Flatwillow Creek-North Willow and Willow Creek drainage divide area landform origins in Musselshell, Fergus, and Petroleum Counties, Montana, USA

· Montana, Musselshell River
Authors

A geomorphic history based on topographic map evidence

Abstract:

The Flatwillow Creek-North Willow Creek and Willow Creek drainage divide area discussed here is located in Musselshell, Fergus and Petroleum Counties, Montana, USA. Although detailed topographic maps of the Flatwillow Creek-North Willow Creek and Willow Creek 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 Flatwillow Creek-North Willow Creek and Willow Creek 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 Flatwillow Creek-North Willow Creek and Willow Creek drainage divide area landform origins in Musselshell, Fergus, and Petroleum Counties, Montana, USA. Map interpretation methods can be used to unravel many geomorphic events leading up to formation of present-day drainage routes and development of other landform features. While each detailed topographic map feature provides detailed evidence to be explained, the solution must be consistent with explanations for adjacent area map evidence as well as solutions to big picture map evidence puzzles. I invite readers to improve upon my solutions 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 Flatwillow Creek-North Willow Creek and Willow Creek drainage divide area landform evidence will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm. This essay is included in the Missouri River drainage basin landform origins research project essay collection.

Flatwillow Creek-North Willow Creek and Willow Creek drainage divide area location map

Figure 1: Flatwillow Creek-North Willow Creek and Willow Creek 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 Flatwillow Creek-North Willow Creek and Willow Creek drainage divide area location map and illustrates a region in Montana. The Missouri River flows southeast from the figure 1 northwest corner to Fort Peck Lake. The Musselshell River flows from the figure 1 west center (south half) edge to Harlowton, Ryegate, Lavina, Roundup, and Melstone and then turns north to flow to Mosby and to join the Missouri River at Fort Peck Lake. The Yellowstone River flows from Big Timber (along the figure 1 west edge near the figure 1 southwest corner) to Greycliff, Columbus, Laurel, Billings, Custer, Forsyth, Miles City, and Terry (located along the figure 1 east edge). Flatwillow Creek originates at the east end of the Big Snowy Mountain region (located in the figure 1 west center area) and flows east, southeast, and northeast to join southeast oriented Box Elder Creek, which flows to the north-oriented Musselshell River. North Willow Creek is a southeast and northeast-oriented Musselshell River tributary originating north of Roundup and joining the Musselshell River north of Melstone. Willow Creek is the unnamed southeast-oriented Musselshell River tributary originating at the Big Snowy Mountains east end and located immediately west of North Willow Creek. Based on 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 deep valleys that eroded headward into a topographic surface at least as high as the figure 1 region highest elevations today. Prior to Musselshell River valley headward erosion (although not much before) the deep Yellowstone River valley eroded southwest to capture southeast-oriented flood waters and to divert flood waters northeast. The Musselshell River valley was the next valley that eroded southwest to capture southeast-oriented flood water and to divert flood flow northeast. The northeast-oriented North Willow Creek valley then eroded southwest to behead and capture southeast-oriented flood flow routes moving flood water to the newly eroded northeast-oriented Musselshell River valley. The southeast-oriented North Willow Creek valley segment eroded headward along a captured southeast-oriented flood flow route and the Willow Creek valley eroded headward along another southeast-oriented flood flow route. Next the Flatwillow Creek valley eroded southwest and northwest to capture southeast-oriented flood flow that had been moving to what was then the newly eroded North Willow Creek valley and headward erosion of the Flatwillow Creek valley beheaded all southeast-oriented flood flow to the Flatwillow Creek-North Willow Creek and Willow Creek drainage divide area discussed in this essay. The North Willow Creek-Musselshell River drainage divide area essay, the Missouri River-Big Dry Creek drainage divide area essay, the Musselshell River-Yellowstone River drainage divide area in Musselshell and Yellowstone Counties essay, and the Musselshell River-Yellowstone River drainage divide area in Treasure and Rosebud Counties essay describe drainage divide areas located near the Flatwillow Creek-North Willow Creek and Willow Creek drainage divide area discussed here and can be found under Musselshell River on the sidebar category list, with the exception of Missouri River-Big Dry Creek essay, which can be found under Big Dry Creek.

