Lake Shetek outlet and is seen in figure 9 below. The Beaver Creek route is located on the Prairie Coteau upland surface  and was probably established as the remnant ice sheet wall in the figure 6 map area melted. There do not appear to be obvious through valleys or other evidence suggesting the Beaver Creek route was initiated by ice-walled and ice-floored or bedrock-floored valleys. The figure 6 map area does contain many poorly drained areas and lake basins. Among the lake basins are several dry lake beds north of the word “LOWVILLE”. Ditches suggest these lake beds have been drained and before being drained may have been shallow intermittent lakes. The lake basins and hummocky topography present is typical of glaciated areas where an ice sheet containing considerable ice-transported debris has melted and deposited the debris in place. Compare this figure 6 map region with the escarpment slope and lowland at the escarpment base seen in figures 5 and 7. Southeast-oriented melt water floods deeply eroded the escarpment slope and the lowland at the escarpment base. Flood waters removed much of the ice sheet transported debris, although coarser grained material was left and the flood waters also probably deposited materials as well. What is important to understanding the large-scale regional geomorphic features is that an ice sheet remnant remained on the Prairie Coteau upland surface after melt water floods sliced the southeast-oriented ice-walled and bedrock-floored valley to the northeast of figure 6. Headward erosion of ice-walled and bedrock-floored valleys eventually detached the Prairie Coteau ice sheet remnant from the main ice sheet (and by chopping up the ice sheet probably greatly hastened the melting process). As the Prairie Coteau ice sheet remnant melted, debris contained within the ice was deposited in place (perhaps with some sliding back and forth) and was not washed away.

Northeast-facing Prairie Coteau escarpment near Tracy, Minnesota

Figure 7: Northeast-facing Prairie Coteau escarpment near Tracy, Minnesota.

• Figure 7 illustrates the region north and slightly west of figure 6 and east and south of figure 5 and includes overlap areas with figure 5 (there is a small gap between figure 6 and figure 7). Tracy, Minnesota is the town located in the figure 7 southeast quadrant. The crest of the northeast oriented escarpment crosses the figure 7 map area diagonally from the northwest corner area to the figure 7 southeast quadrant. The northeast and north oriented Redwood River is located in the figure 7 northwest corner. A small area of the lowland at the escarpment base is located in the figure 7 northeast corner. The northeast-facing escarpment slope occupies the remainder of the figure 7 northeast half and the Prairie Coteau upland surface is located in the figure 7 southwest half. Cottonwood River flows in a northeast direction to the escarpment crest and then flows in a southeast direction along the escarpment crest, before turning to flow in a northeast direction down the escarpment slope. At the escarpment base the Cottonwood River turns again to flow in a southeast direction. Eventually the Cottonwood River reaches the southeast-oriented Minnesota River. In the figure 7 south center area is Long Lake and a southeast-oriented stream that flows to the figure 7 south center edge. That stream flows to Lake Shetek and is the Des Moines River headwaters. Note how the Des Moines River headwaters are linked by a through valley (seen in detail in figure 8 below) with Lake Yankton and also with the southeast-oriented Rock Lake outlet valley. Headward erosion of the northeast-oriented Cottonwood River headwaters valley (southwest from the escarpment crest) has beheaded a southeast-oriented flow route to the Des Moines River valley. Also, the southeast-oriented Des Moines River headwaters are flowing along the crest of a major northeast-facing escarpment. Southeast-oriented drainage routes along the escarpment crest and also at the escarpment base are probably relics from the time southeast-oriented melt water floods carved the immense southeast-oriented ice-walled and bedrock-floored Minnesota River lowland valley. Northeast-oriented drainage routes probably developed after southeast-oriented flood flow had diminished and melt water from the remaining ice wall began to flow down the newly eroded escarpment slope.

Detailed map of Des Moines River headwaters area

Figure 8: Detailed map of Des Moines River headwaters area.

• Figure 8 provides a detailed map of the Des Moines River headwaters area seen in less detail in figure 7 above. Long Lake is located in the figure 8 southeast corner. The Long Lake outlet is located southeast of the figure 8 map area and as seen in figure 7 and figure 9 below flows in a southeast direction to Lake Shetek and the Des Moines River. The Cottonwood River is the northeast-oriented stream located in the figure 8 northwest corner and north of the figure 8 map area, as seen in figure 7 above, flows in a southeast direction along the escarpment crest before it flows down the northeast-facing escarpment slope. The figure 8 map area is located just southwest of the crest of the northeast-facing escarpment, and illustrates topography found west and south of that escarpment crest. The figure 8 topography is hummocky and there are numerous closed depressions now filled with lakes and/or swamps. The region is poorly drained and is typical of areas covered by glacially deposited debris. Note the shallow northwest-southeast oriented through valley linking the Lake Yankton area with the Long Lake area. The through valley is not as pronounced as the through valley linking the Redwood River with the Rock River seen in figures 3 and 4, however it does provide evidence water once flowed from the present day Cottonwood River valley area to the present day Des Moines River drainage basin. Probably when water flowed southeast along that route it was in an ice-walled valley and it is possible the valley floor was also ice. Headward erosion of the northeast-oriented Cottonwood River valley probably captured the southeast-oriented flow and diverted it to the adjacent southeast-oriented Cottonwood River channel. The existence of two southeast-oriented channels along the escarpment crest suggests the figure 8 map region may have been located along the southwest margin of an anastomosing complex of southeast-oriented ice-walled valleys carved into the decaying ice sheet surface at the time the large and deep ice-walled and bedrock-floored Minnesota River lowland valley was carved into the melting ice sheet surface. If so, headward erosion of the deep ice-walled and bedrock-floored valley captured southeast-oriented flow in the present day southeast-oriented Cottonwood River valley segment and diverted that flow in a northeast direction down the newly eroded escarpment slope. That capture enabled the present day northeast-oriented Cottonwood River valley segment to erode headward into the melting ice wall to behead southeast-oriented flow to the Des Moines River channel and divert the water to the Minnesota River valley lowland. Lake basins probably represent where buried remnant ice masses melted and left depressions, which are now filled with water.

