A geomorphic history based on topographic map evidence

Topographic map interpretation methods are used to determine landform origins in the Snake Creek-North Platte River drainage divide area south and east of Alliance, Nebraska. The Snake Creek-North Platte River drainage divide area south and east of Alliance is located in the western Nebraska Sand Hills region and was crossed by immense south oriented melt water floods. The Sand Hills region is interpreted to have developed on what were deltaic sediments deposited where immense floods entered temporarily ponded flood waters. Ponding of flood waters probably resulted from the convergence of flood flow, which had used different flow routes to reach central and eastern Nebraska. Subsequent eolian activity has obscured most drainage history evidence within the Sand Hills region, although evidence can be observed in the Snake Creek drainage basin northwest of Alliance and also along the North Platte River valley. Drainage histories constructed from the available evidence suggest the Snake Creek-North Platte River drainage divide was crossed by an immense southeast-oriented flood, which was captured by headward erosion of the North Platte River valley.

• The purpose of this essay is to use topographic map interpretation methods to explore Snake Creek-North Platte River drainage divide area landform origins south and east of Alliance, Nebraska, 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 the 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 Snake Creek-North Platte River drainage divide area landform origins south and east of Alliance, Nebraska 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.

Snake Creek-North Platte River drainage divide area location map

Figure 1: Snake Creek-North Platte 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 Snake Creek-North Platte River drainage divide area location map. Nebraska is the state occupying most of the figure 1 map area with Wyoming located west of the Nebraska Panhandle and Colorado located south of Wyoming and the Nebraska Panhandle. The South Platte River flows in a northeast direction through Sterling, Colorado to the Colorado northeast corner and then to North Platte, Nebraska, where it joins the North Platte River. The combined North and South Platte Rivers forms the Platte River, which flows in a southeast direction from North Platte to the figure 1 east edge (near the southeast corner). Southeast-oriented drainage south of the South Platte River flows to the east-northeast oriented Republican River located south of the figure 1 map area. The North Platte River flows in a southeast direction from the figure 1 west edge (north half) through Guernsey and Torrington, Wyoming to Scottsbluff and Broadwater, Nebraska before joining the South Platte River near North Platte. Near the figure 1 north edge the Niobrara River flows in a southeast direction from near Lusk, Wyoming to the Agate Fossil Beds National Monument in western Nebraska and then flows in an east and a northeast direction to reach Valentine near the figure 1 northeast corner. Snake Creek is shown on figure 1, but is not labeled and is the stream flowing in a southeast and east direction to Alliance, Nebraska (located southeast of the Agate Fossil Beds National Monument) and then ending near Alliance. Immediately east of Alliance is the Nebraska Sand Hills region, which encompasses most of the figure 1 map east two-thirds located between the Niobrara River and the North Platte (and Platte) Rivers. Other essays describe evidence for the Niobrara River drainage basin located north of the Snake Creek-North Platte River drainage divide area and are found under Niobrara River on the sidebar category list and have also described evidence for the Loup River drainage basin located east of the Snake Creek-North Platte River drainage divide area and which can be found under Loup River on the sidebar category list.  Hundreds of Missouri River drainage basin landform origins research project essays published on this website provide overwhelming evidence for an immense southeast-oriented flood moving across Nebraska. Flood waters were probably derived from a rapidly melting thick North American ice sheet and probably converged with east-oriented flood water (from the same rapidly melting ice sheet), which were flowing east from Colorado and Wyoming. Convergence of these two massive floods probably resulted in the ponding of flood water and deposition of deltaic sedimentary deposits. Sand hills in the Nebraska Sand Hills region are located on those former deltas. The Snake Creek-North Platte River drainage divide area illustrated and discussed here is located in the region south and east of Alliance and north of the North Platte River (located west of north-south highway 61 and east of Broadwater, Nebraska).

