Missouri River-Musselshell River drainage divide area landform origins in Fergus and Petroleum Counties, Montana, USA

Authors

A geomorphic history based on topographic map evidence

Abstract:

The Missouri River-Musselshell River drainage divide area discussed here is located in northeast Fergus and northern Petroleum Counties, Montana, USA. Although detailed topographic maps of the Missouri River-Musselshell 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 Missouri River-Musselshell 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 Missouri River valley captured all southeast-oriented flood flow.

Preface:

The following interpretation of detailed topographic map evidence is provided as evidence in the Missouri River drainage basin landform origins research project, which is compiling similar evidence for all major drainage divides contained within the Missouri River drainage basin and for all major drainage divides with and within certain adjacent drainage basins. The research project is interpreting evidence in the context of a previously unexplored geomorphology paradigm, which is briefly described in the introduction below. Project essays are listed on the sidebar category list under their appropriate Missouri River tributary drainage basin, Missouri River segment drainage basin (by state), and/or state in which the Missouri River drainage basin is located.

Introduction:

  • The purpose of this essay is to use topographic map interpretation methods to explore Montana Missouri River-Musselshell River drainage divide area landform origins. 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 Missouri River-Musselshell River drainage divide area landform evidence in Montana will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm.

Missouri River-Musselshell River drainage divide area location map

Figure 1: Missouri River-Musselshell 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 Missouri River-Musselshell River drainage divide area location map and illustrates a region in Montana. The Missouri River flows east-southeast from the figure 1 west edge to Fort Benton and then to Fort Peck Lake before joining the southeast-oriented Milk River and flowing to the figure 1 east edge. The Musselshell River flows from the Little Belt Mountain area in the figure 1 southwest corner to Harlowton, Lavina, Roundup, and Melstone and then turns north to flow to Mosby and to join the Missouri River at Fort Peck Lake. The Missouri River-Musselshell River drainage divide area discussed here is located north of southeast-oriented Box Elder Creek, which flows to join the north-oriented Musselshell River valley segment near Mosby, and east of where northeast-oriented Armells Creek joins the Missouri River (located west of Fort Peck Lake). The drainage divide area discussed here includes the Missouri River-Crooked Creek drainage divide area (Crooked Creek has been renamed and is now the Sacagawea River on newer maps including detailed topographic maps used in this essay) and the Crooked Creek-Musselshell River drainage divide area northeast of Box Elder Creek.

  • The Missouri River-Musselshel River drainage divide area west of the area discussed is addressed in other essays by looking at drainage divides between various Missouri River and Musselshell River tributaries (including Armells Creek and the Judith River, among others). Based on evidence from the 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 east-oriented Missouri River valley eroded west to the location of UL Bend (where the Musselshell River joins the Missouri River) and the north-oriented Musselshell River valley eroded south and southwest to capture southeast-oriented flood water and to divert flood flow to the northeast. Southeast-oriented tributary valleys eroded headward from the newly eroded north-oriented Musselshell River valley and the east-oriented Crooked Creek (Sacagawea River) valley eroded west to behead southeast-oriented flood flow routes to the newly eroded north-oriented Musselshell River valley. Subsequently the Missouri River valley eroded northwest to capture southeast-oriented flood flow that had been moving to what was then the newly eroded Crooked Creek (or Sacagawea River) valley. Headward erosion of the deep Missouri River valley beheaded all southeast-oriented flood flow to the Missouri River-Musselshell River drainage divide area discussed in this essay.  The Missouri River-Big Dry Creek drainage divide area essay, the Big Dry Creek-Prairie Elk Creek drainage divide area essay, the Musselshell River-Yellowstone River drainage divide area in Musselshell and Yellowstone Counties essay, the Musselshell River-Yellowstone River drainage divide area in Treasure and Rosebud Counties essay, and in the Big Dry Creek-Yellowstone River drainage divide area essay describe drainage divide areas located near the Missouri River-Musselshell River drainage divide area discussed here and can be located under appropriate river names on the sidebar category list.

