A geomorphic history based on topographic map evidence

The Musselshell River-Yellowstone River drainage divide area discussed here is located in Musselshell and Yellowstone Counties, Montana, USA. Although detailed topographic maps of the Musselshell River-Yellowstone River drainage divide area have been available for more than fifty years detailed map evidence has not previously been used to interpret the region’s geomorphic history. The interpretation provided here is based entirely on topographic map evidence. The Musselshell River-Yellowstone River drainage divide area is interpreted to have been eroded during immense southeast-oriented flood events, the first of which flowed on a topographic surface at least as high as the highest points in the present-day drainage divide area. Flood erosion across the drainage divide ended when headward erosion of the deep Musselshell River valley captured all southeast-oriented flood flow.

Introduction:

• The purpose of this essay is to use topographic map interpretation methods to explore Montana Musselshell River-Yellowstone River drainage divide area landform origins in Musselshell and Yellowstone Counties. 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 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 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 similar essays is a thick North American ice sheet, comparable in thickness to the present day Antarctic ice sheet, occupied approximately the North American region usually recognized to have been glaciated and through its weight and erosive actions created a “deep” North American “hole”, 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 Musselshell River-Yellowstone River drainage divide area landform evidence in Musselshell and Yellowstone Counties will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm.

Musselshell River-Yellowstone River drainage divide area location map

Figure 1: Musselshell River-Yellowstone 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 Musselshell River-Yellowstone River drainage divide area location map and illustrates a region in Montana and a small region in northern Wyoming (along the figure 1 south edge). The Yellowstone River flows east from Livingston (near the figure 1 west edge, south half) to Billings and then in a northeast direction to Miles City near the figure 1 east edge. The Musselshell River is located north of the Yellowstone River and enters the figure 1 map area near Martindale, which is located near the figure 1 west center edge. After flowing east-southeast to near Lavina the Musselshell River turns to flow northeast to the Melstone, area and then turns to flow north-northwest to Mosby and the figure 1 north edge. The Musselshell River-Yellowstone River drainage divide area discussed here is located east of a line between Lavina and Billings and west of line between Melstone and Custer. Based on evidence from hundreds of Missouri River drainage basin landform origins research project essays published on this website landform evidence illustrated here is interpreted in the context of an immense southeast-oriented flood flowing across the figure 1 map area and which was systematically captured and diverted northeast by headward erosion of deep valleys eroded into a topographic surface at least as high as the figure 1 region highest elevations today. The northeast-oriented Yellowstone River and Musselshell River valleys were two of the deep valleys that eroded southwest to capture southeast-oriented flood water and to divert the flood flow to the northeast. First the deep Yellowstone River valley head eroded southwest to capture the southeast-oriented flood waters and to divert flood waters to the northeast. Flood waters on the northwest ends of the beheaded flood flow routes reversed flow direction to flow northwest to the newly eroded Yellowstone River valley. By doing so reversed flood flow eroded northwest-oriented tributary valleys. Subsequently the deep Musselshell River valley eroded south, southwest, and west-northwest to capture the southeast-oriented flood flow and to divert the flood waters further to the northeast (to the Missouri River valley, which is located north of the figure 1 map area). Again flood waters on the northwest ends of beheaded flood flow routes reversed flow direction to flow northwest to the newly eroded Musselshell River valley. Detailed maps below provide evidence supporting this interpretation. This essay only addresses Musselshell River-Yellowstone River drainage divide area evidence in Yellowstone and Musselshell Counties. The Rosebud and Treasure County Musselshell River-Yellowstone River drainage divide area essay described the drainage divide area east of the drainage divide area discussed here and can be found under either Musselshell River or Yellowstone River on the sidebar category list.