Snake Creek-North Platte River drainage divide area detailed location map

Figure 2: Snake Creek-North Platte 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 map of the Snake Creek-North Platte River drainage divide area south and east of Alliance. Box Butte, Morrill, Garden, Grant, and Arthur are Nebraska county names and the county boundaries are shown. The North Platte River flows in a southeast direction from the figure 2 west center edge to the figure 2 south edge (east half). Blue Creek is the long southeast-oriented North Platte River tributary, which originates in the Crescent Lake National Wildlife Refuge area. Lost Creek is the labeled southeast-oriented North Platte River tributary immediately west of Blue Creek. Note also the presence of several other shorter southeast-oriented North Platte River tributaries in Garden County. This essay follows the North Platte River valley upstream from Blue Creek to the Broadwater area in eastern Morrill County. Southeast-oriented tributaries to the North Platte River are interpreted as evidence the North Platte River valley eroded headward to capture multiple southeast-oriented flood flow channels, such as might be found in a large-scale anastomosing channel complex. Note northwest-oriented tributaries (or tributary segments) south of the North Platte River. Those northwest-oriented North Platte River tributaries (or tributary segments) are interpreted to have originated as reversals of flood flow on the northwest ends of beheaded southeast-oriented flood flow routes. The North and South Branches of Snake Creek flow in a southeast direction from the figure 2 northwest corner area to join in the Box Butte County southwest corner and then to flow in an east direction to the Box Butte County southeast corner, where Snake Creek as a surface drainage route is lost. Note multiple southeast-oriented Snake Creek tributaries located in Box Butte County. The Snake Creek valley appears to have eroded headward from somewhere east of the Alliance to capture multiple southeast-oriented flood flow routes, similar to those found in a large-scale anastomosing channel complex. The multiple southeast-oriented Snake Creek tributaries provide evidence the Snake Creek valley eroded headward across a southeast-oriented anastomosing channel complex that was moving flood water to what was probably a newly eroded North Platte River valley. Flood waters were probably flowing across deltaic sediments that had been previously deposited as flood waters had entered a ponded flood water area. Flood flow across the figure 2 map area ended when headward erosion of Niobrara River valley (north of he figure 2 map area) beheaded all southeast-oriented flood flow routes to the figure 2 map area. Subsequent to the end of flood flow across the figure 2 map area wind action has created sand hills on the former deltaic sediments to obscure most drainage routed between the Box Butte County Snake Creek valley and the North Platte River valley to the southeast.

Blue Creek-North Platte River drainage divide area

Figure 3: Blue Creek-North Platte River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 3 uses reduced size topographic maps to illustrate the Blue Creek-North Platte River drainage divide area. The North Platte River flows in a southeast direction from the figure 3 west center edge to the figure 3 south edge. Oshkosh is the town located in the North Platte River valley midway between the figure 3 west and south edges. Blue Creek flows from the figure 3 north edge in a southeast direction in the figure 3 north center area and then turns to flow in a south-southeast direction to join the North Platte River just south of the figure 3 south edge. The southeast-oriented stream joining the North Platte River near Oshkosh is Lost Creek. Sand hills cover much of the figure 3 north half, especially in the northeast quadrant and obscure most evidence required to determine drainage histories. Close to the North Platte River valleys are not obscured and figure 3 evidence reveals multiple southeast-oriented tributary valleys to the North Platte River valley. While not in the drainage divide area being studied in this essay, tributary valleys are also visible on the south side of the North Platte River. Rush Creek is the north-oriented tributary in the figure 3 southwest corner (flowing north along the figure 3 west edge). Note the northwest-oriented Rush Creek tributaries. North-oriented tributaries south of Oshkosh also have northwest-oriented tributaries and/or headwaters, which are located south of the figure 3 map area. The southeast-oriented tributaries are evidence the North Platte River eroded headward across multiple southeast-oriented flood flow channels, such as might be found in a southeast-oriented anastomosing channel complex. The northwest-oriented tributaries (or tributary segments) south of the North Platte River are evidence headward erosion of the North Platte River beheaded multiple southeast-oriented flood flow routes. Flood waters on the northwest ends of those beheaded flood flow channels reversed flow direction to flow northwest and north to the newly eroded and deeper North Platte River valley.

