A geomorphic history based on topographic map evidence

The American Fork Musselshell River-Fish Creek drainage divide area discussed here is located northeast of the Crazy Mountains in Montana, USA. Although detailed topographic maps of the American Fork Musselshell River-Fish 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 American Fork Musselshell River -Fish 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 American Fork Musselshell River-Fish Creek 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 American Fork Musselshell River-Fish Creek drainage divide area landform origins, 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 American Fork Musselshell River-Fish Creek drainage divide area landform evidence will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm.

American Fork-Fish Creek drainage divide area location map

Figure 1: American Fork-Fish 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 location map for the American Musselshell River-Fish Creek drainage divide area. 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. The American Fork Musselshell River (not labeled in figure 1) originates in the Crazy Mountains (just south of Loco Peak) and flows northeast to join the Musselshell River near Harlowton. Fish Creek (also not labeled) originates at the east edge of the Crazy Mountains and flows northeast to join the Musselshell River near Ryegate. From Ryegate the Musselshell River flows 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-Fish Creek drainage divide area discussed here includes areas in the northern Crazy Mountains area.

• Essays describing regions near the American Fork Musselshell River-Fish Creek drainage divide area can be found under appropriate river names on the sidebar category list and include the South Fork Judith River-Musselshell River, the Judith River-Musselshell River (Big Snowy Mountains), the South Fork-American Fork Musselshell River, the Fish Creek-Big Coulee Creek (under Musselshell River), the North Fork Smith River-North Fork Musselshell River, and the South Fork Smith River and South Fork Musselshell River drainage divide area landform origins essays and have interpreted landform evidence in the context of immense southeast-oriented flood events. Those essays 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. American Fork Musselshell River- Fish Creek 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 Fish Creek and American Fork valleys 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-oriented Musselshell River tributary valleys, such as the American Fork and Fish Creek valleys, evolved.

American Fork-Fish Creek drainage divide area detailed location map

Figure 2: American Fork-Fish Creek 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 American Musselshell River-Fish Creek drainage divide area. Sweet Grass County, Golden Valley County, and Wheatland County are located in Montana. Green areas are National Forest lands, which generally are located in mountain regions. The Crazy Mountains are located along the figure 2 west edge area. The Musselshell River flows southeast from the figure 2 northwest corner to Harlowton and Ryegate and then east to Cushman along the figure 2 east edge. The American Fork Musselshell River originates in the Crazy Mountains near the figure 2 west center edge and flows northeast to join the Musselshell River east of Harlowton (located in the figure 2 north center area). Fish Creek originates near Porcupine Butte in the Sweet Grass County northwest corner and flows northeast, southeast, and northeast to join the Musselshell River near Ryegate. The discussion in this essay begins in the Crazy Mountains where the America Fork Musselshell River originates. Fish Creek originates just east of the Crazy Mountains. The essay discussion then progresses eastward along the American Fork-Fish Creek drainage divide and then along the Musselshell River-Fish Creek drainage divide. 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). The deep Musselshell River valley eroded headward into the figure 2 map region with the Fish Creek valley and the American Fork valley eroding into Crazy Mountains region to capture southeast-oriented flood waters. Those captured southeast-oriented flood waters were then diverted northeast in the Fish Creek and American Fork valleys to what was then the newly eroded Musselshell River valley.  Headward erosion of the deep Musselshell River valley and its various tributary valleys (including the American Fork and Fish Creek valleys) 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 flood waters deeply eroded the region.