Missouri River-Musselshell River drainage divide area detailed location map

Figure 2: Missouri River-Musselshell 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 Missouri River-Musselshell River drainage divide area discussed in this essay. Fergus and Petroleum Counties are located in Montana. The Missouri River forms the county line in the figure 2 north center and northeast areas and flows southeast, east, and southeast to join the north-oriented Musselshell River at UL Bend and then to flow northeast to Fort Peck Lake in the figure 2 northeast corner. Crooked Creek (Sacagawea River) originates in the northeast corner of Fergus County and flows southeast, northeast, east, and southeast to join the north-oriented Musselshell River just south of UL Bend. Antelope Creek is a northeast oriented Crooked Creek tributary important to the discussion here. The Musselshell River flows north in the figure 2 east half from the figure 2 south edge to Mosby and then to join the Missouri River at UL Bend. Box Elder Creek, which forms the southwest limit of the Missouri River-Musselshell River drainage divide area discussed here, originates in Fergus County north of the Judith Mountains and then flows southeast through Petroleum County to join the north-oriented Musselshell River as a barbed tributary in the figure 2 southeast quadrant. Other southeast-oriented Musselshell River tributaries important in the discussion here include Drag Creek, Dovetail Creek, Blood Creek, Cottonwood Creek, and Cat Creek. This essay begins by looking at evidence along the Missouri River-Crooked Creek (Sacagawea River) drainage divide area. The essay continues by looking at evidence along the Crooked Creek (Sacagawea River)-Drag Creek and Dovetail Creek drainage divide, the Drag Creek-Dovetail Creek drainage divide, Antelope Creek-Blood Creek drainage divide, Blood Creek-Cottonwood Creek drainage divide, and concludes with the Cottonwood Creek-Cat Creek drainage divide area. Almost all Musselshell River tributaries in the region discussed here are southeast-oriented and join the north-oriented Musselshell River as barbed tributaries. The southeast-oriented tributary valleys were eroded by southeast-oriented flood flow moving into the newly eroded north-oriented Musselshell River valley. Southeast-oriented flood flow to what were then actively eroding southeast-oriented tributary valleys was beheaded first by headward erosion of the northeast, east, and southeast-oriented Crooked Creek (Sacagawea River)-Antelope Creek valley and subsequently by headward erosion of the deeper southeast-oriented Missouri River valley

East end of Missouri River-Sacagawea River drainage divide area

Figure 3: East end of Missouri River-Sacagawea River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 3 illustrates the east end of the Missouri River-Sacagawea River (Crooked Creek) drainage divide area. The flooded east and south-oriented Missouri River valley extends from the figure 3 northwest corner to the figure 3 southeast corner where the Missouri River joins the north oriented Musselshell River and then flows northeast. The Sacagawea River (Crooked Creek) flows east along the figure 3 south edge and then turns southeast to join the north oriented Musselshell River as a barbed tributary just south of the figure 3 map area (and also where the Musselshell River joins the Missouri River). Note southeast-oriented tributaries to the east oriented Sacagawea River and the south-oriented Missouri River valley segment. Also note northwest-oriented headwaters of tributaries to the east oriented Missouri River valley. The northwest-southeast orientation of these tributary valleys is evidence the Missouri River-Sacagawea River drainage divide was crossed by multiple southeast-oriented flood flow channels prior to headward erosion of the deep east oriented Missouri River valley. Figure 3 evidence can be explained in the context of an immense southeast-oriented flood moving across the entire figure 3 map area on a topographic surface at least as high as the highest present day figure 3 elevations. Headward erosion of the deep north and northeast oriented Musselshell River-Missouri River valley south and east of figure 3 (and across the figure 3 southeast corner) first captured the southeast-oriented flood flow and diverted the flood waters northeast. The deep Scagawea River valley then eroded northwest and west from newly eroded Musselshell River valley and southeast-oriented flood flow eroded southeast-oriented tributary valleys into the newly eroded Sacagawea River valley north wall. The deep Missouri River valley then eroded north, northwest, and west from the newly eroded Musselshell River valley and captured the southeast-oriented flood flow. Northeast-oriented Missouri River tributary valleys eroded southwest from the actively eroding Missouri River valley head to capture southeast-oriented flood flow, although headward erosion of the Missouri River valley soon beheaded flood flow to shorter northeast-oriented tributary valleys. Flood waters on the northwest ends of beheaded flood flow routes reversed flow direction to flow north and northwest to the newly eroded and deeper east- and southeast oriented Missouri River valley.