Blue Creek headwaters area

Figure 4: Blue Creek headwaters area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 4 illustrates the Blue Creek headwaters in the Crescent Lake National Wildlife Refuge area north and west of the figure 3 map area. The Crescent Lake National Wildlife Refuge extends from the figure 4 center area to the east center area and Crescent Lake (slightly to the southeast of the figure 4 center) is where south-southeast oriented Blue Creek originates. Blue Creek flows from Crescent Lake to the figure 4 south edge. Lakes in the figure 4 map area provide evidence of a high water table underlying the sand hills region. However, other than Blue Creek and Jones Run (an east-oriented stream linking Swan Lake, Jones Lake, and Blue Lake with Crescent Lake) the figure 4 map area shows no other surface drainage routes. Sand hills obscure all evidence of pre-sand hills valleys. Sand in the sand hills was probably derived from flood transported sediments, deposited in large deltas where flood waters entered extensive, but temporary flood formed lakes. The large temporary lakes formed where immense floods from the northwest converged with massive floods from the west (flood waters in both cases were from a rapidly melting ice sheet to the north, although flood waters used different flow routes to reach Nebraska). The large temporary lakes may have covered significant regions in central and Nebraska and coarsest grained flood transported sediments would have been deposited as deltaic sediments at the northwest and western ends of the temporary lakes. Further east finer grained sediments probably settled out. Present day valley systems, including the Platte River valley, the North Platte River valley, the Loup River valley, the Snake Creek valley, and the Niobrara River valley were not eroded until the temporary lakes had drained from the region and probably played a significant role in helping to drain the ponded flood waters. Headward erosion of present day valley systems probably eroded the flood deposited sediments, although enough flood deposited deltaic sediments remained that subsequent eolian activity was able to create the sand hills, which have obscured most of the flood eroded valleys. In North Dakota and in Manitoba (and in other areas) sand hills are common on what published reports describe as glacial melt water flood deposited deltas formed in glacial lakes. [Note, the thick ice sheet that melted fast paradigm interprets those glacial lakes as places where large melt water floods converged and ponded flood waters, just as the paradigm sees flood waters converging and developing temporary lakes in Nebraska. The paradigm recognizes the existence of temporary lakes (or lake-like rivers, in those northern locations and also of flood deposited deltas where sand hills are presently located.] East of the Nebraska Sand Hills, where sand hills have not developed, finer grained flood deposited sediments probably still remain, although they were probably deeply eroded during final flood stages by headward erosion of flood eroded valleys .

Lost Creek-North Platte River drainage area

Figure 5: Lost Creek-North Platte River drainage area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 5 provides an easier to read map of the Lost Creek-North Platte River drainage divide area seen in less detail in the figure 3 map above. Oshkosh is the town located near the figure 5 southeast corner. The North Platte River flows in a southeast direction from the figure 5 west center edge to the figure 5 south edge (just west of Oshkosh). Lost Creek is the southeast and south-southeast stream flowing to join the North Platte River near Oshkosh. Mutton Creek is the south-oriented North Platte River tributary in the figure 5 center area and Coldwater Creek is the south- and southwest-oriented stream in the figure 5 northwest quadrant (near the west edge). While Coldwater Creek enters the North Platte River as a southwest-oriented stream, its headwaters are southeast-oriented. North of the Platte River valley there is a northwest-southeast orientation of landscape features and drainage routes, which probably reflects erosion by southeast-oriented flood water that once flowed across that upland surface (although some hills may be sand hills and may not be flood eroded residuals). Headward erosion of the deep North Platte River valley captured the southeast-oriented flood flow and tributary valleys eroded headward from the newly eroded North Platte River valley north wall. While many of these tributary valleys are southeast-oriented for their entire length, some such as Coldwater Creek are different. A close look at the drainage divide between Coldwater Creek and the southwest-oriented North Platte River tributary immediately the southeast reveals the southwest-oriented valleys eroded headward across southeast-oriented flood flow routes. Figure 5a below provides a detailed map of that drainage divide and reveals shallow northwest-southeast oriented through valleys eroded into the drainage divide. These through valleys are not spectacular, but they exist and are evidence the Coldwater Creek valley eroded headward across multiple southeast-oriented flood flow routes. South of the North Platte River in the figure 5 southwest quadrant the north-oriented stream with the wooded valley is Rush Creek. East of Rush Creek note northwest-oriented Watt Canyon, which drains directly to the North Platte River valley. The northwest-oriented Watt Canyon valley was initiated by a reversal of flood waters on the northwest end of a southeast-oriented flood flow channel, which North Platte River valley headward erosion beheaded.

Figure 5a: Detailed map of drainage divide southeast of Coldwater Creek. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Browns Creek-Lower Dugout Creek drainage divide area

