A geomorphic history based on topographic map evidence

The South Fork-American Fork Musselshell River drainage divide area east of Cottonwood Creek discussed here is located in Montana, USA and includes areas in the northern Crazy Mountains. Although detailed topographic maps of the South Fork-American Fork 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 South Fork-American Fork 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 South Fork-American Fork Musselshell River drainage divide ended when headward erosion of the Missouri River valley captured all southeast-oriented flood flow.

Introduction:

• The purpose of this essay is to use topographic map interpretation methods to explore South Fork-American Fork Musselshell River drainage divide area landform origins (east of Cottonwood Creek), 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 the South Fork-American Fork Musselshell River drainage divide area landform evidence will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm.

South Fork-American Fork Musselshell River drainage divide area location map

Figure 1: South Fork-American Fork 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 location map for the South Fork-American Musselshell River drainage divide area east of Cottonwood Creek. Figure 1 illustrates an area in central Montana. The South Fork Musselshell River begins north of the Crazy Mountains. The South Fork Musselshell River (not labeled on figure 1) flows in a northeast direction to join the North Fork Musselshell River near Martinsdale, and then to flow as the Musselshell River east-southeast and northeast to Roundup and Melstone. At Melstone the Musselshell River turns to flow north-northwest to join the east-oriented Missouri River. The Missouri River originates at Three Forks (located near the figure 1 southwest corner and flows north and northwest through Canyon Ferry Lake (a large reservoir) and the Gates of the Rocky Mountains to Wolf Creek before turning to flow northeast. Northeast of Loma the Missouri River turns to flow southeast and east-northeast and southeast to Fort Peck Reservoir, which is the large reservoir in the figure 1 northeast corner area. The Yellowstone River flows northeast from the figure 1 south edge to Livingston and Big Timber, Montana and then southeast and northeast to Billings and Custer. Cottonwood Creek is the unlabeled stream originating at north end of the Crazy Mountains near Loco Peak and flowing north to join the South Fork Musselshell River near Martinsdale. The American Fork Musselshell River originates in the Crazy Mountains (just south of Loco Peak) and flows northeast to join the Musselshell River near Harlowton. The South Fork-American Fork Musselshell River drainage divide area discussed here is located east of Cottonwood Creek and includes areas in the northern Crazy Mountains area.

• Essays describing regions near the South Fork-American Fork Musselshell River drainage divide area include the South Fork Judith River-Musselshell River essay, the Judith River-Musselshell River (Big Snowy Mountains) essay, the North Fork Smith River-North Fork Musselshell River essay, and the South Fork Smith River and South Fork Musselshell River essay and have interpreted landform evidence in the context of immense southeast-oriented flood events. Those essays can be found under appropriate river names on the sidebar category list and s suggest mountain ranges such as the Crazy Mountains initially did not present obstacles to southeast-oriented flood movements and suggest the mountain ranges emerged as flood waters crossed the region. Such emergence could have occurred if the mountains were buried in easily eroded sediments and/or ice, which the flood waters removed, and/or if the mountain ranges were uplifted as flood waters eroded the region. South Fork-American Fork Musselshell River-drainage divide evidence is interpreted likewise (i.e., flood waters initially moved across a topographic surface where the present day Crazy Mountains were not obstacles to southeast-oriented flood flow).

• Headward erosion of the deep Yellowstone River valley and subsequently the deep Musselshell River valley captured southeast-oriented flood flow as the Big Belt and Crazy Mountains began to emerge. Southeast oriented flood water was channeled between the Big Belt and Crazy Mountains as flood waters flowed south to what was then the newly eroded Yellowstone River valley. Headward erosion of what was then the deep south-southeast oriented Shields River valley (unlabeled river at Wilsall in figure 1) was along this major south-oriented flood route. The Shields River valley also eroded headward into the present day Crazy Mountains area to capture southeast-oriented flood flow still moving in that region. At about the same time the deep Musselshell River valley eroded headward into the region and the American Fork eroded southwest into the Crazy Mountain area to capture some of the same southeast-oriented flood flow routes headward erosion of the Shields River valley was capturing. As headward erosion of the Musselshell River valley and its various tributary valleys progressed the Musselshell River-Shields River drainage divide was created and drainage divides between newly eroded northeast and north-oriented Musselshell River tributary valleys evolved.

