A geomorphic history based on topographic map evidence

Abstract:

The Cheyenne River drainage basin is located in western South Dakota and eastern Wyoming and includes the Black Hills upland region. The Cheyenne River originates in eastern Wyoming and flows in a southeast and northeast direction around the Black Hills south end, while a major Cheyenne River tributary, the Belle Fourche River, flows in a northeast and southeast direction around the Black Hills north end. East of the Black Hills the Cheyenne River flows in a northeast direction to join the south oriented Missouri River as a barbed tributary. This overview essay provides highlights from more detailed essays which use topographic map evidence to illustrate and describe Cheyenne River drainage basin landform origins. The detailed essays can be found under Cheyenne River on this website’s sidebar category list. The north and northeast oriented Powder River and Little Missouri River drainage basins are located west of the Wyoming Cheyenne River drainage basin. South of the Cheyenne River drainage basin are the North Platte and Niobrara River drainage basins in Wyoming and the White and Bad River drainage basins in South Dakota. In South Dakota the southeast and northeast oriented Moreau River drainage basin is north of the Cheyenne River drainage basin. Through valleys eroded into the high Black Hills upland surface link north-oriented Belle Fourche River tributary valleys with south, southeast, and east-oriented Cheyenne River tributary valleys. Evidence consisting of southeast-northwest oriented Cheyenne and Belle Fourche River tributaries and northwest-southeast oriented through valleys crossing present day drainage divides, including drainage divides in the high Black Hills, is interpreted to mean a deep Cheyenne River valley eroded headward around the Black Hills south end while a deep Belle Fourche River valley eroded headward around the Black Hills north end to capture immense southeast-oriented floods, which once flowed across the Black Hills upland surface. Flood waters were derived from a rapidly melting thick North American ice sheet and deeply eroded the region surrounding the Black Hills. The ice sheet was located in a deep “hole” located north and east of the Cheyenne River drainage basin area and as the ice sheet roots melted deep northeast and east oriented valleys, including the Cheyenne River valley, eroded headward from the deep “hole” to capture the immense southeast-oriented ice marginal floods. Delayed crustal warping triggered by the thick ice sheet weight and deep flood water erosion of ice sheet margin areas may also have contributed to emergence of the present day Black Hills as a high upland region.

Figure 1: Cheyenne River drainage basin location map in northeast Wyoming and western South Dakota (select and click on maps to enlarge). National Geographic Society map digitally presented using National Geographic Society TOPO software.

Cheyenne River drainage basin drainage history.

Cheyenne River headwaters are located in eastern Wyoming east of the north and northeast-oriented Powder River drainage basin and north of the southeast oriented North Platte River drainage basin and east-oriented Niobrara River drainage basin. The Belle Fourche River, Dry Fork Cheyenne River, and Lightning Creek (from north to south) are the major northeast-oriented Cheyenne River tributaries. The Belle Fourche River flows in a northeast direction along the Black Hills west flank and then turns to flow in a southeast direction along the Black Hills northeast flank. The Dry Fork Cheyenne River joins southeast oriented tributaries, including Black Thunder Creek, and turns to flow in a southeast direction to join Lightning Creek and then to flow in a northeast direction again. The Cheyenne River then joins southeast oriented Beaver Creek, which is flowing along the Black Hills southwest margin, and turns to flow in a southeast and northeast direction around the Black Hills south end. Once east of the Black Hills the Cheyenne River flows in a northeast direction to join southeast-oriented tributaries coming from the Black Hills upland region and then to join the southeast- and east-oriented Belle Fourche River, which has flowed around the Black Hills north end. After joining the Belle Fourche River the Cheyenne River flows in an east-northeast direction to join the south and southeast-oriented Missouri River, which flows to the south-oriented Mississippi River, which then flows to the Gulf of Mexico. The Cheyenne River drainage basin completely encompasses the Black Hills upland region with the Cheyenne River flowing in a southeast and northeast direction around the Black Hills south end and its tributary Belle Fourche River flowing in a northeast and southeast direction around the Black Hills north end. North and west of the Belle Fourche River drainage basin in northeast Wyoming is the northeast-oriented Little Missouri River drainage basin. North of the Belle Fourche and Cheyenne River drainage basins in South Dakota is the southeast, east, and northeast oriented Moreau River drainage basin. South and east of the Cheyenne River drainage basin in South Dakota are the White and Bad River drainage basins. All drainage basins mentioned here are included in the Missouri River drainage basin, with the north and northeast-oriented Powder River flowing to the northeast-oriented Yellowstone River, which joins the Missouri River in northwest North Dakota and the northeast-oriented Little Missouri River joining the Missouri River in west-central North Dakota. The Moreau, Cheyenne, Bad, and White River join the Missouri River in South Dakota and the North Platte River joins the Platte River, which joins the Missouri River in Nebraska.