Des Moines River in Lake Shetek area

Figure 9: Des Moines River in Lake Shetek area.

• Figure 9 illustrates the Des Moines River headwaters in the Lake Shetek area and is south and east of figure 7 and north and east of figure 6 and includes overlap areas with figures 6 and 7. The southeast-oriented Long Lake outlet is located in the figure 9 north center area and flows to the northwest-southeast oriented lake named The Inlet, which is really a Lake Shetek Bay. The Lake Shetek outlet, which is the southeast-oriented Des Moines River, is located in the figure 9 southeast quadrant and flows by the town of Currie. The northeast-oriented stream joining the Des Moines River near Currie is Beaver Creek, which was seen in figures 3 and 6. Like the figure 8 map area the figure 9 map area is located immediately south and west of the northeast-facing escarpment slope. The southeast-oriented Judicial Ditch (located in the figure 9 northeast corner) is located in a shallow southeast-oriented valley near the escarpment crest. The northeast-oriented stream flowing from Robbins Slough (located northeast of Lake Shetek near the figure 9 east edge) flows to northeast-oriented Willow Creek, which flows down the escarpment slope (see figure 10 below). In other words, the Des Moines River-Minnesota River drainage divide is located between Lake Shetek and Robbins Slough. The presence of a major drainage divide in this location is further evidence the southeast-oriented Des Moines River headwaters route was established by southeast-oriented melt water flow at a time when ice was still present on the Prairie Coteau upland surface and probably at approximately the same time as the much larger and deeper ice-walled and bedrock-floored Minnesota River lowland valley was sliced into the decaying ice sheet’s surface. The southeast-oriented Des Moines River route was probably initiated as a southeast-oriented channel at the southwest margin of what was probably a large southeast-oriented anastomosing complex of ice-walled valleys sliced into the rapidly melting ice sheet surface. Melt water flood flow became concentrated in southeast-oriented channels further to the northeast and was able to erode the deeper and larger southeast-oriented Minnesota River lowland ice-walled and bedrock-floored valley. Being located in an ice-walled valley along the escarpment crest the southeast-oriented Des Moines River channel was not captured (southeast of the figure 8 map area) and continued to carry melt water southeast on the Prairie Coteau upland surface.

Escarpment northeast of Des Moines River headwaters area

Figure 10: Escarpment northeast of Des Moines River headwaters area

• Figure 10 illustrates the escarpment northeast of the Des Moines River headwaters area and east of the figure 9 map area (and includes overlap areas with figure 9). Currie, Minnesota is the town in the figure 10 southwest corner and the Des Moines River is the southeast-oriented river at Currie. Walnut Grove is the town along the figure 10 north edge and Willow Creek is the northeast-oriented stream flowing down the northeast-facing escarpment to Walnut Grove. Note how Willow Creek originates at the escarpment crest. A southeast-oriented tributary joins Willow Creek in HOLLY township and receives water from the southeast-oriented Judicial Ditch in the figure 10 northwest quadrant and from the Robbins Slough northeast-oriented outlet. Westbrook is the town located along the figure 10 south edge. The northeast-oriented stream flowing from Lake Julia (located just west of Westbrook) to the figure 10 northeast corner is Dutch Charley Creek. Dutch Charley Creek originates a short distance south of the figure 10 map area. Highwater Creek is the northeast-oriented stream in the figure 10 southeast corner area and also originates a short distance south of the figure 10 map area. Southwest of the Dutch Charley Creek and Highwater Creek headwaters is the southeast-oriented Des Moines River. A shallow northeast-oriented through valley links the Dutch Charley Creek headwaters with the Des Moines River valley and suggests headward erosion of the Dutch Charley Creek valley across the Prairie Coteau upland surface may have almost captured the Des Moines River. Probably that headward erosion occurred while considerable melting ice remained on the Prairie Coteau upland surface, and once the ice had melted flow in the Dutch Charley Creek valley was not great enough to erode the valley deeper. Presence of the southeast-oriented Des Moines River along the crest of the northeast-facing escarpment is evidence the escarpment was eroded at the same time as the Des Moines River route was established. As previously described the escarpment is what remains of the southwest wall of an immense southeast-oriented ice-walled and bedrock-floored valley sliced into the surface of a rapidly melting thick ice sheet. The adjacent Des Moines River valley was probably initiated as a shallower and smaller parallel ice-walled and bedrock-floored valley by the same southeast-oriented melt water floods. The ice wall between the larger and deeper ice-walled and bedrock-floored Minnesota River valley and the smaller and shallower ice-walled and bedrock-floored Des Moines River valley prevented water in the Des Moines River valley from moving northeast to the newly eroded northeast-facing escarpment slope. Once the ice wall had melted, ice sheet deposited debris established the present day Des Moines River-Minnesota River drainage divide.

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.