South Fork-American Fork Musselshell River drainage divide area detailed location map

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

Figure 2 provides a somewhat more detailed map of the South Fork-American Musselshell River drainage divide area east of Cottonwood Creek. Meagher County and Wheatland County are located in Montana. Green areas are National Forest lands, which generally are located in mountain regions. The Little Belt Mountains are located along the figure 2 north edge. The Castle Mountains are located southeast of White Sulphur Springs in Meagher County. The Crazy Mountains are located in the figure 2 south center edge area. The American Fork Musselshell River originates in the Crazy Mountains near the figure 2 south center edge and flows northeast to join the Musselshell River east of Harlowton (located in the figure 2 east center area). The South Fork Musselshell River begins east of Reservation Mountain, which is located south of the Castle Mountains and the town of Loweth, and flows northeast and joins the northeast-oriented North Fork Musselshell River near Martinsdale to form the southeast-oriented Musselshell River. Cottonwood Creek flows north from the Bald Ridge area in the Crazy Mountains to join the South Fork Musselshell River between Lennep and Martinsdale. This essay illustrates and discusses evidence east of Cottonwood Creek, north of the American Fork, and south of the South Fork Musselshell River and the Musselshell River and includes areas in the Crazy Mountains.

• Note south-oriented drainage routes south and southeast of Ringling and west of the Crazy Mountains (e.g. Potter Creek). Those streams flow to the south-oriented Shields River, which joins the Yellowstone River near Livingston. The Shields River originates in the Crazy Mountains as a west and southwest-oriented stream near the county line south of Bald Ridge. The southwest-oriented stream at Ringling is Sixteenmile Creek, which flows to the figure 2 southwest corner area and then turns west to join the northwest-oriented Missouri River (west of figure 2). The northwest-oriented Smith River (which flows to the figure 2 northwest corner) eventually joins the northeast-oriented Missouri River (see figure 1). Drainage divides illustrated and discussed in this essay include the Shields River-Musselshell River drainage divide in the Crazy Mountains. The discussion in this essay begins north of the Crazy Mountains where Cottonwood Creek joins the South Fork Musselshell River. The first five maps below illustrate evidence north and east of the Crazy Mountains. The final three maps look at evidence along what are today high Crazy Mountains area drainage divides. Evidence is interpreted in the context of immense southeast-oriented flood events, which deeply eroded the entire figure 2 region. The flood occurred at a time when flood waters initially could move on a topographic surface where the present day mountain ranges did not interfere with water movements. As flood waters eroded the figure 2 map region the Crazy Mountains emerged (perhaps by a combination of deep erosion of surrounding materials and of uplift occurring as flood waters eroded the region). Flood waters were channeled to flow southeast along the present day Smith River alignment to the present day Shields River drainage basin and then to what was then the newly eroded Yellowstone River valley. The deep Musselshell River valley eroded headward into the figure 2 map region with the American Fork valley eroding into Crazy Mountains region. Headward erosion of the deep Musselshell River valley and its various tributary valleys (including the South Fork Musselshell River valley) systematically captured the southeast-oriented flood flow and diverted the flood waters northeast and east. Flood capture events appear to have been greatly aided by emergence of the Crazy Mountains as a significant upland region as flood waters deeply eroded the region.