Musselshell River-Yellowstone River drainage divide area detailed location map

Figure 2: Musselshell River-Yellowstone River drainage divide area detailed location map. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 2 illustrates a somewhat more detailed map of the Musselshell River-Yellowstone River drainage divide area discussed here. Yellowstone, Musselshell, Treasure and Golden Valley Counties are located in Montana. The northeast-oriented Yellowstone River flows from the Laurel area on the figure 2 south edge (west half) northeast through Billings to Myers, Montana near the figure 2 east edge. The Musselshell River flows from Ryegate near the figure 2 west edge (in Golden Valley County) through Lavina and Roundup to Melstone near the figure 2 north edge (east half). The Musselshell River-Yellowstone River drainage divide area discussed here is located east of a line which extends from Lavina to Billings and west of a line which extends from Melstone to Custer and is located entirely within Musselshell and Yellowstone Counties. Figure 2 shows numerous southeast-oriented Yellowstone River and Musselshell River tributaries and some northwest-oriented tributaries. This southeast and northwest drainage alignment is evidence the northeast oriented Yellowstone River valley (and the east and northeast-oriented Musselshell River valley) eroded headward to capture southeast-oriented flood flow. The southeast-oriented tributary valleys were eroded by southeast-oriented flood flow moving into the newly eroded valleys and the northwest-oriented tributary valleys were eroded by reversals of flood flow on the northwest ends of beheaded southeast-oriented flood flow routes. Because flood waters move in and erode anastomosing (or inter-connected) channels reversed flood flow on a beheaded flood flow route could capture flood flow from yet to be beheaded flood flow routes. Such captures of yet to be beheaded flood flow could enable the reversed flood flow routes to erode much deeper and larger northwest-oriented valleys than might otherwise be possible. Often evidence for such flow reversals and captures can be found on detailed topographic maps. Detailed maps below start with the northeast end of the Musselshell River-Yellowstone River drainage divide area, along the Yellowstone County-Treasure County boundary, and then progress southwest along the drainage divide area and conclude by looking at the  drainage divide in southwest Musselshell County and adjacent areas of western Yellowstone County.

Hawk Creek-Deadman Creek drainage divide area

Figure 3: Hawk Creek-Deadman Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 3 illustrates the Hawk Creek-Deadman Creek drainage divide area near the east end of the Musselshell River-Yellowstone River drainage divide area discussed here. Hawk Creek flows northeast from the figure 3 southwest corner and turns to flow northwest to the figure 3 northwest corner and then continues  to join the northeast-oriented Musselshell River as a barbed tributary at Musselshell, Montana. Deadman Creek is located in the figure 3 south center area and flows southeast to the figure 3 south edge and then to join southeast-oriented Buffalo Creek, which flows to the northeast-oriented Yellowstone River. Southeast-oriented drainage along the figure 3 south and east edges flows to the Yellowstone River. Note how southeast-oriented drainage originates almost at the edge of the northeast-oriented Hawk Creek valley. Also note erosional residuals standing above the present day Hawk Creek-Yellowstone River drainage divide. The southeast orientations of Yellowstone River tributaries and the northwest-orientation of the Hawk Creek valley along with the through valleys eroded across the present day Musselshell River-Yellowstone River drainage divide all provide evidence southeast-oriented flood waters once moved across the drainage divide in multiple channels such as might be found in a southeast-oriented anastomosing channel complex. This evidence suggests flood waters were flowing on a topographic surface at least as high as the highest figure 3 elevations today (in other words, the deep Musselshell River valley to the north and northwest did not yet exist). Headward erosion of the deep Yellowstone River valley captured the southeast-oriented flood flow and diverted the flood waters northeast. Flood flow entering the newly eroded Yellowstone River valley eroded the southeast-oriented Yellowstone River tributary valleys and the southeast-sloping surface into which those valleys were eroded. Subsequently headward erosion of the deep Musselshell River valley captured the southeast-oriented flood flow and diverted the flood waters further to the northeast. Flood waters on the northwest ends of beheaded flood flow routes reversed flow direction to flow northwest into the newly eroded and deep northeast-oriented Musselshell River valley. The northwest-oriented Hawk Creek valley was eroded by such reversed flood flow. The northeast-oriented Hawk Creek valley segment eroded southwest from northwest-oriented reversed flood flow to capture yet to be beheaded southeast-oriented flood flow further to the south and southwest (remember, the deep Musselshell River valley was eroding headward from the northeast to the southwest and southeast-oriented flood flow routes were beheaded in sequence from the northeast to the southwest). The drainage divide between Hawk Creek and Deadman Creek is approximately 120 meters higher than where Hawk Creek joins the Musselshell River and 200 meters higher than where Buffalo Creek joins the Yellowstone River. These elevations provide some minimum estimates of the amount erosion accomplished when the deep Yellowstone  River and Musselshell River valleys eroded headward into the figure 3 map region.