Figure 6: Browns Creek-Lower Dugout Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 6 illustrates the Browns Creek-Lower Dugout Creek drainage divide area north of Broadwater located west and north of the figure 5 map area. The southeast-oriented North Platte River is located in the figure 6 southwest corner. Broadwater is the town located in the North Platte River valley. Lower Dugout Creek is the North Platte River tributary flowing in a southeast, southwest, and south direction from the upland surface north of Broadwater to the North Platte River valley. Note the southeast-oriented Lower Dugout Creek headwaters on the upland surface and also southeast-oriented headwaters of Lower Dugout Creek tributaries (figure 8 below provides a more detailed map of the Lower Dugout Creek headwaters area). East of Lower Dugout Creek is an unnamed south-oriented North Platte River tributary, which also has southeast-oriented headwaters. In the figure 5 center area two southeast-oriented streams are shown as flowing for short distances in southeast-oriented valleys (the streams and valleys appear to end and do not continue to the North Platte River). West of Lower Dugout Creek is south-oriented Browns Creek, which flows near the figure 6 west edge. Browns Creek headwaters in the figure 6 northwest corner area are also southeast-oriented. The upland surface northeast of the North Platte River valley includes northwest-southeast oriented valleys, suggesting the region has been eroded by southeast-oriented flood water. The southeast orientations of headwaters of North Platte River tributaries is evidence the tributary valleys eroded north into the upland surface to capture southeast-oriented flood flow and to divert the flood waters into the newly eroded and deeper North Platte River valley. Based on figure 6 map evidence it is reasonable to suggest the North Platte River valley eroded headward along a major southeast-oriented flood flow route. The southeast-oriented tributary headwaters valley suggest flood flow on the adjacent upland surface was parallel to the North Platte River valley, which explains why tributary valleys have not eroded deeply into the upland surface. Figure 7 below provides a more detailed map of the Lower Dugout Creek-North Platte River drainage divide.

Detailed map of the Lower Dugout Creek-North Platte River drainage divide area

Figure 7: Detailed map of the Lower Dugout Creek-North Platte River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 7 provides a detailed map of the Lower Dugout Creek-North Platte River drainage divide area north of Broadwater, which was seen in less detail in figure 6 above. The southeast-oriented North Platte River can just barely be seen in the figure 7 southwest corner. Broadwater is the town located in the figure 7 south center edge area. Lower Dugout Creek flows in a south-southwest direction from the figure 7 north edge to join irrigation canals in the North Platte River valley. Note the large southeast-oriented valley heading in section 16 (north of Broadwater) and extending in a southeast direction to figure 7 south edge (and then to the North Platte River). Note how that large southeast-oriented valley is linked by a large northwest-southeast oriented through valley with the Lower Dugout Creek valley and also the presence of streamlined northwest-southeast oriented erosional residuals located northeast and east of Broadwater (section 22 and just south of the section 21 southwest corner). The northwest-southeast oriented through valley is evidence large volumes of flood water flowed in a southeast direction across section 16 to section 22 and then to the North Platte River. This southeast-oriented flood flow probably occurred as the North Platte River valley was eroding headward into the figure 7 map region. Headward erosion of the south-southeast oriented Lower Dugout Creek valley from what was probably the actively eroding North Platte River valley head beheaded the southeast-oriented flood flow route through sections 16 and 22 and diverted the water more directly to what was then the North Platte River valley head. Note the presence of additional though valleys linking the Lower Dugout Creek valley with the unnamed south-oriented North Platte River tributary valley to the west. Through valleys can be seen in sections 8 and 17, providing evidence headward erosion of the unnamed tributary valley beheaded southeast-oriented flood flow to the newly eroded Lower Dugout Creek valley. A close look at section 11 reveals shallow through valleys across the upland surface linking the Lower Dugout Creek valley with the south-oriented section 11 and section 14 valley. Those through valleys provide evidence headward erosion of the Lower Dugout Creek valley beheaded southeast-oriented flood flow to that section 11 and section 14 valley.

Detailed map of Lower Dugout Creek headwaters area

Figure 8 provides a detailed map of the Lower Dugout Creek headwaters area and is located north of the figure 7 map area and shows an area seen in less detail in figure 6 above. The northwest-southeast oriented hills in sections 13 and 18 in the figure 8 northeast quadrant may be sand hills and probably are not streamlined erosional residuals. The southeast-oriented Lower Dugout Creek valley headcut rim is located in section 16 in the figure 8 northwest quadrant. A significant secondary southeast-oriented headcut rim is located in section 23 in the figure 8 center. The valley heading in section 24 is linked to the south oriented North Platte River located east of Lower Dugout Creek and seen in sections 17 and 14 in figure 7 above. The valley originating in section 19 continues in a southeast direction east of the figure 8 map area, but then is lost and is not a continuous valley to the North Platte River valley. Note that while Lower Dugout Creek headwaters are south-oriented along the figure 8 south edge and is south-southwest oriented figure 7, the heads of the Lower Dugout Creek headwaters valleys are all southeast-oriented. The Lower Dugout Creek valley is a small-scale example of how a south-oriented valley eroded north from a newly eroded deep valley wall to capture southeast-oriented flood water on the adjacent upland surface. Note how headward erosion of each of the major Lower Dugout Creek headcuts is arranged so as to capture different southeast-oriented flood flow routes or channels. In other words, flood waters were eroding all of the southeast-oriented headcuts at the same time. Flood flow to the Lower Dugout Creek headcuts probably ended when headward erosion of the Browns Creek valley (or headcut) beheaded flood flow routes to the Lower Dugout Creek headcuts. Headward erosion of these North Platte River tributary valleys occurred prior to development of the regional sand hills, although probably occurred after deltaic sediments had been deposited in the region. A complication in this region is distinguishing between wind developed landforms and flood eroded landforms, as both appear to have the same northwest-southeast orientation. Most hills on the upland surface are probably wind developed sand hills and not flood eroded landforms, the valleys are probably flood eroded and predate the sand hills.

