A geomorphic history based on topographic map evidence

This overview essays provides highlights from more detailed essays which illustrate and describe  topographic map evidence to interpret landform origins within the Montana Musselshell River drainage basin and between the Musselshell River drainage basin and adjacent drainage basins. All interpretations in this essay and the related detailed essays are based on topographic map evidence, examples of which are illustrated here. The detailed essays can be found under Musselshell River on this website’s sidebar category list. Musselshell River headwaters are located in the Little Belt, Castle, and Crazy Mountains of central Montana and Musselshell River tributaries originate in the Big Snowy, Little Snowy, and Judith Mountains. East of Martinsdale, where the North and South Forks meet, the Musselshell River flows in a southeast, northeast, and then north direction to join the northeast and east oriented Missouri River. Evidence presented in this overview essay and the related detailed essays demonstrates a deep Musselshell River valley eroded headward to capture an immense southeast-oriented flood, which was flowing across the entire present day Musselshell River drainage basin on a high level topographic surface to what was then an actively eroding, deep, and new east and northeast-oriented Yellowstone River valley. Flood waters were derived from a rapidly melting thick North American ice sheet and the deep Musselshell-Missouri River valley eroded headward from the ice sheet margin, suggesting flood waters were being captured by headward erosion of deep northeast-and north-oriented valleys which eroded headward from the ice sheet margin to divert the large ice marginal melt water floods into space the ice sheet had once occupied. Topographic map evidence documents flood waters moved across the present day Judith, Big and Little Snowy, Little Belt, Castle, and Crazy Mountains and those Rocky Mountain outlier mountain ranges emerged either as flood waters deeply eroded the surrounding areas and/or as crustal warping uplifted the Rocky Mountain outlier mountain ranges. Southeast-oriented flood flow into the present day Musselshell River drainage basin ended when headward erosion of the Missouri River valley (to the north and west) beheaded and reversed all southeast-oriented flood flow routes to what was then the newly eroded Musselshell River valley.

Montana Musselshell River drainage basin drainage history

The Musselshell River drainage basin is located in central Montana and the Musselshell River originates in the northern Crazy Mountains and southern Little Belt Mountains and flows in an east-southeast and east-northeast direction to near Melstone where it turns abruptly to flow in a north-northwest and north direction to join the northeast and east-oriented Missouri River. Musselshell River tributaries from the west and north are often southeast oriented and/or have significant southeast oriented segments and/or tributaries. South of the Musselshell River drainage basin is the northeast, southeast, northeast, and east-oriented Yellowstone River drainage basin. Between the Musselshell River headwaters and the Yellowstone River valley are the high Crazy Mountains. West of the Crazy Mountains the Musselshell River headwaters are linked by a large north-south oriented through valley with the south-oriented Shields River, which flows to the Yellowstone River. East of the Crazy Mountains Yellowstone River tributaries from the north tend to be southeast-oriented and are linked by through valleys to Musselshell River tributary valleys. North and west of the Musselshell River drainage basin is the northwest, northeast, southeast, east-northeast, and east-southeast oriented Missouri River drainage basin. Between the Missouri River drainage basin and the Musselshell River drainage are the Little Belt Mountains, Big Snowy Mountains, Judith Mountains, and the Judith Basin.

• Drainage history interpretations in this Musselshell River drainage basin landform origins overview essay and in the related detailed essays are based entirely on topographic map evidence. Each of the detailed essays illustrates and describe a specific Musselshell River drainage divide area and includes location maps and eight or more detailed topographic maps showing examples of landform features found in that region. Evidence interpreted usually includes orientations of valleys present and through valleys linking drainage routes on opposite sides of major drainage divides. Significant through valleys between the Little Belt and Big Snowy Mountains and between the Big Snowy Mountains and Judith Mountains link north-oriented Missouri River tributaries with east and southeast-oriented Musselshell River tributaries. These through valleys can be identified on large region maps such as figure 1, although the linkages become much more evident on topographic maps, especially mosaics of detailed topographic maps. However, each of the nearly 200 different detailed topographic maps included in the detailed essays documents many other smaller scale through valleys providing evidence the deep Musselshell River valley eroded headward across a massive southeast-oriented flood, which was flowing to what was probably a newly eroded and deep Yellowstone River valley and beheaded flood flow routes supplying flood waters to actively eroding Yellowstone River tributary valleys. Headward erosion of the deep northwest, northeast, southeast, east-northeast, and east-southeast oriented Missouri River valley subsequently beheaded and reversed flood flow to the newly eroded Musselshell River valley and its actively eroding tributary valleys. Perhaps what is most intriguing are linkages between Missouri River tributaries and Musselshell River tributaries, which can be found in the high Crazy Mountains, Little Belt Mountains, Big Snowy Mountains, and Judith Mountains. Similar high level linkages between the Yellowstone River tributaries and Musselshell River tributaries can also be found in the high Crazy Mountains. These linkages suggest flood water flowed across what are today high mountain ranges, which means flood waters deeply eroded the surrounding regions and/or the mountain ranges were uplifted as the flood occurred.