American Fork and Fish Creek headwaters area

Figure 3 illustrates the headwaters areas of the American Fork and of Fish Creek. The Crazy Mountains upland region is located in the figure 3 west half. Porcupine Butte is located in the figure 3 east center. The South Fork of the American Fork originates in the figure 3 southwest quadrant and flows northeast, east, and northeast to join the Middle Fork in the figure 3 center area. From the figure 3 center the American Fork flows northeast to the figure 3 northeast corner. Fish Creek originates as a northwest-oriented stream on the Porcupine Butte west side and then flows northeast and east to the figure 3 east edge (north of Porcupine Butte). Sweet Grass Creek is the northeast and southeast oriented stream located in the figure 3 south center area and southeast quadrant. Sweet Grass Creek flows south to the Yellowstone River while the American Fork and Fish Creek are Musselshell River tributaries, and the Musselshell River flows north to the Missouri River. The Musselshell River-Yellowstone River drainage divide is located between the South Fork of the American Fork and Sweet Grass Creek valleys and also between the Fish Creek elbow of capture (west of Porcupine Butte) and the Sweet Grass Creek valley. Figure 3 evidence suggests headward erosion of the southeast-oriented Sweet Grass Creek valley beheaded northeast-oriented flow to the Fish Creek valley and also probably to the northeast-oriented American Fork valley. Figure 3 evidence suggests prior to this capture by headward erosion of the southeast-oriented Sweet Grass Creek valley multiple northeast-oriented anastomosing flood flow routes were emerging from the Crazy Mountains upland region and moving large quantities of water northeast to what was then probably the newly eroded and deep Musselshell River valley.

• How could a large northeast-oriented flood be emerging from what is today a high mountain region and then be partially captured to flow southeast? First, the Crazy Mountains topography once looked quite different from it does today and an immense southeast-oriented flood entered the region. The northeast-oriented Sweet Grass Creek, American Fork, Middle Fork, and Lebo Fork (of Big Elk Creek located in the figure 3 northwest quadrant) in that order eroded headward into what is today the Crazy Mountains upland region to capture the southeast-oriented flood flow and to divert the flood waters northeast to what was then the newly eroded and deep Musselshell River valley. At the same time the deep south-oriented Shields River valley eroded north, northeast, and east into the same region to capture the southeast-oriented flood flow. The west oriented drainage in the figure 3 west center edge area is the headwaters area of the west, southwest, and south-oriented Shields River. Headward erosion of the deep Shields River valley beheaded southeast-oriented flood flow to what was then the newly eroded northeast-oriented Sweet Grass Creek valley and the newly eroded American Fork valley system (see South Fork-American Fork Musselshell River drainage divide essay for illustrations and discussion of region west and northwest of figure 3) . As headward erosion of the deep Shields River valley was beheading flood flow routes to the Sweet Grass Creek and South Fork American Fork valleys headward erosion of the southeast-oriented Sweet Grass Creek valley was capturing what is today the northeast-oriented Sweet Grass Creek headwaters valley.

• Headward erosion of the deep Sweet Grass Creek, American 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.

American Fork-Sweet Grass Creek drainage divide at Fish Creek headwaters

Figure 4: American Fork-Sweet Grass Creek drainage divide at Fish Creek headwaters. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 4 illustrates a detailed map of the American Fork-Sweet Grass Creek drainage divide area and of the Fish Creek headwaters area seen in less detail in figure 3 above. Northeast and southeast-oriented Sweet Grass Creek is located along the figure 4 south center edge area. The American Fork flows northeast from the figure 4 southwest corner to the figure 4 north center edge. Hummocky topography around Rein Lake appears to be a glacial moraine of some type. The Sweet Grass Creek and American Fork valleys have eroded this moraine, although the valleys probably existed before the moraine was deposited. Fish Creek originates on Porcupine Butte and flows northwest in section 16 into section 9 where it turns to flow northeast to the figure 4 east edge. Prior to being captured by headward erosion of the southeast-oriented Sweet Grass Creek valley water moving northeast in the northeast-oriented Sweet Grass Creek valley continued northeast into what is now the northeast-oriented Fish Creek valley and probably into the northeast-oriented American Fork valley. Further, the water also probably moved northeast into the unnamed northeast-oriented valley in the figure 4 northeast corner. The convergence and divergence of flow routes here suggests an anastomosing channel pattern characteristic of channel complexes developed during large flood events. There is evidence the Sweet Grass Creek valley was glaciated, although the glaciation appears occurred after the valley was formed, which means flood events responsible for eroding the valley occurred before glaciation altered the valley. The northwest-oriented Fish Creek headwaters suggest flood erosion of the figure 4 map area deeply eroded the region surrounding Porcupine Butte and initially flood waters flowed on a topographic surface at least as high as the top of Porcupine Butte. The northwest-oriented Fish Creek headwaters valley was probably eroded by a reversal of flood water on the northwest end of a beheaded southeast-oriented flood flow route. If so, the southeast-oriented flood flow route was probably beheaded by a deep valley eroding headward from what was then the newly eroded and deep Musselshell River valley. Further, at that time the prevailing elevation in the figure 4 map area was at least as high as the Porcupine Butte top. Flood water erosion was responsible for lowering the figure 4 map area topographic surface to produce the topographic surface seen today (with minor alterations produced by what appears to have been subsequent glaciation in the Sweet Grass Creek valley).