Dry Fork Hawk Creek-Hibbard Creek drainage divide area

Figure 4: Dry Fork Hawk Creek-Hibbard Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 4 illustrates the Dry Fork Hawk Creek-Hibbard Creek drainage divide area south and west of the figure 3 map area and includes overlap areas with figure 3. Southeast-oriented Deadman Creek is located in the figure 4 northeast corner. Southeast-oriented drainage flowing to the figure 4 east edge flows to the northeast-oriented Yellowstone River. Hawk Creek flows east-southeast in the figure 4 northwest corner area and then turns northeast in the figure 4 north center area to flow into the figure 3 map area above. South of Hawk Creek is the east-southeast and northeast oriented Dry Fork Hawk Creek. Southeast-oriented drainage south of the Dry Fork Hawk Creek flows to the Yellowstone River. Note how headward erosion of the northeast-oriented Hawk Creek valley segment captured southeast-oriented flood flow and in doing so beheaded multiple southeast-oriented flood flow routes moving flood waters into what was then the newly eroded and deep northeast-oriented Yellowstone River valley. Yet to be beheaded and reversed southeast-oriented flood waters were captured by headward erosion of the northeast-oriented Hawk Creek valley and were moved northeast to newly  beheaded and reversed flood flow on the northwest-oriented Hawk Creek valley segment alignment, which was moving reversed flood flow to the actively eroding deep northeast-oriented Musselshell River valley head. Subsequently headward erosion of the deep northeast-oriented Musselshell River valley head beheaded flood flow routes to the east-southeast oriented Hawk Creek headwaters valley and then to the east-southeast oriented Dry Fork Hawk Creek headwaters valley. Figure 5 below illustrates the Dry Fork Hawk Creek-Hibbard Creek drainage divide area in more detail. The flood water source cannot be determined from evidence presented here. However, hundreds of Missouri River drainage basin research project essays when taken as a group can be used to trace flood flow toward the flood water source and also downstream to see where flood waters were moving. A logical flood water source would be rapid melting of a thick North American ice sheet located in approximately the North American location usually recognized to have been glaciated. If that thick North American ice sheet had created a deep “hole” through crustal warping caused by its weight and by deep glacial erosion, headward erosion of deep northeast-oriented valleys would be a logical geologic process to capture the immense southeast-oriented melt water floods and divert flood waters further and further northeast into space in the deep “hole” being opened up as the thick ice sheet was rapidly melting. The northern Missouri River drainage basin would then be the deeply eroded southwest wall of the thick North American ice sheet created deep “hole”.