Flatwillow Creek-North Willow Creek and Willow Creek drainage divide area detailed location map

Figure 2: Flatwillow Creek-North Willow Creek and Willow Creek 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 Flatwillow Creek-North Willow Creek and Willow Creek drainage divide area discussed in this essay. Golden Valley and Musselshell Counties are located in Montana. The unnamed county in the figure 2 northwest quadrant is Fergus County and the unnamed county east of Fergus County is Petroleum County. The Musselshell River flows southeast from the figure 2 west edge (south half) to Ryegate and Lavina in Golden Valley County and then northeast to Roundup, Delphia, Musselshell, and Melstone in Musselshell County, before flowing north-northwest and north-northeast along the county line to the figure 2 north edge. North Willow Creek originates just east of the Little Snowy Mountains (located east of the Big Snowy Mountains) and flows southeast from the county line towards Roundup, but instead of flowing to the Musselshell River turns east-northeast and finally northeast to leave Musselshell County and join the Musselshell River in the Petroleum County southeast corner. Willow Creek originates in the Little Snowy Mountains and flows generally southeast to join the northeast-oriented Musselshell River near Roundup. Flatwillow Creek originates in the eastern Big Snowy Mountains and flows north of the Little Snowy Mountains before turning southeast and then northeast and east to join southeast-oriented Box Elder Creek, which flows to the north-oriented Musselshell River. Figure 2 shows several southeast-oriented North Willow Creek tributaries from the north, suggesting the North Willow Creek valley eroded southwest to capture multiple southeast-oriented flood flow routes such as might be found in a southeast-oriented anastomosing channel complex and the Flatwillow Creek valley eroded headward to capture the southeast-oriented flood flow to the North Willow Creek-Musselshell River drainage divide area (essay) immediately south of the Flatwillow Creek-North Willow Creek and Willow Creek drainage divide area discussed here. The Musselshell River-Yellowstone River drainage divide area in Musselshell and Yellowstone Counties and the Musselshell River-Yellowstone River drainage divide area in Treasure and Rosebud Counties essays also document a large southeast-oriented flood coming from northwest of the present day Musselshell River. This essay begins by looking at evidence at the east end of the Flatwillow Creek-North Willow Creek and Willow Creek drainage divide area and proceeds west along the drainage divide area to the Little Snowy Mountains region and concludes by looking at the Flatwillow Creek-Willow Creek drainage divide in the Little Snowy Mountains.

Flatwillow Creek-North Maginnis Creek drainage divide area

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

Figure 3 illustrates the Flatwillow Creek-Maginnis Creek drainage divide area at the eastern end of the Flatwillow Creek-North Willow Creek drainage divide area. The Musselshell River flows north-northwest and north-northeast in the figure 3 east half. Flatwillow Creek flows east along the figure 3 north edge and then turns southeast and east to join the Musselshell River. North Willow Creek flows northeast and joins the Musselshell River in the figure 3 southeast corner. Maginnis Creek flows southeast from the figure 3 center west area to the figure 3 south edge (east half) and joins northeast-oriented North Willow Creek south of the figure 3 south edge. Maginnis Creek is just one of several southeast-oriented North Willow Creek tributaries seen in figure 3. Also note northwest-oriented Musselshell River tributaries in the figure 3 northeast corner area. This northwest-southeast drainage alignment is evidence the North Willow Creek and Flatwillow Creek valleys eroded headward across multiple southeast oriented flood flow routes such as might be found in a southeast-oriented anastomosing channel complex. Southeast-oriented flood water flowing across the figure 3 map area at that time was probably moving to what was then the newly eroded Musselshell River valley. Headward erosion of the North Willow Creek valley then captured the southeast-oriented flood flow and diverted the flood waters more directly to the north-oriented Musselshell River valley. Next headward erosion of the Flatwillow Creek valley captured the southeast-oriented flood flow and diverted the flood waters more directly to the north-oriented Musselshell River valley. Northwest-oriented Musselshell River valley tributary valleys were probably eroded by reversals of southeast-oriented flood flow on the northwest ends of beheaded flood water channels. Flood waters on the northwest ends of those beheaded flood flow routes reversed flow direction to flow northwest into the newly eroded north-northeast-oriented Musselshell River valley.