Figure 2: Map showing Cheyenne River tributary orientations west of Black Hills. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 2 illustrates Cheyenne River tributary orientations in northeast Wyoming, west of the Black Hills. The north-south Wyoming-South Dakota state line is located near the figure 2 east edge. Green areas along the east edge are National Forest lands, which are generally located in the Black Hills upland region. Weston is a Wyoming county. Campbell County is located west of Weston County, Niobrara County is located south of Weston County, and Converse County is located west of Niobrara County and south of Campbell County. The Dry Fork Cheyenne River drains in a northeast direction to the Converse County northeast corner and then into southern Weston County where it joins southeast-oriented Black Thunder Creek and the Cheyenne River then flows in a southeast direction into northern Niobrara County before joining northeast-oriented Lance Creek (northeast-oriented Lightning Creek is a Lance Creek tributary) near Mule Creek Junction. From the Mule Creek Junction area the Cheyenne River flows in a northeast, east, and southeast direction to and along the edge of the Black Hills upland near the South Dakota state line (and figure 2 east edge). The Belle Fourche River flows in a northeast direction across Campbell County to the Weston County northwest corner and figure 2 north center edge. Note how most Belle Fourche River tributaries from the northwest are oriented in a southeast direction and from the southeast are oriented in a northwest direction. Also note the southeast-oriented orientation of Cheyenne River tributaries flowing from the Belle Fourche River-Cheyenne River drainage divide. This southeast and northwest orientation of Belle Fourche River and Cheyenne River tributaries is evidence the Cheyenne River valley captured flow from multiple southeast-oriented channels such as might be found in a large-scale southeast-oriented anastomosing channel complex. The tributary orientation is also evidence the northeast-oriented Belle Fourche River valley subsequently eroded headward across the same southeast-oriented channels and beheaded the flow routes to what were then actively eroding southeast-oriented Cheyenne River tributary valleys. The northwest-oriented Belle Fourche River tributaries are evidence flood waters on northwest ends of beheaded flood flow channels reversed flow direction to erode northwest-oriented Belle Fourche River tributary valleys. Topographic map evidence presented in the detailed essays found under Black Hills region on the sidebar category list much better illustrate this northwest-southeast orientation of Belle Fourche River and Cheyenne River tributaries and also shows through valleys linking the northwest-oriented Belle Fourche River tributary valleys with the southeast-oriented Cheyenne River tributary valleys. The through valleys provide further evidence of a large southeast-oriented anastomosing complex captured by Cheyenne River valley headward erosion and subsequently captured by Belle Fourche River valley headward erosion.