Detailed map of Dry Fork Hawk Creek-Hibbard Creek drainage divide area

Figure 5: Detailed map of Dry Fork Hawk Creek-Hibbard Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 5 illustrates a detailed map of the Dry Fork Hawk Creek-Hibbard Creek drainage divide illustrated in less detail in figure 4 above. Southeast-oriented drainage east of Steamboat Butte (east half of figure 5 southwest quadrant) flows to southeast-oriented Hibbard Creek, which flows to the northeast-oriented Yellowstone River. The Dry Fork Hawk Creek flows east and northeast in the figure 5 north half. Note multiple through valleys notched into the present day Dry Fork Hawk Creek-Hibbard Creek drainage divide. Two of the easiest through valleys to identify are those used by the two roads crossing the drainage divide. A close look at the drainage divide reveals many other similar through valleys, although elevations of the floors of each valley vary. The multiple through valleys provide evidence the Dry Fork Hawk Creek-Hibbard Creek drainage divide was eroded by multiple channels of southeast-oriented flood flow moving from what is today the Dry Fork Hawk Creek valley to what was then the actively eroding southeast-oriented Hibbard Creek drainage basin (and the northeast-oriented Yellowstone River valley). The multiple channels also suggest flood waters were eroding an ever-changing complex of anastomosing (or interconnected) channels that were being captured by headward erosion of the Hibbard Creek valley. Southeast-oriented flood flow to the Hibbard Creek valley moving across this figure 5 map area came from northwest of the present day deep northeast-oriented Musselshell River valley, which means the deep northeast-oriented Musselshell River valley northwest of the figure 5 map area did not yet exist. Southeast-oriented flood flow into the actively eroding Hibbard Creek drainage basin was beheaded from east to west by headward erosion of the east-oriented Dry Fork Hawk Creek valley, which diverted the flood flow northeast and then northwest to what was then the deep and actively eroding northeast-oriented Musselshell River valley head. As the deep and actively eroding Musselshell River valley head progressed to the southwest it beheaded southeast-oriented flood flow to the newly eroded Dry Fork Hawk Creek valley and ended flood flow to the figure 5 map region.

Halfbreed Creek-Railroad Creek drainage divide area near Dunn Mountain

Figure 6: Halfbreed Creek-Railroad Creek drainage divide area near Dunn Mountain. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 6 illustrates the Halfbreed Creek-Railroad Creek drainage divide area near Dunn Mountain west and slightly south of the figure 4 map area and includes overlap areas with figure 4. East-oriented headwaters of Hawk Creek are located in the figure 6 northeast corner. South of those headwaters are east-southeast oriented headwaters of Dry Fork Hawk Creek, which are located southeast of Three Buttes. Southeast-oriented drainage in the figure 6 southeast quadrant flows to southeast-oriented Railroad Creek, which flows to the northeast-oriented Yellowstone River. Northwest of Dunn Mountain is northwest-oriented drainage to northwest-oriented Rehder Coulee, which flows to northwest-oriented Halfbreed Creek, which in turn flows to the northeast-oriented Musselshell River near Roundup, Montana. North-oriented drainage north of Dunn Mountain also flows to the northeast-oriented Musselshell River. Dunn Mountain stands approximately 450 meters higher than the present day elevation where Halfbreed Creek joins the Musselshell River and approximately 570 meters higher than the elevation where Railroad Creek joins the Yellowstone River. Again multiple through valleys cross the present day Musselshell River-Yellowstone River drainage divide and provide evidence multiple channels of southeast-oriented flood waters once flowed across the drainage into what was then the actively eroding Railroad Creek drainage basin and the actively eroding Yellowstone River valley. Figure 7 below illustrates in detail through valleys in the Dunn Mountain area. The flood waters came from somewhere northwest of the present day deep Musselshell River to the north and northwest of figure 6, which means at the time flood waters crossed the present day Musselshell River-Yellowstone River drainage divide the deep northeast-oriented Musselshell River valley did not exist. The deep northeast-oriented Musselshell River valley eroded headward into the region north of figure 6 to capture the southeast-oriented flood flow and to divert the flood water northeast and north to what was then an actively eroding Missouri River valley (which may have flowed northeast rather than east and southeast in the Montana northeast corner). Headward erosion of the deep Musselshell River valley and the reversal of flood flow on the northwest ends of beheaded southeast-oriented flood flow routes created the present day Musselshell River-Yellowstone River drainage divide and also eroded north and northwest-oriented Musselshell River tributary valleys.