Flatwillow Creek-Mud Springs Creek drainage divide area

Figure 4: Flatwillow Creek-Mud Springs Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 4 illustrates the Flatwillow Creek-Mud Springs Creek drainage divide area south and west of the figure 3 map area and includes overlap areas with figure 3. Flatwillow Creek flows northeast in the figure 4 northwest corner. Southeast of Flatwillow Creek is north-northeast and north oriented Alkali Creek, which joins Flatwillow Creek north of the figure 4 map area. Mud Springs Creek flows southeast to the figure 4 east center edge and joins northeast-oriented North Willow Creek east of the figure 4 map area. East-southeast oriented Howard Coulee is located along the figure 4 south edge and drains to northeast-oriented North Willow Creek. The Flatwillow Creek-North Willow Creek drainage divide is a southwest-northeast ridge or upland extending from the figure 4 southwest corner to the figure 4 north edge. Southeast of the drainage divide are numerous southeast-oriented tributaries to Howard Coulee and North Willow Creek. Northwest of the drainage divide are short northwest-oriented Alkali Coulee tributaries. The northwest-southeast tributary alignment is evidence the drainage divide was created when headward erosion of the northeast-oriented Flatwillow Creek and Alkali Creek valley beheaded and reversed multiple southeast-oriented flood flow routes moving flood waters to what was then the newly eroded northeast-oriented North Willow Creek valley. The parallel Alkali Creek and Flatwillow Creek valleys suggest the two valleys were eroded at the approximately the same time as channels in a northeast-oriented anastomosing channel complex that was capturing flood waters from a southeast-oriented anastomosing channel complex. Southeast-oriented North Willow Creek tributary valleys were eroded headward along southeast-oriented flood flow routes. Northwest-oriented Alkali Creek tributary valleys were eroded by reversals of flood flow on the northwest ends of beheaded southeast-oriented flood flow routes. The flood water reversed flow direction to flow northwest to newly eroded Alkali Creek and Flatwillow Creek valleys. The Flatwillow Creek valley was subsequently eroded deeper than the Alkali Creek valley by flood waters captured from points further west as illustrated in figures shown below.

Flatwillow Creek-Little Wall Creek drainage divide area

Figure 5: Flatwillow Creek-Little Wall Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 5 illustrates the Flatwillow Creek-Little Wall Creek drainage divide area south and west of the figure 4 map area and includes overlap areas with figure 4. Flatwillow Creek flows northeast across the figure 5 northwest corner. Headwaters of north-northeast oriented Alkali Creek are located in the figure 5 north center area. Southeast-oriented Howard Coulee drains to the figure 5 east center edge and then to northeast-oriented North Willow Creek. Little Wall Creek flows northeast from the figure 5 west center edge along what is probably a southwest-northeast oriented hogback and then turns to flow southeast to the figure 5 south edge and to join northeast-oriented North Willow Creek south of the figure 5 map area. Again note the numerous southeast-oriented Howard Coulee  and Little Wall Creek tributaries. The southeast-orientation of these tributaries is evidence the northeast-oriented North Willow Creek valley eroded headward to capture multiple southeast-oriented flood flow routes and subsequently the northeast-oriented Flatwillow Creek valley eroded headward to capture southeast-oriented flood flow routes that had been moving flood waters to what was then the newly eroded North Willow Creek valley. Note how west of the Alkali Creek headwaters are headwaters of northwest-oriented Flatwillow Creek tributaries. The proximity of headwaters of these two different drainage routes suggests the northwest-oriented Flatwillow Creek tributaries beheaded flood flow moving to the north-northeast oriented Alkali Creek valley and diverted the flood water to the northeast-oriented Flatwillow Creek valley. Also note how the northeast-oriented Little Wall Creek valley segment is in alignment with the north-northeast oriented Alkali Creek valley. The Little Wall Creek elbow of capture was probably developed when headward erosion of the southeast-oriented Little Wall Creek valley beheaded flood flow that was beginning to move northeast toward what was then the Alkali Creek valley head located further to the northeast. This evidence suggests the North Willow Creek, Little Wall Creek, Alkali Creek, and Flatwillow Creek valleys were all eroded at approximately the same time, although headward erosion of the North Willow Creek valley was first.