Figure 3: Map showing Cheyenne River tributary orientations in southern Black Hills and east and south of Black Hills. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 3 illustrates Cheyenne River tributaries in the southern Black Hills and also south and east of the Black Hills. Again green areas in the figure 3 west half are National Forest lands and are generally located in the Black Hills upland region. The red area in Custer County is Custer State Park in the north and Wind Cave National Park in the south, both of which are located in the Black Hills upland area, Red areas in the figure 3 east half represent Badlands National Park areas. Orange shaded areas in the figure 3 southeast quadrant represent Pine Ridge Indian Reservation lands. The northeast-oriented river flowing through the Pine Ridge Indian Reservation (in Shannon County) is the White River. Note how the Cheyenne River flows in a southeast direction to Edgemont near the figure 3 southwest corner and then flows around the Black Hills upland region south edge and turns to flow in a northeast direction between the Black Hills upland and the Badlands National Park regions. Note also how Cheyenne River tributaries from the Black Hills upland region are generally oriented in a southeast direction and from the southeast are generally oriented in a northwest direction. The southeast and northwest orientation of Cheyenne River tributaries is just the same as west of the Black Hills upland and suggests the northeast-oriented Cheyenne River valley in figure 3 also eroded headward across multiple southeast oriented flood flow channels such as might be found in a large-scale anastomosing channel complex. If so, the southeast oriented flood the northeast-oriented Cheyenne River valley captured east of the Black Hills was flowing across what is now the Black Hills upland region. Today the Black Hills upland surface is more than 1000 meters higher than regions surrounding the Black Hills upland and Black Hills peaks rise even higher. Detailed essays found under Cheyenne River on the sidebar category list provide evidence the flood waters did flow across the Black Hills upland surface. If flood waters flowed across the Black Hills upland present day topography did not exist. Either the region surrounding the Black Hills was buried in easily eroded sediments and a very deep Cheyenne River valley was eroded around the Black Hills south end and a very deep Belle Fourche River valley was eroded around the Black Hills north end or the Black Hills were uplifted as flood waters flowed across the region. Or perhaps some combination of these two events occurred. The best evidence for flood flow movements across the Black Hills upland surface are present day through valleys eroded into the Black Hills upland surface linking north-oriented Belle Fourche River tributary valleys with southeast and east oriented Cheyenne River tributary valleys such as those illustrated in the next two figures..

Figure 4 provides an example of the evidence suggesting anastomosing southeast-oriented flood flow channels were eroded into the Black Hills upland surface and illustrates the drainage divide between north-oriented Spearfish Creek, which flows to the Belle Fourche River north of the Black Hills, and southeast-oriented Rapid Creek, which flows to the Cheyenne River east of the Black Hills. Northwest-oriented Clayton Draw in the figure 4 northwest quadrant drains to north-oriented Spearfish Creek as do other north-oriented valleys in the figure 4 northwest quadrant. East-southeast and south-southeast oriented South Fork Rapid Creek can be seen in the figure 4 south center area. East-oriented Tilson Creek in the figure 4 northeast quadrant is another Rapid Creek tributary. Note how Clayton Draw is linked by through valleys with both the south-southeast oriented South Fork Rapid Creek valley and the east-oriented Tilson Creek valley. The spot elevation at the Clayton Draw-South Fork Rapid Creek drainage divide on the through valley floor is 6604 feet and hills on either of the valley rise to elevations greater than 6740 feet (the map contour interval is 20 feet). In other words the through valley, which is a water eroded valley, is approximately 140 feet deep. Note another through valley just to the north in the northwest corner of section 28, which has a higher elevation, but is also a water eroded through valley. Also north of Besant Park along the figure 4 north edge still another through valley links the south- and east-oriented Rapid Creek valley with the north-oriented Spearfish Creek valley. And the list of through valleys in the figure 4 map area could continue and by enlarging the map area more through valleys could be found. The through valleys provide evidence of multiple anastomosing flood flow channels eroded into the Black Hills upland surface by an immense southeast-oriented flood which flowed across what are now the high Black Hills. Flood waters were flowing in a southeast direction to what was then the actively eroding and deep northeast-oriented Cheyenne River valley, which at that time was eroding headward in the area east of the Black Hills. At that time the deep Belle Fourche River valley did not exist. Headward erosion of the deep Belle Fourche River and its north-oriented tributary valleys such as the Spearfish Creek valley captured the southeast-oriented flood flow channels and diverted flood waters in a north direction to the newly eroded Belle Fourche River valley.