Cottonwood Creek-Miller Creek drainage divide area

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

Figure 3 illustrates the Cottonwood Creek-Miller Creek drainage divide area south of the northeast-oriented South Fork Musselshell River and the southeast-oriented Musselshell River. The South Fork Musselshell River flows from the figure 3 west center edge area to join the southeast oriented North Fork Musselshell River just south of the Martinsdale Colony in the figure 3 north center edge area. The Musselshell River flows southeast from the Martinsdale Colony area to the figure 3 east edge. Cottonwood Creek flows north-northwest from the figure 3 south edge to join the northeast-oriented South Fork Musselshell River near Groveland. Miller Creek flows northeast from the figure 3 south edge to join the Musselshell River near the figure 3 east edge. Between Cottonwood Creek and Miller Creek, Little Elk Creek flows northeast from the figure 3 south edge to join northeast-oriented Miller Creek near the figure 3 east edge. Whetstone Ridge in the figure 3 northwest corner area is the southeast end of the Castle Mountains upland region. Gordon Butte appears to be some type of circular igneous intrusion and hills in the  figure 3 south center edge area and the figure 3 southeast corner are also probably composed of erosion resistant rock. Elongate ridges southeast of Gordon Butte appear to be hogbacks, although valleys between them are water eroded features. Note how those valleys today represent through valleys linking the north-northwest oriented Cottonwood Creek valley with the southeast-oriented Musselshell River valley. The presence of the through valleys provides evidence that headward erosion of the north-northwest oriented Cottonwood Creek valley beheaded multiple northeast-oriented channels moving water northeast to what was probably a newly eroded southeast-oriented Musselshell River valley. The multiple channels suggest a northeast-oriented anastomosing channel complex, which was captured by headward erosion of the Cottonwood Creek valley. The north-northwest orientation of the Cottonwood Creek valley suggests it was eroded by a reversal of flood flow on the northwest end of a beheaded southeast-oriented flood flow route. In other words, the figure 3 map area was deeply eroded by southeast-oriented flood waters that for a time were channeled between Gordon Butte and the hill to the southeast to flow northeast to the newly eroded Musselshell River valley. Headward erosion of the deep South Fork Musselshell River then captured the flood flow and moved it north-northwest and northeast around Gordon Butte.

Musselshell River-American Fork drainage divide area

Figure 4: Musselshell River-American Fork drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 4 illustrates the Musselshell River-American Fork Musselshell River drainage divide area east of the figure 3 map area and includes overlap areas with figure 3. The Musselshell River flows southeast from the figure 4 west edge (north half) to Twodot and then northeast and east to Harlowton and the figure 4 east edge. Little Elk Creek flows northeast to join the Musselshell River near the figure 4 west edge. Big Elk Creek is the north and north-northwest oriented tributary joining the Musselshell River near Twodot. Note multiple southeast-oriented Musselshell River tributaries from the north including southeast-oriented Haymaker Creek, which uses the same northwest-southeast oriented alignment as northwest-oriented Big Elk Creek. Lebo Creek flows northeast in the figure 4 southeast quadrant and the American Fork Musselshell River flow northeast across the figure 4 southeast corner. Figure 4, like figure 3, contains evidence of significant northeast-oriented drainage to the Musselshell River valley and also of southeast-oriented drainage from the north. The multiple southeast-oriented tributaries from the north provide evidence the Musselshell River valley eroded headward across numerous southeast-oriented flood flow routes, such as might be found in a southeast-oriented anastomosing channel complex. Northeast-oriented Musselshell River tributaries provide evidence headward erosion of the Musselshell River valley was not simple, but instead consisted of headward erosion of one northeast-oriented tributary valley after another. Each northeast-oriented tributary valley was eroding headward to capture southeast-oriented flood flow that was moving to the previously eroded northeast-oriented tributary valley. The north-northwest oriented Big Elk Creek valley provides evidence headward erosion of the Musselshell River valley beheaded and reversed a major southeast-oriented flood flow route that may have earlier been captured by headward erosion of the Lebo Creek valley (and before that by  American Fork valley headward erosion). A large abandoned southeast-oriented headcut in the figure 4 southwest corner provides the best evidence for an immense southeast-oriented flood across the figure 4 map area. That headcut is better illustrated in figures 5 and 6 below and further provides evidence the northeast-oriented valleys eroded headward across the figure 4 map area to capture an immense southeast-oriented flood.