Flatwillow Creek-North Willow Creek drainage divide area at Devils Basin

Figure 6: Flatwillow Creek-North Willow Creek drainage divide area at Devils Basin. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 6 illustrates the Flatwillow Creek-North Willow Creek drainage divide area at Devils Basin located west of the figure 5 map area and includes overlap areas with figure 5. Flatwillow Creek flows southeast and east across the figure 6 north half. The east end of the Little Snowy Mountains is located along the figure 6 west edge just south of Flatwillow Creek. Southeast-oriented streams flowing through the Devils Basin are headwaters of North Willow Creek, which flows southeast before turning to flow northeast. Southeast-oriented Willow Creek flows across the figure 6 southwest corner and headwaters of southeast-oriented Jones Creek are located east of the Little Snowy Mountains. Jones Creek flows from the east end of the Little Snowy Mountains to join Willow Creek south of the figure 6 map area. Willow Creek flows southeast to join the northeast-oriented Musselshell River south of figure 6. Ridges in the Devils Basin area are probably hogbacks and reflect the presence of a geologic structure, although valleys between the ridges were eroded by running water. Multiple southeast-oriented drainage routes leading to the figure 6 south edge provide evidence of multiple southeast-oriented flood flow routes to what was then the newly eroded northeast-oriented Musselshell River valley. Headward erosion of the northeast-oriented North Willow Creek valley then captured the southeast-oriented flood flow routes moving across the Devils Basin area, although southeast-oriented flood flow continued to move around the Little Snowy Mountains east end to what was then the actively eroding southeast-oriented Jones Creek valley. Headward erosion of the Flatwillow Creek valley next captured the that southeast-oriented flood flow and diverted the flood water east and northeast and in the process beheaded flood flow routes to Jones Creek and North Willow Creek. It is tempting to end the Flatwillow Creek-North Willow Creek and Willow Creek drainage divide history at this location. West of figure 6 are the Little Snowy Mountains and Flatwillow Creek flows north of the Little Snowy Mountains. Willow Creek, which is seen in the figure 6 southwest corner, originates in the Little Snowy Mountains and for that reason the Flatwillow Creek-North Willow Creek and Willow Creek drainage divide history is still incomplete.

Flatwillow Creek-Jones Creek drainage divide area

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

Figure 7 illustrates the east end of the Little Snowy Mountains and better illustrates the Flatwillow Creek-Jones Creek drainage divide seen in figure 6. Figure 7 is located west and north of the figure 6 map area and includes overlap areas with figure 6. Jones Creek flows southeast in the figure 7 southeast quadrant and at the Little Snowy Mountains east end is linked by a through valley with the southeast oriented Flatwillow Creek valley. West of southeas toriented Jones Creek is southeast-oriented Whoop-up Creek, which begins near Bald Butte and flows southeast to join southeast-oriented Willow Creek, which is located west of Whoop-up Creek. Southwest of Willow Creek are southeast-oriented Minerva Creek and Pole Creek. Pole Creek flows southeast to join the northeast-oriented Musselshell River south of the figure 7 map area and Minerva Creek is a Pole Creek tributary. North of Bald Butte is the southeast- and northeast-oriented South Fork Flatwillow Creek, which joins the southeast-oriented North Fork Flatwillow Creek to form Flatwillow Creek, which flows southeast to the figure 7 east edge. North of Flatwillow Creek is evidence of southeast-oriented tributaries to Flatwillow Creek. Evidence along the Flatwillow Creek-North Willow Creek drainage divide area east and northeast of figure 7 suggests the Flatwillow Creek valley eroded headward across southeast-oriented flood flow, which probably was moving to what was then the newly eroded northeast-oriented Musselshell River valley. Figure 7 evidence suggests headward erosion of the Flatwillow Creek valley also beheaded a southeast-oriented flood flow route moving flood water to the Jones Creek valley, which is a tributary to the Willow Creek valley. Figure 7 evidence also reveals that Whoop-up Creek and Willow Creek (and also Minerva Creek and Pole Creek) are also southeast-oriented and the South Fork Flatwillow Creek valley is located north of them. The only difference is Whoop-up Creek and Willow Creek begin in the Little Snowy Mountains and the South Fork Flatwillow Creek has eroded a deep valley headward into the Little Snowy Mountains upland region. Could the southeast-oriented flood waters have flowed over the present day Little Snowy Mountains? Figures 8, 9 and 10 below look at additional topographic map evidence and consider the possibility.