Figure 5: Topographic map illustrating more through valleys on Black Hills upland surface linking northwest oriented East Spearfish Creek (flowing to north-oriented Spearfish Creek and then to Belle Fourche River north of Black Hills) with southeast-oriented North Fork Rapid Creek and northeast-oriented Elk Creek (flowing to Cheyenne River east of Black Hills). United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 5 illustrates additional through valleys eroded across the high Black Hills upland surface. The northwest-oriented stream flowing to the figure 5 northwest corner is East Spearfish Creek, which north and west of figure 5 flows to north-oriented Spearfish Creek, which joins the southeast-oriented Belle Fourche River north of the Black Hills. The north-oriented stream in the figure 5 north center area is Whitewood Creek, which is another Belle Fourche River tributary. The northeast-oriented stream flowing to the figure 5 northeast corner is Elk Creek, which east of figure 5 turns to flow in a southeast and east direction to eventually join the northeast-oriented Cheyenne River east of the Black Hills. The southeast-oriented stream originating near Dumont in the figure 5 south center area and flowing to the figure 5 south edge (east half) is the North Fork Rapid Creek, which south of the figure 5 map area joins east-oriented Rapid Creek, which flows to the northeast-oriented Cheyenne River east of the Black Hills. The figure 5 map contour interval is 20 feet and the through valley linking the northwest-oriented East Spearfish Creek valley with the southeast-oriented North Fork Rapid Creek valley has an elevation of between 6140 and 6160 feet. A hill immediately to the north rises to more than 6400 feet and a hill to the south rises to more than 6500 feet. In other words the through valley is at least 240 feet deep and may have been even deeper when eroded. Further the through valley is interesting because a through valley now drained to the northwest by Keough Draw parallels the northwest-oriented East Spearfish Creek through valley. In addition to this northwest-southeast oriented through valley a north-south oriented through valley links the north-oriented Whitewood Creek valley with the southeast-oriented North Fork Rapid Creek valley. The elevation of this through valley floor is hard to read on figure 6, but appears to be slightly higher than 6200 feet. Hills on either side of the through valley rise to elevations greater than 6400 feet. Still another through valley is located in the northwest corner of section 9 (in the figure 5 southeast quadrant) and links the southeast-oriented North Fork Rapid Creek valley with the northeast-oriented Elk Creek valley. The floor of this through valley has an elevation of between 6140 and 6160 feet and hills on either side rise to elevations greater than 6400. These through valleys are water eroded features and only make sense in the context of a flood formed anastomosing channel complex eroded into the Black Hills upland surface.

• Why would immense southeast-oriented floods be flowing across what is now the high Black Hills upland surface and how could those southeast-oriented flood waters flow across the high elevations? The Cheyenne River drainage basin history began with development of a North American ice sheet comparable in size to the present day Antarctic Ice Sheet, if not larger. The ice sheet was thick, probably several kilometers thick, and was located in a deep “hole”, which the ice sheet had formed by a combination of deep glacial erosion and crustal warping caused by the ice sheet weight. When at its maximum size the ice sheet stood high above the pre-glacial surface, but also had roots that extended well below the pre-glacial surface, which no longer exists. The Cheyenne River drainage basin was probably located south and west of ice sheet’s southwest margin, although evidence for the ice sheet’s southwest margin has probably been removed by deep melt water flood erosion. The pre-glacial surface under the ice sheet was completely destroyed by deep glacial erosion and the pre-glacial surface adjacent to the ice sheet and elsewhere on the North American continent was deeply eroded by deep melt water flood erosion and was also probably significantly altered by crustal warping caused by the thick North American ice sheet presence.