East end of Carroll Coulee-Sacagawea River drainage divide area

Figure 4: East end of Carroll Coulee-Sacagawea River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 4 illustrates the east end of the Carroll Coulee-Sacagawea River (Crooked Creek) drainage divide area located west of the figure 3 map area and includes overlap areas with figure 3. The east-northeast oriented Sacagawea River flows from the figure 4 south center area through the figure 4 southeast quadrant. The east-oriented Missouri River is located along the figure 4 north edge. Carroll Coulee is located in the figure 4 northwest quadrant and drains east and northeast to the Missouri River. Note southeast-oriented Sacagawea River tributaries from the north and northwest-oriented headwaters of Carroll Coulee tributaries from the south. Detailed maps show shallow saddles or through valleys eroded across the ridge that now serves as the Carroll Coulee-Sacagawea River drainage divide. The northwest-southeast orientation of tributary valleys and the shallow through valleys notched into the present day drainage divide provide evidence southeast-oriented flood flow once moved across the present day Carroll Coulee-Sacagawea River drainage divide. Figure 4 evidence can be explained in the context of southeast-oriented flood flow moving across the entire 4 map area on a topographic surface at least as high as the highest figure 4 elevations today. Headward erosion of the deep east-northeast oriented Sacagawea River valley into that high level topographic surface captured the southeast-oriented flood flow and diverted the water east to the north and northeast-oriented Musselshell River-Missouri River valley. Subsequently headward erosion of the deep east-oriented Missouri River-Carroll Coulee valley along the figure 4 north edge and into the figure 4 northwest quadrant captured the southeast-oriented flood flow before it could reach the newly eroded Sacagawea River valley. Flood waters on the northwest ends of the beheaded flood flow routes reversed flow direction to flow northwest into the newly eroded Carroll Coulee-Missouri River valley. The reversed flood flow eroded the northwest-oriented Carrol Coulee tributary valleys and created the present day Carroll Coulee-Sacagawea River drainage divide. At the same time, although slightly slower than the Carroll Coulee valley headward erosion, the deep Missouri River valley eroded headward and beheaded all flood flow to the newly eroded Carroll Coulee valley.