Flatwillow Creek-Willow Creek drainage divide area in Little Snowy Mountains

Figure 8: Flatwillow Creek-Willow Creek drainage divide area in Little Snowy Mountains. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 8 illustrates the Flatwillow Creek-Willow Creek drainage divide area in the Little Snowy Mountains. The figure 8 map area is located west of the figure 7 map area and includes overlap areas with figure 7. Higher areas in the figure 8 southwest corner are the east end of the Big Snowy Mountains. The North Fork Flatwillow Creek flows east and northeast in the figure 8 northwest quadrant and then east along the figure 8 north edge. The South Fork Flatwillow Creek flows southeast in the figure 8 west center area and then turns north and northeast to flow to the figure 8 east edge. Note the northwest-southeast-oriented through valley linking North Fork Flatwillow Creek headwaters with South Fork Flatwillow Creek headwaters. That through valley provides evidence headward erosion of the North Fork valley beheaded southeast-oriented flow to the South Fork Flatwillow Creek valley and is supportive of the southeast-oriented flood flow moving across the Little Snowy Mountains hypothesis. Bald Butte is located in the figure 8 east center area and headwaters of southeast-oriented Whoop-up Creek can be seen along the figure 8 east center edge. Southwest of Bald Butte is the southeast-oriented Willow Creek valley, although following Willow Creek upstream leads to a northeast oriented valley located south of South Bench. Headwaters of southeast-oriented Minerva Creek and Pole Creek are located southwest of Willow Creek in the figure 8 southeast quadrant. Northeast and southeast oriented headwaters of Cameron Creek begin at Mc Lean Lake in the figure 8 south center. Cameron Creek is a southeast-oriented Pole Creek tributary. Leaving aside the Little Snowy Mountains elevations the drainage route relationship suggest headward erosion of the northeast and southeast-oriented Willow Creek valley beheaded southeast-oriented flood flow to the southeast-oriented Minerva Creek and Pole Creek valleys. Also headward erosion of northeast-oriented South Fork Flatwillow Creek valley beheaded southeast-oriented flood flow to the Willow Creek valley and also to the Cameron Creek valley and headward erosion of the northeast-oriented North Fork Flatwillow Creek valley segment beheaded southeast-oriented flood flow to the South Fork Flatwillow Creek valley. Further, figure 8 evidence shows what could be considered a large northwest-southeast oriented through valley located between the Big Snowy Mountains in the figure 8 southwest corner area and Bald Butte near the figure 8 east center edge. In other words, if the southeast-oriented flood flow hypothesis is correct, flood waters flowed across a topographic surface at least as high as the top of Bald Butte and eroded a large northwest-southeast oriented valley between the Bald Butte and the Big Snowy Mountains. Figures 9 and 10 provide two detailed maps to further check this southeast-oriented flood hypothesis.