American Fork-Fish Creek drainage divide area

Figure 5: American Fork-Fish Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 5 illustrates the American Fork-Fish Creek drainage divide are north and east of the figure 4 map area and includes overlap areas with figure 4. The American Fork flows northeast and east from the figure 5 west edge to the figure 5 center and then northeast to the figure 5 northeast corner. Fish Creek flows northeast in the figure 5 southwest corner and then east-southeast to the figure 5 south center and finally northeast and east to the figure 5 east edge. Big Elk Creek is the east- and north-oriented stream in the figure 5 northwest quadrant (northwest of Lebo Lake). Lebo Creek is the northeast oriented stream between Lebo Lake and the American Fork. The north edge of Porcupine Butte is located along the figure 5 south edge in the figure 5 southwest quadrant. While major figure 5 drainage routes appear to be diverging northeast oriented streams, these streams appear to be linked by interconnected valleys suggesting they originated as channels in a large northeast oriented anastomosing channel complex. Further, there is evidence, especially in the figure 5 west center area, the northeast-oriented valleys were eroded headward across southeast-oriented valleys. The South Fork-American Fork Musselshell River drainage divide essay provides much stronger evidence for earlier southeast oriented flood flow coming from the region immediately northwest of the figure 5 northwest corner. Based on this evidence the figure 5 map area is interpreted to have been eroded initially by southeast-oriented flood flow, which was captured by headward erosion of what were probably an anastomosing complex of deep northeast- and north-oriented valleys eroding headward from what was probably the newly eroded and deep Musselshell River valley, which had eroded headward into the region north and east of figure 5.

Detailed map of American Fork-Fish Creek drainage divide area

Figure 6: Detailed map of American Fork-Fish 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 American Fork-Fish Creek drainage divide area seen in less detail in figure 5 above. The American Fork is located in the figure 6 top half and flows northeast and east from the figure 6 west edge to the figure 6 northeast corner. Northwest-oriented Fish Creek headwaters on Porcupine Butte are located in the east half of the figure 6 southwest quadrant and Fish Creek then flows northeast around the north end of Porcupine Butte and then east-southeast into the figure 6 southeast quadrant and to the figure 6 east edge. Northeast-oriented Spring Creek is located between the American Fork and Fish Creek and flows from the figure 6 center area to join the American Fork east of the figure 6 northeast corner area. Note how Spring Creek flows in an independent valley, but how that independent valley is linked by through valleys and otherwise with the American Fork valley. Also note the multiple east-oriented valleys between the Spring Creek valley and the Fish Creek valley. This complex of diverging and converging valleys provides evidence of a large anastomosing channel complex that had eroded headward from what was then the newly eroded and deep Musselshell River valley towards a major flood water source where the American Fork and Sweet Grass Creek now emerge from the Crazy Mountains upland region. Volumes of water required to erode these valleys were much greater than melting of alpine glaciers could produce. These valleys were eroded by an immense southeast-oriented flood that crossed the present day Crazy Mountain region and was captured by headward erosion of deep northeast oriented Musselshell River tributary valleys. Flood flow across the figure 6 map area ended as headward erosion of northeast and north-oriented Musselshell River tributary valleys further to the northwest beheaded southeast-oriented flood flow and as headward erosion of the Shields River valley captured southeast-oriented flood flow further to the west. Headward erosion of the deep valleys and the flood flow captures were probably greatly aided by emergence of the Crazy Mountains upland region as flood waters eroded the region. Emergence of the Crazy Mountains may have occurred as flood waters removed easily eroded sediments and/or ice that had surrounded the Crazy Mountains and/or as the Crazy Mountains were uplifted as flood waters eroded the region. Probably some combination occurred.