Little Elk Creek-Big Elk Creek drainage divide area

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

Figure 5 illustrates the region south and east of figure 3 and south and west of figure 4 and includes overlap areas with both figures 3 and 4. Little Elk Creek flows northeast across the figure 5 northeast corner. Big Elk Creek flows northeast from the figure 5 south center edge to the figure 5 east center area and then turns north and north-northwest to flow to the figure 5 north edge. Miller Creek flows northeast between Little Elk Creek and Big Elk Creek. Lebo Creek flows northeast in the figure 5 southeast corner and the northeast-oriented American Fork is located just southeast of the figure 5 southeast corner. Cinnamon Peak is the northeast high point in the Crazy Mountains upland region. Note the large escarpment-surrounded basin in which the Martin and C Bar J Reservoir is located (figure 5 center area). That escarpment-surrounded basin is a large abandoned headcut eroded by a southeast-oriented flood flow route moving immense quantities of flood waters into what was then the newly eroded and deep Big Elk Creek valley. The Big Elk Creek valley, as previously mentioned, was eroded headward along a beheaded and reversed southeast-oriented flood flow route that had been captured by headward erosion of what was then the deep Musselshell River valley. Reversed flood flow on the newly beheaded Big Elk Creek alignment captured a major yet to be beheaded (by Musselshell River valley and tributary valley headward erosion) southeast-oriented flood flow route. That yet to be beheaded southeast-oriented flood flow route was moving on a topographic surface at least as high as the present day drainage divide between Miller Creek and Big Elk Creek. Flood waters flowing into the newly eroded Big Elk Creek valley eroded the large headcut northwest into the northeast-oriented Big Elk Creek valley wall. Headward erosion of the southeast-oriented headcut ceased when headward erosion of the Musselshell River and Miller Creek valley captured the southeast-oriented flood flow route and diverted the flood waters northeast to the newly eroded Musselshell River valley. Headward erosion of the Little Elk Creek valley next captured the flood flow and as seen in figure 3 headward erosion of the Musselshell River, South Musselshell River, and Cottonwood Creek valley subsequently captured all southeast-oriented flood flow.

Detailed map of Little Elk Creek-Big Elk Creek drainage divide area

Figure 6: Detailed map of Little Elk Creek-Big Elk Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 6 provides a detailed map of the large abandoned headcut seen in less detail in figure 5 above. Big Elk Creek flows northeast in the figure 6 southeast quadrant. This abandoned headcut is typical of similar escarpment-surrounded basins or abandoned headcuts located throughout the Great Plains and Rocky Mountain regions. Another large abandoned headcut down flood from this figure 6 location is located at Hoskin Basin (see figure 6 in the Painted Robe Creek-Yellowstone River drainage divide area essay).  Some of the abandoned headcuts are much larger than this figure 6 abandoned headcut. Each such feature is a large escarpment-surrounded basin and was eroded by an immense flood of water, which was subsequently captured and diverted to flow in another direction. These large abandoned headcuts can be located on detailed topographic maps and used to provide an independent test of the drainage histories being constructed by essays in this Missouri River drainage basin research project series. In the case of the figure 6 abandoned headcut it provides evidence of southeast-oriented flood flow moving into what must have been a deep and newly eroded northeast-oriented Big Elk Creek valley. As noted in the figure 5 discussion the deep Big Elk Creek valley had eroded headward from the what was then the newly eroded Musselshell River valley near Twodot. The large Big Elk Creek valley downstream from this figure 6 map area was eroded at least in part by flood waters that eroded this abandoned headcut (however, figure 8 illustrates additional sources of flood water that helped erode the Big Elk Creek valley). Flood erosion of this figure 6 abandoned headcut ended when headward erosion of the northeast-oriented Miller Creek and Little Elk Creek valleys captured all of the southeast-oriented flood flow. Figure 5 and 6 evidence provides some clues as the quantities of flood waters involved and also as to the amount erosion those flood waters accomplished. Figures 8, 9, and 10 present evidence of even greater flood water erosion, which implies still greater flood water volumes.