South Fork Flatwillow Creek-Willow Creek drainage divide area at South Bench

Figure 9: South Fork Flatwillow Creek-Willow Creek drainage divide area at South Bench. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 9 illustrates the South Fork Flatwillow Creek-Willow Creek drainage divide area at South Bench seen in less detail in figure 8 above. South Fork Flatwillow Creek flows northeast, southeast, and northeast across the figure 9 north half. Willow Creek flows northeast from the figure 9 southwest corner and then in the figure 9 center area turns southeast to flow to the figure 9 south edge. South Bench is the erosion surface which today serves as the South Fork Flatwillow Creek-Willow Creek drainage divide and the South Fork Flatwillow Creek valley is approximately 500 feet deeper than the present day South Bench surface. The Willow Creek valley is relatively shallow in the figure 9 southwest quadrant, but deepens downstream and is almost 500 feet deep as it crosses the word “Mountains”. Bald Butte is located along the figure 9 east edge (south half) and the top is approximately 400 feet higher than the lowest points on the South Bench drainage divide and approximately 1600 feet higher than the Flatwillow Creek valley to the east. If the flood erosion hypothesis is correct flood waters eroded a 1600-foot deep valley east of Bald Butte, a 400-foot (plus?) deep broad valley between the present day Big Snowy Mountains and Bald Butte, a 500-foot deep valley Willow Creek valley into the Little Snowy Mountains region and then a 500-foot deep South Fork Flatwillow Creek valley into the Little Snowy Mountains that captured all southeast-oriented flood flow to the Willow Creek valley. A flood capable of such erosion would require immense volumes of water, although much greater amounts of water would be required to erode the entire region described by this essay series. The source of such an immense quantity of southeast-oriented flood waters cannot be determined from evidence presented here. However, essays in this series 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. In addition, such a flood water source may explain uplift of the mountains regions, such as the Little Snowy Mountains, during an immense southeast-oriented flood. A thick North American ice sheet in deep “hole” created in part due to the ice sheet’s weight would probably create crustal warping elsewhere on the continent, especially along ice sheet margins. Rapid erosion of overlying material might trigger localized uplifts of geologic structures such as those observed in the Little Snowy Mountains region.

South Fork Flatwillow Creek-Willow Creek drainage divide area at Willow Creek headwaters

Figure 10: South Fork Flatwillow Creek-Willow Creek drainage divide area at Willow Creek headwaters. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 10 illustrates the South Fork Flatwillow Creek-Willow Creek drainage divide area at the Willow Creek headwaters area also shown in less detail in figure 8 above. The South Fork Flatwillow Creek flows northeast and north in a deep valley from the figure 10 west edge (south half) to the figure 10 north edge (west half). Willow Creek flows northeast in a shallow valley across the figure 10 center area to the figure 10 northeast corner. Note the west-oriented through valley linking the northeast oriented Willow Creek valley with the north-northeast oriented South Fork Flatwillow Creek valley. A similar, but less obvious, through valley can be seen section 19 in the figure 10 north center area linking the north-oriented South Fork Flatwillow Creek valley with a northeast oriented Willow Creek tributary valley. These through valleys provide evidence headward erosion of the northeast- and north-oriented South Fork Flatwillow Creek valley beheaded at least two different flow routes to the Willow Creek valley. This evidence and other evidence presented in this essay helps build a case that southeast-oriented flood waters did cross the present day Little Snowy Mountains and did erode the landscape features seen today. Obviously, when flood waters crossed the Flatwillow Creek-North Willow Creek and Willow Creek drainage divide the regional topography looked very different from it does today. There were no Little Snowy Mountains. Either the Little Snowy Mountains were buried under easily eroded sedimentary cover (or some other easily removed material such as ice) that flood waters completely removed or the Little Snowy Mountains were uplifted as flood waters flowed across the region. Evidence for deep regional erosion has been presented in almost every essay in this Missouri River drainage basin landform origins research project essay series. However, evidence presented here and in other essays shows flood waters crossing what are today high mountain regions. The height of those mountain regions is difficult to explain solely by deep erosion of surrounding areas. For that reason the best interpretation of the evidence probably needs to include uplift of mountain regions as flood waters deeply eroded those mountain regions. Rapid removal of significant amounts of overlying bedrock might trigger localized uplift as flood waters continued to flow across a region and the Little Snowy Mountains might be a location where such a localized uplift occurred.

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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