Snake Creek tributaries in the region northwest of Alliance

Figure 9: Snake Creek tributaries in the region northwest of Alliance. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 9 illustrates Snake Creek tributaries northwest of Alliance and is north of the figures 6, 7, and 8 map areas. Snake Creek is located is south of the figure 9 map area and is seen in figure 10 below. Box Butte Creek in the figure 9 northeast quadrant (near the north edge) flows in a southeast direction parallel to the Snake Creek tributaries and then turns to flow in a northeast direction to join the east-oriented Niobrara River, which flows across the region north of the figure 9 map area. All other streams in the figure 9 map area flow to east-oriented Snake Creek or directly to the sand hills located immediately east of Alliance. From the northeast to the southeast the named streams in figure 9 are Hemingford Creek, North Fork (Berea Creek), Berea Creek, North Branch (Point of Rocks Creek), and Point of Rocks Creek. Note the many southeast-oriented tributaries to these named southeast-oriented streams and how some of those tributaries flow in a southeast direction and then turn to flow east (for example), while another tributary originates on the same alignment and flows in a southeast direction further to the southeast. This pattern suggests the tributary valleys (and the major valleys as well) originated as channels in a southeast-oriented anastomosing channel complex. The entire figure 9 map area appears to have been eroded by southeast-oriented flood flow moving in a large-scale southeast-oriented anastomosing channel complex. Headward erosion of the east-oriented Niobrara River valley to the north and northwest of the figure 9 map area would have beheaded flood flow to the figure 9 map area (see figure 1). The figure 9 evidence, which lies outside the Snake Creek-North Platte River drainage divide area region south and east of Alliance, is simple and easy to see. Drainage routes are not obscured by sand hills and drainage histories can be constructed. The problems come just to the southeast of this figure 9 map area as is seen in figure 10 below. In a very short distance, the figure 9 drainage routes disappear into the Nebraska Sand Hills region. In other words, the figure 9 map area is located just west of where the sand hills begin.

Snake Creek-North Platte River drainage divide area southeast of Alliance

Figure 10: Snake Creek-North Platte River drainage divide area in region southeast of Alliance. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 10 uses reduced size maps to illustrate the region east and south of Alliance and includes overlap areas with figure 9. Alliance is the city in the figure 10 northwest quadrant. Snake Creek is the east-oriented stream south of Alliance and flows to the Alliance airport area (southeast from Alliance) and then disappears in the sand hills. Note how other southeast-oriented streams in the figure 10 northwest corner area also disappear as they enter the sand hills region. Lakes in the sand hills indicate the water table is high, which suggests there is ground water to the east and/or southeast. However, all evidence of surface drainage routes is obscured by the sand hills, which severely limits evidence for reconstructions of previous drainage routes. Reconstructions must be made using evidence from regions surrounding the sand hills area. Figure 9 illustrated one such area located immediately northwest of the drainage divide area being studied. Figures 3-8 along the North Platte River valley illustrated the region immediately south of the drainage divide area of concern. Since evidence within the Snake Creek-North Platte River drainage divide area south and east of Alliance cannot be used for drainage history reconstructions the figures 3-9 evidence must be used. Based on the figures 3-9 evidence an immense southeast-oriented flood flowed across the Snake Creek-North Platte River drainage divide area south and east of Alliance at the time the North Platte River valley eroded headward into the region. The southeast-oriented flood water was probably flowing across flood deposited deltaic sediments, which had been previously deposited as flood waters entered a large, but temporary flood created lake. Headward erosion of the North Platte River valley may have helped drain the flood created lake. Flood waters then probably eroded southeast-oriented anastomosing channels into the flood deposited deltaic sediments. Headward erosion of the east-oriented Niobrara River valley then captured the southeast-oriented flood flow and diverted the water east and northeast to what was then the newly eroded Missouri River valley in northeast Nebraska. Eolian activity then developed sand hills on the flood deposited deltaic sediments and obscured flood eroded valleys across the Snake Creek-North Platte River drainage divide area south and east of Alliance.

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.