Big Elk Creek-American Fork drainage divide area

Figure 7: Big Elk Creek-American Fork drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 7 illustrates the region southeast of the figure 5 map area and includes overlap areas with figure 5. The abandoned headcut illustrated in figure 6 is located in the figure 7 northwest corner area. Big Elk Creek flows northeast, east, and north from the figure 7 southwest corner area to the figure 7 north center edge. Crooked Creek flows northeast from the figure 7 southwest corner to join the east-oriented Big Elk Creek segment. The American Fork Musselshell River flows northeast from the figure 7 southwest quadrant to the figure 7 east center edge. Lebo Creek originates in the figure 7 southwest quadrant (north of the American Fork) and flows northeast to the figure 7 northeast corner area. Fish Creek flows northeast across the figure 7 southeast corner. The Fish Creek-Big Coulee Creek drainage divide area essay (found under Musselshell River) illustrated and described evidence southeast of the figure 7 map area. Note the northwest-southeast oriented ridges and through valleys linking various northeast-oriented drainage routes. The through valleys provide evidence southeast-oriented flood waters first moved to what was then a newly eroded and deep northeast-oriented American Fork valley. Next flood waters were captured by headward erosion of the deep northeast-oriented Lebo Creek valley and finally the southeast-oriented flood waters were captured by headward erosion of the deep north-oriented Big Elk Creek valley. Figure 7 evidence suggests these flood captures occurred in fairly rapid succession and that for a time at least flood waters were probably flowing simultaneously in all three valleys (and perhaps also spilling over to the northeast-oriented Fish Creek valley and maybe to additional valleys further to the southeast). Southeast-oriented flood flow across the figure 7 map area did not end until headward erosion of the Musselshell River and Miller Creek valleys beheaded all southeast-oriented flood flow. It is possible significant northeast-oriented flood flow continued to move into the figure 7 map area from sources further to the southwest. Figure 8 follows Big Elk Creek and American Fork upstream to Crazy Mountains source areas.

Big Elk Creek and American Fork headwaters area

Figure 8: Big Elk Creek and American Fork headwaters area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

The figure 8 map area is south and west of the figures 7 and 5 map areas and includes overlap areas with figure 5. Big Elk Creek originates in the Loco Mountain area (figure 8 north center area) and flows northeast to the figure 8 north edge. Major Big Elk Creek tributaries from the northeast to the southwest are Buzzard Creek, North Fork, Middle Fork, Blacktail Creek, and Lebo Fork. Note how these tributaries are southeast-oriented or in the case of Lebo Fork have southeast-oriented headwaters. The American Fork flows northeast from the figure 8 south center area to the figure 8 northeast corner area. Note how many American Fork tributaries have southeast-oriented headwaters. The large west oriented valley in the figure 8 west center area is drained by west oriented Shields River headwaters. Note how Shields River tributaries from the south are northwest-oriented. Also note in the figure 8 northwest corner area a northwest-oriented drainage route linked by a through valley to southwest-oriented Crandall Creek, which flows to the west oriented Shields River. That northwest-oriented drainage route is the Middle Fork of Cottonwood Creek and is better illustrated in the South Fork Musselshell River-Cottonwood Creek drainage divide area essay. Note the deep, but high level, through valleys (or passes) eroded across the present day Shields River-Big Elk Creek and Shields River-American Fork drainage divides. Those through valleys (or passes) are water eroded features and provide evidence large volumes of water once moved across what is today a high mountain ridge. Flood waters were probably moving east from the present day Shields River drainage basin to the American Fork drainage basin and later to the Big Elk Creek drainage basin, although it is probable flood waters were moving into both the American Fork and Big Elk Creek valleys simultaneously. At that time the west and south oriented Shields River valley had not yet eroded headward into the figure 8 map area. Headward erosion of the deep west and south oriented Shields River valley captured the southeast-oriented flood water and diverted flood flow west and south to what was then the newly eroded Yellowstone River valley. In doing so headward erosion of the deep Shields River valley ended flood flow into the American Fork and Big Creek valleys and also created present day Shields River-American Fork and Shields River-Big Elk Creek drainage divides. Southeast-oriented flood water continued to flow into the newly reversed Shields River valley from northwest of figure 8 until headward erosion of the deep South Fork Musselshell River valley also captured that southeast-oriented flood flow.