• Events important to Cheyenne River drainage basin history began as the ice sheet was rapidly melting and had melted to the point that in the south it no longer stood high above the surrounding non-glaciated surface, which had probably already been significantly lowered by deep melt water erosion. Immense melt water floods were flowing in a southeast direction along the ice sheet’s southwest margin and were just beginning to deeply erode the region between the Black Hills and the ice sheet’s southwest margin, which at that time was located north and east of today’s Cheyenne River drainage basin. At that time the Black Hills did not stand high above the ice sheet surface, or for that matter above the surface located between the Black Hills and the ice sheet southwest margin. Initially immense floods of melt water flowing from the rapidly melting ice sheet flowed in a southeast direction between what were then the emerging Rocky Mountains and ice sheet southwest margin. Flood waters were probably derived from immense southeast and south-oriented supra-glacial melt water rivers which carved giant ice-walled and ice-floored (later bedrock-floored) canyons into the decaying ice sheet surface and which flowed to the ice sheet’s southwest margin in present day Alberta and then in a southeast direction along the decaying ice sheet’s southwest margin. The surface on which the southeast-oriented floods were moving was significantly higher than the ice sheet floor elevation and probably no longer exists.

• How did the deep “hole” the ice sheet had been occupying open up so as to permit headward erosion of the deep northeast-oriented Cheyenne River valley? Remember, the ice sheet was thick and had deep roots. The ice sheet roots may have extended more than a kilometer below the pre-glacial surface on which the ice sheet had formed. The deep “hole” had probably been formed by a combination of deep glacial erosion of the pre-glacial surface underlying the ice sheet and of crustal warping caused by the weight of an ice sheet several kilometers thick. The crustal warping, which almost certainly did not occur instantaneously, probably also affected regions elsewhere on the continent and may have contributed to uplift of the Black Hills and other North American mountain ranges and high plateau areas as flood waters flowed across them. In other words, not only was the rapidly decaying ice sheet located in a deep “hole” that was opening up as the ice sheet melted, but delayed crustal warping caused by the ice weight was raising mountain ranges and high plateaus regions south and west of the ice sheet margin. The combination of these two events created a situation where the gigantic southeast-oriented melt water river flowing along the decaying ice sheet’s southwest margin was systematically captured by headward erosion of deep northeast-oriented valleys, which were eroding headward from the decaying ice sheet location.

Figure 6: Cheyenne River-White River drainage divide area in the Sage Creek and Scenic Basin areas located east of the Black Hills. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 6 provides an example of topographic map evidence illustrated and discussed in detailed essays found under Cheyenne River, White River, and Bad River on the sidebar category list and which describe drainage divides east of the Black Hills. The northeast oriented Cheyenne River is located in the figure 6 northwest corner and the northeast and east-oriented White River is located in the figure 6 southeast corner. Note how Cheyenne River tributaries are northwest-oriented and White River tributaries are southeast-oriented. Also note how through valleys link northwest-oriented Cheyenne River tributary valleys with southeast-oriented White River tributary valleys. The Badlands National Park area in the figure 6 north center area is the Sage Creek Basin where Sage Creek flows in a northwest direction through what appears to be a water gap eroded across a higher level erosion surface defined by Quinn Table (southwest of the Sage Creek valley). Southeast of  northwest-oriented Sage Creek Basin is Conata Basin, which is drained by southeast-oriented streams to the northeast and east-oriented White River. Through valleys, such as Sage Creek Pass, cross the Sage Creek Basin-Conata Basin drainage divide. West and south of Sage Creek Basin is the similar Scenic Basin, which is also drained by northwest-oriented Cheyenne River tributaries, which also flow through narrow valleys across the high level erosion surface defined by Quinn Table, 71 Table, and Kube Table. Again, the Cheyenne River-White River drainage divide is not located on that high level erosion surface, but on the floor of the basins to the southeast. Figure 6 evidence tells a fascinating story about headward erosion of the deep and east-oriented White River valley into the region to capture massive southeast-oriented flood flow, which was flowing across what is today the high Black Hills upland surface. The Sage Creek and Scenic Basins were eroded headward by southeast-oriented flood flow from the newly eroded White River valley while the northeast oriented White River valley eroded headward across the southeast-oriented flood flow routes. Following closely behind White River valley headward erosion was headward erosion of the deep northeast-oriented Cheyenne River valley, which captured southeast-oriented flood flow to what were then the actively eroding southeast-oriented Sage Creek and Scenic Basin headcuts. At first the newly formed Cheyenne River-White River drainage divide was located along what are now the abandoned Sage Creek and Science Basin headcut  rim. However, White River valley headward erosion was slightly in advance of Cheyenne River valley headward erosion and the actively eroding White River valley was capturing large volumes of flood waters moving around the south end of what was then the emerging  Black Hills upland. These immense water volumes overwhelmed the newly eroded northeast- and east-oriented White River valley causing some flood water to spill across the newly formed Cheyenne River-White River drainage divide. This spillage eroded the northwest-oriented “water gaps” across the higher level erosion surface defined by Quinn, 71, and Kube Tables. Shortly thereafter Cheyenne River valley headward erosion captured flood flow moving around the Black Hills south end and flow in the White River valley greatly diminished. As flow in the White River valley diminished flood waters filling the Sage Creek and Scenic Basin areas drained to the northwest and southeast to create present day drainage routes in those basins.