Sand Creek-Sacagawea River drainage divide area

Figure 5: Sand Creek-Sacagawea River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 5 illustrates the Sand Creek-Sacagawea River (Crooked Creek) drainage divide west of the figure 4 map area and includes overlap areas with figure 4. Southeast-oriented Little Crooked Creek is located in the figure 5 south center area and flows to the Sacagawea River (Crooked Creek) south of the figure 5 map area. Other southeast-oriented drainage along the figure 5 south edge also flows to the Sacagawea River. The east-southeast oriented Missouri River is located in the figure 5 north half. Sand Creek is the northeast-oriented Missouri River tributary flowing from the figure 5 west center area. Armells Creek is the northeast-oriented Missouri River tributary located in the figure 5 northwest corner. Carroll Coulee flows east in the figure 5 east center area. Note southeast-oriented Sand Creek tributaries and how headwaters of those tributaries are linked to headwaters of northwest-oriented Armells Creek tributaries and also note the northwest-oriented Sand Creek tributaries and how headwaters of those tributaries are linked to southeast-oriented Sacagawea River tributaries (or Carroll Creek tributaries). Figure 5 evidence can also be explained in the context of an immense southeast-oriented flood flowing across the entire figure 5 map area on a topographic surface at least as high as the highest figure 5 elevations today. Headward erosion of the deep Sacagawea River valley south of the figure 5 map area first captured the southeast-oriented flood flow. Next headward erosion of Missouri River valley-Carroll Creek valley captured flood flow to the newly eroded Sacagawea River valley, although headward erosion of the Carroll Creek valley was halted when headward erosion of the Missouri River-Sand Creek valley beheaded all southeast-oriented flood flow routes supplying flood water to what had been the actively eroding Carroll Creek valley head. Subsequently headward erosion of the Missouri River-Armells Creek valley beheaded southeast-oriented flood flow routes to the newly eroded Sand Creek valley.

Sacagawea River-Dovetail Creek drainage divide area

Figure 6: Sacagawea River-Dovetail Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 6 illustrates the Sacagawea River- Dovetail Creek and the Sacagawea River-Drag Creek drainage divide area south of the figures 3 and 4 map areas and includes overlap areas with figures 3 and 4. East-northeast and east-oriented Sacagawea River (Crooked Creek) is located in the figure 6 north half. Southeast-oriented Dovetail Creek flows through the figure 6 south center area to the figure 6 south edge. Southeast-oriented Drag Creek flows the figure 6 east center area to the figure 6 east edge. Dovetail Creek and Drag Creek flow to the north oriented Musselshell River (see figure 7 below). Note how tributaries from the north to southeast-oriented Dovetail Creek and southeast-oriented Drag Creek are predominantly southeast-oriented. Also note northwest-oriented headwaters to north and northeast-oriented Sacagawea River tributaries. The Sacagawea River-Dovetail Creek and the Sacagawea River-Drag Creek drainage divides are well-defined ridges although a close look at figure 6 (and/or at more detailed topographic maps of the drainage divide areas) reveals numerous through valleys eroded across the ridge. The through valleys and the tributary orientations provide evidence multiple channels of southeast-oriented flood flow once moved across the present day Sacagawea River-Dovetail Creek and the Sacagawea River-Drag Creek drainage divides. Figure 6 evidence can be explained by an immense southeast-oriented flood flowing across the entire figure 6 map area on a topographic surface at least as high as the highest figure 6 elevations today. Headward erosion of the deep north oriented Musselshell River valley east of the figure 6 map area captured the flood flow and diverted the flood waters north and northeast. The southeast-oriented Drag Creek and Dovetail Creek valleys then eroded headward from the newly eroded north oriented Musselshell River valley. Headward erosion of the deep east-northeast and east-oriented Sacagawea River valley and its north- and northeast-oriented tributary valleys then beheaded southeast-oriented flood flow routes supplying flood waters to the then actively eroding Drag Creek and Dovetail Creek valleys. Flood flow to the Drag Creek valley was beheaded first and flood flow to the Dovetail Creek valley was beheaded second as the Sacagawea River valley eroded west-southwest. .