American Fork-Fish Creek drainage divide area near Devils Pocket

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

Figure 7 illustrates the American Fork-Fish Creek drainage divide area east and north of the figure 6 map area and includes overlap areas with figure 6. Fish Creek is located in the figure 7 south center edge area. The Musselshell River flows southeast in the figure 7 northeast quadrant. The American Fork flows northeast from the figure 7 west edge (south half) to the figure 7 north edge and joins the Musselshell River north of the figure 7 map area. Lebo Creek flows northeast in the figure 7 northwest quadrant and joins the American Fork near the figure 7 north edge. Mud Creek flows southeast, northeast and east in the figure 7 southeast quadrant. Timber Creek flows northeast from the Devils Pocket area to the Musselshell River. The Devils Pocket and the hill east of the Red Basin appear to be eroded domal structures surrounded by hogback ridges. Note through valleys eroded between these structures and across the hogback ridges providing evidence of water movements across present day drainage divides. The eroded structures provide evidence of deep erosion in the figure 7 map area. The through valleys provide evidence present day valleys eroded headward across an earlier drainage system that probably involved multiple southeast-oriented channels such as might be found in a southeast-oriented anastomosing channel complex. In other words, the figure 7 map area evidence provides further evidence supporting the interpretation that an anastomosing complex of deep east and northeast-oriented tributary valleys eroded headward from what was then the newly eroded and deep Musselshell River valley to capture flood waters moving in a large-scale southeast-oriented anastomosing channel complex. Again, flood events recorded by this figure 7 evidence required much greater volumes of water than melting of alpine glaciers could create. Flood waters were coming from beyond the Crazy Mountain region and were able to deeply erode the figure 7 map area. It is also possible the eroded structures seen in figure 7 were being uplifted as flood waters eroded the region. Removal of great thicknesses of overlying rock material might trigger crustal uplifts and if flood waters were derived from rapid melting of a thick North American ice sheet, the ice sheet weight might also have triggered crustal uplift elsewhere on the continent.

Detailed map of Timber Creek drainage basin area near Devils Pocket

Figure 8 provides a more detailed map of the Timber Creek drainage basin seen in less detail in figure 7 above. The northeast-oriented American Fork joins the southeast-oriented Musselshell River in the figure 8 northwest quadrant. Devils Pocket is located along the figure 8 south edge (west half). Timber Creek flows northeast from north of Devils Pocket and then north-northeast to join the Musselshell River in the figure 8 north center area. Note how there are parallel north-northeast oriented valleys adjacent to the Timber Creek valley providing evidence of multiple north-northeast oriented flood flow routes. Also note the multiple east-oriented valleys in the figure 8 southeast quadrant. Further note multiple valleys in the figure 8 northwest quadrant and also in the Devils Pocket area. These multiple valleys provide excellent evidence figure 8 map area was eroded by large floods that both flowed north and east into what must have been the newly eroded and deep southeast-oriented Musselshell River valley. Areas north of the Musselshell River valley (and visible in figure 8) also appear to have been eroded by large volumes of flood water, probably moving southeast into the valley (the South Fork Judith River-Musselshell River and Judith River-Musselshell River (Big Snowy Mountains drainage divide area essays describe evidence north of the Musselshell River here). The valleys are easiest to explain if the deep Musselshell River was eroding headward to capture southeast-oriented flood flow from the north, and to capture north- and northeast-oriented flood flow from the south. East-oriented valleys in the figure 8 southeast quadrant probably were eroded as the deep southeast-oriented Musselshell River valley head was eroding into the figure 8 map area. When the deep Musselshell River valley head reached the figure 8 north center area the north-northeast oriented valleys were eroded and beheaded the east-oriented flood flow routes. Subsequently headward erosion of the Musselshell River valley head and the northeast-oriented American Fork valley beheaded flood flow routes to the Timber Creek valley complex.