Figure 7: Cheyenne River-Bad River drainage divide area west of south and southeast-oriented Missouri River. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 7 completes this overview of the Cheyenne River drainage basin landform origins by illustrating orientations of Cheyenne River and Bad River tributaries flowing from the Cheyenne River-Bad River drainage divide west of the southeast-oriented Missouri River. The Cheyenne River is the northeast-oriented river forming the northern border of Haakon and Stanley Counties in figure 7 and the Bad River is the northeast-oriented river forming the Haakon and Stanley County southern borders. Note how Bad River tributaries from the north are almost all oriented in a southeast direction and how most Cheyenne River tributaries from the south are oriented in a northwest direction. Also note how Cheyenne River tributaries from the north are oriented in a southeast direction. This northwest-southeast orientation of Cheyenne River and Bad River tributaries is evidence the northeast-oriented Bad River valley eroded headward across multiple southeast-oriented flood flow channels such as might be found in a southeast-oriented anastomosing channel complex. The tributary orientation is also evidence the northeast-oriented Cheyenne River valley subsequently eroded headward across the same southeast-oriented anastomosing channel complex and beheaded the flood flow channels moving flood water to the newly eroded northeast-oriented Bad River valley. Northwest-oriented Cheyenne River (and Bad River) tributary valleys were eroded by reversals of flood flow on northwest ends of beheaded flood flow channels. Also note the Missouri River meander at the Cheyenne River mouth. The Missouri River flows in a south, southwest, northwest, and southeast direction to reach the northeast-oriented Cheyenne River valley. The northwest and southeast Missouri River valley segments represented by that meander were eroded along former southeast-oriented flood flow channels. The northwest-oriented meander segment was created by a reversal of flood flow on the northwest end of a beheaded southeast-oriented flood flow channel. Topographic maps shown in this Cheyenne River drainage basin detailed essays illustrate this drainage alignment much better and also illustrate through valleys linking northwest-oriented Cheyenne River tributary valleys with southeast-oriented Bad River tributary valleys.