Detailed map of Halfbreed Creek-Railroad Creek drainage divide area near Dunn Mountain

Figure 7: Detailed map of Halfbreed Creek-Railroad Creek drainage divide area near Dunn Mountain. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 7 illustrates a detailed map of the Rehder Coulee-Railroad Creek drainage divide area near Dunn Mountain, which was shown in less detail in figure 6 above. Railroad Creek flows to the figure 7 southeast corner. Rehder Coulee drains to the figure 7 northwest corner. Note the shallow through valley eroded into the high Dunn Mountain ridge linking southeast-oriented Railroad Creek with northwest-oriented Rehder Coulee. That through valley, even though it is now notched into the highest ridge in the region, provides evidence southeast-oriented flood water once flowed across the Dunn Mountain upland ridge. In other words, at one time southeast-oriented flood flow moved across the figure 7 map area on a topographic surface at least as high as the highest Dunn Mountain elevations today. Headward erosion of deep southeast-oriented valleys from what was then the newly eroded and deep northeast-oriented Yellowstone River valley was responsible for eroding the region southeast of the present day Musselshell River-Yellowstone River drainage divide. Reversed flow on the northwest ends of beheaded southeast-oriented flood flow routes was responsible for eroding the region northwest of the present day drainage divide. Headward erosion of the Musselshell River valley was responsible for beheading the southeast-oriented flood flow routes and the Musselshell River valley eroded headward from the northeast to the southwest. Flood waters moved in anastomosing (or interconnected) channels, meaning reversed flood flow in one channel could capture yet to be beheaded flood flow from adjacent channels. Headward erosion of the deep Musselshell River valley beheaded the southeast-oriented flood flow channels one channel at a time, which meant each beheaded and reversed flood flow channel had opportunities to capture yet to beheaded flood flow from adjacent channels further to the southwest. Note northeast-oriented valleys linking the various north and northwest-oriented drainage routes in figure 7. These northeast-oriented valleys were eroded by yet to be beheaded southeast-oriented flood flow that had been captured by reversed flood flow in newly beheaded flood flow channels. Captures of yet to be beheaded flood flow played a significant role in eroding the area northwest of the present day drainage divide.

Goulding Creek-Razor Creek drainage divide area

Figure 8: Goulding Creek-Razor Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 8 illustrates the Golding Creek-Razor Creek drainage divide area west and south of the figure 6 map area and includes overlap areas with figure 6. The Musselshell River-Yellowstone River drainage divide in figure 8 extends northeast from the figure 8 southwest corner. Drainage northwest of the drainage divide flows to north-oriented Goulding Creek, which flows to the northeast-oriented Musselshell River. Note northwest-oriented Goulding Creek tributary valleys originating along the drainage divide. Drainage southeast of the drainage divide flows to southeast-oriented Razor Creek, which flows to the northeast-oriented Yellowstone River. Note south and southeast-oriented Razor Creek tributary valleys originating along the drainage divide. Also note shallow through valleys linking headwaters of the northwest-oriented Goulding Creek tributaries with the headwaters of the south and southeast-oriented Razor Creek tributaries. These shallow through valleys provide evidence multiple channels of southeast-oriented flood flow once moved across the present day Goulding Creek-Razor Creek drainage divide. To do so flood waters flowed on a topographic surface at least as high as the highest elevations on the drainage divide surface today, which means the north-oriented Goulding Creek valley did not exist at that time, nor did the deep northeast-oriented Musselshell River valley exist at that time. Headward erosion of the deep Musselshell River valley beheaded the southeast-oriented flood flow and the north-oriented Goulding Creek valley was eroded south to capture yet to be beheaded southeast-oriented flood flow moving in channels west and south of what was then the actively eroding Musselshell River valley head. Northwest-oriented Goulding Creek tributary valleys were eroded by reversals of flood flow on southeast-oriented flood flow routes beheaded by Goulding Creek valley headward erosion. Because the Goulding Creek valley eroded from north to south each major reversed flow channel captured yet to be beheaded southeast-oriented flood flow from adjacent channels further to south. Such captured yet to be beheaded flood flow played a significant role in eroding northwest-oriented Goulding Creek tributary valleys.