Musselshell River-Fish Creek drainage divide area west of Ryegate

Figure 9: Musselshell River-Fish Creek drainage divide area west of Ryegate. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 9 illustrates the Musselshell River-Fish Creek drainage divide area west of Ryegate and is located east and south of figure 7 and includes overlap areas with figure 7. The Musselshell River flows southeast from Shawmut in the figure 9 northwest corner area to Ryegate along the figure 9 east edge. Fish Creek flows northeast from the figure 9 southwest corner area to join the Musselshell River just east of the figure 9 map area. East and northeast-oriented Mud Creek joins the Musselshell River south of Deadmans Reservoir. Note the southeast and northwest-oriented Fish Creek tributaries and the southeast-facing escarpment located along the Mud Creek-Fish Creek drainage divide. The southeast-northwest orientation of Fish Creek tributaries is evidence the Fish Creek valley eroded headward across southeast-oriented flood flow. The northwest-oriented tributary valleys were eroded by reversals of flood flow on the northwest ends of beheaded flood flow routes. The southeast-facing escarpment was eroded by southeast-oriented flood flow flowing into the newly eroded and deep northeast-oriented Fish Creek valley. Southeast-oriented flood flow across the present day drainage divide and the escarpment face ended when headward erosion of the northeast-oriented Mud Creek valley captured the flood waters and diverted the flood waters northeast to what was then the newly eroded Musselshell River valley. A close look at the Mud Creek-Fish Creek drainage divide area reveals shallow through valleys eroded across the drainage divide. Some of these shallow through valley are associated with northwest-oriented Mud Creek tributary valleys, which were eroded by reversals of flood flow as the Mud Creek valley beheaded flood flow routes to the Fish Creek valley. West of Indian Butte is a large northwest-oriented Mud Creek tributary valley with a significant northeast-oriented tributary valley. The northeast-oriented tributary valley provides evidence reversed flood flow responsible for eroding the northwest-oriented valley captured significant yet to be beheaded southeast-oriented flood flow and diverted that captured flood water northeast and then northwest into the newly eroded northeast-oriented Mud Creek valley. Because flood waters moved in anastomosing (or interconnected channels) such captures of yet to be beheaded flood flow were common and helped erode significant reverse flow valleys. This evidence supports the interpretation the northeast-oriented Musselshell River tributary valleys were eroded in sequence as the deep Musselshell River valley eroded headward across the region. Figure 10 below provides a detailed map the area south of Barber and provides further evidence the Musselshell River valley eroded headward into the figure 9 map region.

Detailed map of Musselshell River-Fish Creek drainage divide area west of Ryegate

Figure 10: Detailed map of Musselshell River-Fish Creek drainage divide area west of Ryegate. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 10 illustrates the Musselshell River-Fish Creek drainage divide area south of Barber seen in less detail in figure 9 above. The southeast-oriented Musselshell River flows through the figure 10 north center area and northeast quadrant. East and northeast-oriented Fish Creek is located in the figure 10 south center area and southeast quadrant. Indian Point is the labeled high point located in the figure 10 west center area. Indian Point appears to located on a large north-dipping hogback that extends west to east across most of the figure 10 map area. The hogback may be related to a structure pre-dating the immense southeast-oriented flood or it may be related to a structure that developed as flood waters eroded the figure 10 map area. If the latter, the structure may have been caused by crustal warping triggered by removal of significant overburden and/or by crustal warping caused by the presence of a thick continental ice sheet in a nearby region. Note multiple southeast-oriented Fish Creek tributaries and streamlined erosional residuals between some of the tributary valleys. The erosional residuals provide evidence of a former anastomosing channel complex in the Fish Creek valley. The multiple southeast-oriented tributaries provide evidence the Fish Creek valley eroded headward across a large southeast-oriented flood. The large valley eroded across the west to east oriented hogback just east of Indian Point is much wider in the south than in the north. This evidence suggests the valley was eroded at a time when the Musselshell River valley was not as deep as it is now, although final erosion occurred as the Musselshell River valley was deepened to its present depth. If interpreted correctly the valley probably was eroded as the Musselshell River valley initially eroded headward across the figure 10 map region. At that time the Musselshell River valley and Fish Creek valley depths were not as deep as they are now. Both valleys were subsequently eroded deeper by flood waters captured from points further upstream and deepening of the Musselshell River valley probably occurred slightly before deepening of the Fish Creek valley. As a final note,  future workers will probably significantly improve on my interpretations with respect to flood water movements, although I doubt the figure 10 evidence can be explained without a major flood event.

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.