• Why are the Bad River and Cheyenne River valleys northeast oriented when the Cheyenne and Bad Rivers flow to the southeast oriented Missouri River? In the case of the Cheyenne River and Bad River valleys they eroded headward from a huge southeast and south-oriented ice-walled and ice-floored (later bedrock-floored) canyon which was carved by an immense southeast and south-oriented supra-glacial melt water river north and east of the ice sheet southwest margin. The Missouri Escarpment in North and South Dakota and in Saskatchewan is today what remains of that giant canyon’s west and southwest wall. The ice floor of that giant southeast and south-oriented ice-walled canyon was significantly lower in elevation than the bedrock surface south and west of the decaying ice sheet margin and the huge melt water river flowing in that southeast and south-oriented ice-walled canyon represented the region’s major drainage route, which captured the immense southeast-oriented ice-marginal floods by eroding deep northeast and east-oriented tributary valleys headward across the ice sheet’s southwest margin and then headward into the adjacent bedrock surface. These deep northeast-oriented valleys diverted immense southeast-oriented ice-marginal floods into space the ice sheet had once occupied. Melting of what had been the thick ice sheet roots progressively lowered both the ice sheet surface, the ice-walled canyon floor, and the surrounding bedrock surface, creating a situation where new and even deeper northeast-oriented valleys repeatedly eroded headward to capture the immense southeast-oriented ice-marginal melt water floods.

• Between the Missouri River in figure 7 and the Missouri Escarpment, which is located east of the figure 7 map area, is the Missouri Coteau, which is a region covered with what appear on topographic maps to be glacial deposits. The Missouri Coteau area ranges in width, but in South Dakota can be more than 100 kilometers wide and is also crossed by what appear to be east and northeast oriented valleys. The Missouri Coteau, is illustrated and described in detailed essays found under SD Missouri River and James River on the sidebar category list  and is the location of what was at one time the decaying ice sheet’s detached southwest margin. The ice sheet’s southwest margin was detached when the deep southeast and south-oriented ice-walled canyon (of which the Missouri Escarpment is a remnant of the southwest and west wall) became a bedrock-floored canyon. This detached ice sheet southwest margin initially formed an ice barrier between the deep ice-walled canyon floor and the southeast-oriented ice-marginal floods flowing on the higher elevation bedrock surface south and west of the decaying ice sheet margin. In time flood waters from the southeast-oriented ice-marginal floods broke through the detached ice sheet margin barrier and deep east and northeast oriented valleys eroded headward from the deeper ice-walled canyon floor to capture the immense southeast oriented ice-marginal floods and to divert flood waters into space being opened up in the deep “hole” the melting ice sheet had once occupied. Late in the ice sheet melt down history headward erosion of deep ice-walled and bedrock-floored canyons on the ice sheet floor intersected to create new northeast and north oriented flood water outlets which captured the south-oriented flood flow which had been moving to the Gulf of Mexico. The new flood water outlets moved flood waters to the Gulf of Saint Lawrence and later to what is now Hudson Bay. These northeast-oriented and north-oriented flood flow routes were located in ice-walled and bedrock-floored canyons located between decaying thick ice sheet remnants.

• The deep northeast-oriented valleys we see today were probably eroded late in the ice sheet melt down history and were probably preceded by earlier, but similar northeast-oriented valleys which also diverted massive southeast-oriented ice-marginal floods onto the decaying ice sheet surface. Valleys were eroded headward in sequence, with valleys located in the east and south being eroded headward first. Subsequently a major climate change triggered by diversion of south-oriented melt water floods to become north oriented melt water floods caused a new thin ice sheet to form. The diversion of the south-oriented melt water floods from the Gulf of Mexico to the Gulf of Saint Lawrence and Hudson Bay occurred as ice sheet melting opened up north and northeast-oriented drainage routes (or ice-walled and bedrock-floored canyons) between decaying thick ice sheet remnants. Flood water diversion drastically changed ocean circulation patterns and resulted in a cooling of Northern Hemisphere climates. The climate change also caused north-oriented flood waters to freeze on the former ice sheet floor, forming a wet based thin ice sheet with rejuvenated thick ice sheet remnants embedded in it. This new thin ice sheet blocked the new northeast-oriented rivers, which caused northeast-oriented drainage to spill over drainage divides along the new ice sheet’s southwest margin, which created segments of the southeast-oriented Missouri River valley in South and North Dakota and northeast Montana. The Missouri River valley location today approximates the location of decaying thick ice remnants (the Missouri Coteau marks where those remnants were located) that also formed the thin sheet’s southwest margin.

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.