Big picture view of Goulding Creek-Razor Creek drainage divide area

Figure 9: Big picture view of Goulding Creek-Razor Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 8 uses reduced maps to provide a big picture view of the Goulding Creek-Razor Creek drainage divide area illustrated in figure 8 as well as the Halfbreed Creek-Railroad Creek drainage divide area illustrated in figure 6. The northeast-oriented Musselshell River is located in the figure 9 northwest corner. The northwest oriented stream flowing from the figure 9 west center area to flow north along the figure 9 west edge to the Musselshell River is Dean Creek. The north-oriented Musselshell River tributary with numerous northwest-oriented tributaries located east of Dean Creek is Goulding Creek. Halfbreed Creek flows northwest along the highway to the figure 9 north edge and then to the northeast-oriented Musselshell River. Dunn Mountain is the upland located in eastern end of the figure 9 center area and northwest-oriented Rehder Coulee can be seen draining from the Dunn Mountain area to northwest-oriented Halfbreed Creek located by the north-oriented highway. The Dunn Mountain upland forms northwest rim of an arc-shaped southeast-oriented escarpment-surrounded basin now drained by headwaters of southeast-oriented Railroad Creek. The Dunn Mountain upland area also forms the north rim of a larger southeast-oriented escarpment-surrounded basin now drained by headwaters of southeast-oriented Razor Creek. The southeast-oriented Railroad Creek and Razor Creek escarpment-surrounded basins are large abandoned headcuts that were eroded by southeast-oriented flood waters moving into what was then the newly eroded northeast-oriented Yellowstone River valley. The size of these abandoned headcuts provides evidence as to the magnitude of the southeast-oriented flood involved. The southeast-oriented flood waters were flowing on a topographic surface at least as high as the highest figure 9 elevations today, which means the deep northeast-oriented Musselshell River valley did not exist. Probably the high level topographic surface extended far to the southeast and the deep northeast-oriented Yellowstone River valley was eroded into that topographic surface. Flood waters then eroded the newly eroded Yellowstone River valley north wall and as flood waters were doing so headward erosion of the deep northeast-oriented Musselshell River valley captured the flood flow and diverted the flood water further to the northeast. Figure 10 below illustrates the Hay Basin area located in the figure 9 southwest corner.

Dean Creek-West Fork Razor Creek drainage divide area at Hay Basin

Figure 10 illustrates the Dean Creek-Razor Creek drainage divide area at the northeast end of Hay Basin, which was illustrated in the figure 9 big picture view above. East-oriented Fulcher Creek flows to northwest-oriented Dean Creek in the figure 10 northwest quadrant. South and south-southeast oriented headwaters of the West Fork of Razor Creek begin in the figure 10 northeast quadrant and flow to southeast-oriented West Fork Razor Creek located in the figure 10 east center. Hay Basin is a remarkable southeast-oriented basin, which also is an abandoned southeast-oriented headcut. In the case of Hay Basin flood waters were stripping the landscape and had not eroded as deep as in the case of the much deeper Railroad Creek abandoned headcut or in the much deeper Razor Creek abandoned headcut, which is partially seen east of Hay Basin. The large abandoned headcuts located along the Musselshell River-Yellowstone River drainage divide in Musselshell and Yellowstone Counties drainage divide area provide evidence of an immense southeast-oriented flood that was first captured by headward erosion of the deep northeast oriented Yellowstone River valley and then while flood waters were still eroding the newly eroded Yellowstone River north valley wall the flood waters were captured by headward erosion of the deep northeast and north-oriented Musselshell River valley and diverted further to the northeast. The immense southeast-oriented flood can be explained by rapid melting of a thick North American ice sheet. Headward erosion of the deep northeast oriented Yellowstone River valley and the deep northeast- and north-oriented Musselshell River valley can be explained if the rapidly melting thick North American ice sheet was located in a deep “hole” and if deep valleys were eroding headward into that deep “hole’s” southwest wall to capture the immense southeast-oriented floods and to divert the flood waters further and further northeast into space the rapidly melting ice sheet had once occupied.

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.