Lebo Fork-Shields River drainage divide area

Figure 9: Lebo Fork-Shields River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 9 illustrates the Lebo Fork (of Big Elk Creek)-Shields River drainage divide area seen in less detail in figure 8 above. The west-oriented Shields River is located in the figure 9 southwest quadrant. Southwest-oriented Shields River tributaries are Dugout Creek and Lodgepole Creek. The Lebo Fork (of Big Elk Creek) flows southeast in the figure 9 north center area and then turns northeast to flow to the figure 9 northeast corner area. Southeast-oriented drainage in the figure 9 southeast corner flows to the northeast oriented American Fork. The Shields River-Musselshell River drainage divide today is the high drainage divide marked as the National Forest boundary. A close look at that drainage divide reveals numerous saddles (or passes) eroded across it. One of the deepest passes or saddles links headwaters of southwest-oriented Lodgepole Creek with the Lebo Fork elbow of capture (where southeast-oriented Lebo Fork turns to become northeast-oriented Lebo Fork). That deep pass originated as a valley when southeast oriented flood water captured by headward erosion of the deep northeast-oriented Big Elk Creek valley moved west in the present day Shields River valley and then northeast to what was then the newly eroded Big Elk Creek valley. The southeast-oriented Lebo Fork headwaters were eroded by southeast-oriented flood flow moving to the northeast-oriented Lebo Fork valley. It is probable that after headward erosion of the deep Shields River valley southeast oriented flood waters spilled from the southeast-oriented Lebo Fork valley into what was then the newly eroded Shields River valley (and eroding the southwest-oriented Lodgepole Creek valley in the process).

• Headward erosion of the deep Lebo Fork and Shields River valleys was probably greatly aided by emergence of the Crazy Mountains as a significant upland region as flood waters deeply eroded the surrounding region. Emergence of the mountains may have been as flood waters removed easily eroded materials, such as easily eroded sediments and/or ice, from around the mountains, and/or as the mountains were uplifted. Why would a mountain range be uplifted while an immense southeast-oriented flood was rapidly eroding the adjacent region? While the source of the southeast-oriented flood waters described in this essay cannot be determined from evidence presented here, 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 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 flood waters further and further 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 mountains regions during an immense southeast-oriented flood. A thick North American ice sheet, in a deep “hole” created in part by the ice sheet’s weight, would probably cause crustal warping elsewhere on the continent, especially along ice sheet margins. Rapid erosion of significant amounts of overlying bedrock material might also trigger localized uplift.

Shields River-South Fork of American Fork drainage divide area

Figure 10: Shields River-South Fork of American Fork drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 10 illustrates the Shields River-South Fork of the American Fork Musselshell River drainage divide area seen is less detail in figure 8 above. Figure 10 includes overlap areas with figure 9. The west-oriented Shields River is located in the figure 10 northwest quadrant. The southeast-oriented Middle Fork of the American Fork drainage basin is located in the figure 10 northeast quadrant. The South Fork of the American Fork flows northeast in the figure 10 south center area and then east-northeast and southeast in the figure 10 southeast quadrant. The Ranger District boundary marked by a dashed black line is located along the Shields River-South Fork of the American Fork drainage divide. Note northwest oriented Shields River tributaries flowing from that drainage divide. The north-south Ranger District boundary in section 25 and north is the drainage divide between the west-oriented Shields River and the east-oriented American Fork of the Musselshell River. The Ranger District boundary east of section 26 is the drainage divide between the Middle Fork and the South Fork of the American Fork of the Musselshell River. While perhaps not as obvious as in figure 9 there are saddles or notches eroded into the present day high ridges that serve as the major drainage divides. Those saddles or notches are what remains of valleys eroded by multiple southeast-oriented anastomosing flood flow channels that once crossed the figure 10 map area. Flood waters were captured by headward erosion of the South Fork of the American Fork valley and subsequently by headward erosion of the Middle Fork valley. Northwest-oriented Shields River tributary valleys were eroded by reversals of flood flow as the Shields River valley eroded headward into the figure 10 map area to capture the southeast-oriented flood flow. The Shields River beheaded southeast-oriented flood flow routes one channel at a time and because the flood flow routes were anastomosing, reversed flood flow on newly beheaded flood flow routes usually was able to capture yet to be beheaded flood flow from adjacent flood flow routes. Such captures of yet to be beheaded flood flow enabled reversed flow flood waters to erode significant northwest and west-oriented valleys.

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.