Drag Creek-Dovetail Creek drainage divide area

Figure 7: Drag Creek-Dovetail Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 7 illustrates the Drag Creek-Dovetail Creek southeast of the figure 6 map area and includes overlap areas with figure 6. The north-oriented Musselshell River is located in the figure 7 east half and the valley has been flooded (by the Fort Peck Lake reservoir) in the northeast corner. Southeast-oriented Drag Creek flows from the figure 7 northwest corner to join the north-oriented Musselshell River as a barbed tributary in the figure 7 east center area. Further south, southeast-oriented Dovetail Creek flows from the figure 7 west edge to join the north-oriented Musselshell River as a barbed tributary in the figure 7 southeast quadrant. Note northwest-oriented Musselshell River tributaries from the east. Also note southeast-oriented Drag Creek and Dovetail Creek tributaries from the north and southeast-oriented (barbed) Musselshell River tributaries north of Drag Creek. Headward erosion of the Drag Creek valley beheaded some flood flow routes to the Dovetail Creek valley indicating Dovetail Creek valley headward erosion may have begun slightly in advance of Drag Creek headward erosion. Figure 7 evidence can again be interpreted in the context of an immense southeast-oriented flood flowing across the entire figure 7 map area on a topographic surface at least as high as the highest figure 7 elevations today. Headward erosion of the deep north-oriented Musselshell River valley captured the southeast-oriented flood flow and flood waters on the northwest ends of the beheaded southeast-oriented flood flow routes reversed flow direction to flow northwest and to erode the northwest-oriented Musselshell River tributary valleys seen along the figure 7 east edge. At the same time large southeast-oriented Musselshell River tributary valleys were eroded by the southeast-oriented flood waters flowing into the newly eroded north-oriented Musselshell River valley. Headward erosion of the east-northeast oriented Sacagawea River valley (as seen in figure 6) then beheaded the southeast-oriented flood flow routes to what were then the actively eroding Drag and Dovetail Creek valleys.

Antelope Creek-Blood Creek drainage divide area

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

Figure 8 illustrates the Antelope Creek-Blood Creek drainage divide southwest of the figure 6 map area and includes overlap areas with figure 6. Antelope Creek originates southeast of the southwest-northeast oriented highway segment and flows east, northeast, northwest, and then east-northeast across the figure 8 north center area and finally flows northeast to the figure 8 north edge. North and east of figure 8 Antelope Creek joins the east-northeast and southeast oriented Sacagawea River (Crooked Creek). Blood Creek flows southeast from the figure 8 center area to Valentine and then to the figure 8 east edge (south half) and continues to join the north-oriented Musselshell River (see figure 9 below). Sage Creek is a southeast-oriented Blood Creek tributary located south of Blood Creek in the figure 8 southeast corner. The southeast-oriented stream flowing from the figure 8 west center edge to the figure 8 south center edge is Box Elder Creek, which also flows to the north-oriented Musselshell River as a barbed tributary. Note how Antelope Creek headwaters captured southeast-oriented flood flow that had been moving to the southeast-oriented Blood Creek valley. A close look at figure 8 reveals an Antelope Creek tributary headwaters begin almost on the edge of the Antelope Creek valley (figure 8a below provides a detailed map of the location). Note in figure 8a below the through valley linking the Antelope Creek valley with the southeast-oriented Blood Creek tributary valley. Also note on figure 8 other through valleys linking the Antelope Creek valley with various southeast-oriented drainage routes. Figure 8 (and figure 8a) evidence can again be explained in the context of an immense southeast-oriented flood that was first captured by headward erosion of the deep north-oriented Musselshell River valley. The Blood Creek and Box Elder Creek valleys and their tributary valleys then eroded northwest from the newly eroded north-oriented Musselshell River valley. Headward erosion of the Blood Creek valley beheaded southeast-oriented flood flow to the southeast-oriented Sage Creek valley. Subsequently headward erosion of the Sacagawea River-Antelope Creek valley beheaded southeast-oriented flood flow to what had been the actively eroding Blood Creek valley. Apparently the northeast-oriented Antelope Creek valley was not deep enough to capture southeast-oriented flood flow in the Box Elder Creek valley.

Figure 8a: Detailed map of Antelope Creek-Blood Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Blood Creek-Cottonwood Creek drainage divide area

Figure 9: Blood Creek-Cottonwood Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 9 illustrates the Blood Creek-Cottonwood Creek drainage divide area south of the figure 7 map area and southeast of the figure 8 map area and there is a very narrow overlap area with figure 7. The north-oriented Musselshell River is located along the figure 9 east edge. Blood Creek flows east across the figure 9 north half to join the Musselshell River in the figure 9 northeast corner area. East-southeast oriented Cottonwood Creek is located in the figure 9 south half and joins the north-oriented Musselshell River as a barbed tributary just south of the figure 9 southeast corner (see figure 10 below). Figure 9 evidence is similar to figure 7 evidence and can be explained by the same immense southeast-oriented flood being captured by headward erosion of the north-oriented Musselshell River valley and then headward erosion of the Cottonwood Creek valley and the Blood Creek valley from the newly eroded Musselshell River valley. Headward erosion of the Blood Creek valley beheaded southeast-oriented flood flow to the Cottonwood Creek valley indicating that for a time Cottonwood Creek valley headward erosion may have been slightly in advance of Blood Creek valley headward erosion. The source of the southeast-oriented flood waters cannot be determined from evidence presented here. However, the hundreds of Missouri River drainage basin landform origins research project essays  when taken as a group can be used to trace flood waters both up flood to source areas and down flood to see where flood waters were going. A logical flood water source would be rapid melting of a thick North American ice sheet located in a deep “hole” occupying approximately the North American location usually recognized to have been glaciated. The deep “hole” would have been created by deep glacial erosion and by crustal warping caused by the ice sheet weight. Such a flood water source would not only explain the immense southeast-oriented floods this essay series describes, but would also explain why deep valleys were eroding headward to capture the southeast-oriented flood waters and diverting the flood waters further and further to the northeast and north into space in the deep “hole” the rapidly melting thick ice sheet had once occupied.

Cottonwood Creek-Cat Creek drainage divide area

Figure 10: Cottonwood Creek-Cat Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 10 illustrates the Cottonwood Creek-Cat Creek drainage divide area south of the figure 9 map area and includes overlap areas with figure 9. The north-oriented Musselshell River is located along the figure 10 east edge. Southeast-oriented Cottonwood Creek flows from the figure 10 northwest corner to join the north-oriented Musselshell River as a barbed tributary. Cat Creek flows southeast from the figure 10 northwest quadrant to join the north-oriented Musselshell River as a barbed tributary just east of the figure 10 southeast corner. Southeast-oriented Box Elder Creek can be seen in the figure 10 southwest corner. Streamlined ridges in the Cat Creek drainage basin suggest Cat Creek is flowing across a northwest-southeast oriented geologic structure and the region east of the town of Cat Creek is identified on more detailed topographic maps as the West Dome Cat Creek Oil Field, further suggesting the presence of northwest-southeast oriented hogback ridges. However, the region has been eroded in a manner that suggests southeast oriented flood waters moved across the Cat Creek drainage basin in a southeast oriented anastomosing channel complex. Figure 10a below illustrates a more detailed topographic map of the region northwest of the town of Cat Creek to show the present day pattern of through valleys. Note multiple through valleys linking southeast oriented Cat Creek with southeast- and north-oriented Long Coulee. In figure 10 above the southeast-oriented Long Coulee valley turns abruptly north to join the north-northwest oriented Musselshell River valley. The multiple through valleys suggest headward erosion of the southeast-oriented Cat Creek valley beheaded multiple channels of flood flow which had been captured by headward erosion of the north-oriented Long Coulee valley. Had resistant rock units not been encountered, the north-oriented Long Coulee valley segment probably would have succeeded in eroding south and would today be the Musselshell River valley. However, the presence of resistant rock units slowed headward erosion of the north-oriented Long Coulee valley and the present day Musselshell River valley to east was eroded instead. The southeast-oriented Cat Creek valley then eroded headward from the successful eastern valley along flood flow routes south of the actively eroding Long Coulee valley head and then eroded northwest to behead southeast-oriented flood flow routes that had previously been captured by headward erosion of the north-oriented Long Coulee valley.

Figure 10a: Anastomosing channel evidence in Cat Creek drainage basin. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

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.

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