Figure 2: Reduced size topographic maps illustrating Musselshell River-Yellowstone drainage divide area east of Melstone (figure 2 southwest corner). Musselshell River flows in northeast and north-northwest direction in figure 2 west half. Southeast-oriented streams flowing to figure 2 east and south edge are Yellowstone River tributaries. Ingomar is the town near the southeast corner and Sumatra is northwest of Ingomar. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Evidence suggesting the deep Musselshell River valley eroded headward across massive southeast-oriented floods is relatively easy to observe on topographic maps east of the various mountain ranges in the Musselshell River drainage basin. Figure 2 uses reduced size topographic maps to illustrate the Musselshell River-Yellowstone River drainage divide area east of Melstone, which is the town located in the figure 2 southwest corner. The Musselshell River flows in a northeast direction to Melstone and then turns to flow in a north-northwest direction. The railroad line (now abandoned) east of Melstone follows the Home Creek valley, which is a west-oriented Musselshell River tributary. West of Sumatra a through valley links the west-oriented Home Creek valley with the south-southeast oriented Muggins Creek valley, which is  a south-southeast oriented Yellowstone River tributary. South of Sumatra a northwest-southeast oriented through valley links the Muggins Creek valley with the southeast-oriented West Fork of Froze to Death Creek valley, and Froze to Death Creek is a southeast-oriented Yellowstone River tributary. North of Sumatra southeast-oriented Yellowstone River tributary valleys are linked by through valleys with valleys of northwest and west-oriented Musselshell River tributaries. The multiple northwest-southeast oriented through valleys and northwest-southeast oriented drainage alignment provide evidence the Musselshell River valley eroded headward across multiple southeast-oriented flood flow channels such as might be found in a large-scale southeast-oriented anastomosing channel complex. Study of through valleys linking the various southeast-oriented Yellowstone River tributary valleys provides further evidence supporting this anastomosing channel complex interpretation. Headward erosion of the deep Musselshell River valley beheaded the southeast-oriented flood flow channels in sequence from north to south. Flood waters on northwest and west ends of the beheaded flow channels reversed flow direction to erode the northwest- and west-oriented Musselshell River tributary valleys.

Topographic map evidence presented in this essay and in the related detailed essays also suggests the southeast-oriented floods were immense and deeply eroded the Musselshell River drainage divide area. Figure 3 uses reduced size maps to illustrate the Musselshell River-Yellowstone River drainage divide area in the Hay Basin, Hoskin Basin, and Commanche Flat area located west and south of the figure 2 map area. Painted Robe Creek is the northeast oriented stream in the figure 3 northwest corner and flows to the northeast oriented Musselshell River. Northwest and north oriented streams flowing to the figure 3 north edge (east half) are Musselshell River tributaries. Southeast-oriented streams flowing to the figure 3 east and south edges are Yellowstone River tributaries. Hoskin Basin is a deep southeast-oriented escarpment-surrounded basin now drained by southeast-oriented Crooked Creek. The escarpment surrounding Hoskin Basin is a giant abandoned headcut, which was eroded by massive sheets of southeast-oriented flood water which were flowing into what at that time was the newly eroded and deep Yellowstone River valley. Hay Basin, which is located upstream on the flood flow route, is a shallower escarpment-surrounded basin eroded into the cap rock surface and is also an abandoned headcut. Comanche Flat is also a flood eroded feature, which was eroded as flood waters removed all bedrock materials above what is today the Comanche Flat cap rock surface. The size of the Hoskin Basin abandoned headcut and evidence the flood was extended westward across the Comanche Flat surface suggests the amount of flood water involved was immense. Further, evidence along the Musselshell River-Yellowstone River drainage divide suggests the flood waters eroded what is today the deep northeast-, east, and northeast-oriented Yellowstone River valley headward slightly in advance of eroding the deep Musselshell River valley headward, with headward erosion of the deep Musselshell River valley beheading flood flow routes to the newly eroded Yellowstone River valley.

• Why would immense southeast-oriented floods be flowing across what is now central Montana? The Musselshell River drainage basin history began with development of a North American ice sheet comparable in size to the present day Antarctic Ice Sheet, if not larger. The ice sheet was thick, probably several kilometers thick, and was located in a deep “hole”, which the ice sheet had formed by a combination of deep glacial erosion and crustal warping caused by the ice sheet weight. When at its maximum size the ice sheet stood high above the pre-glacial surface, but also had roots that extended well below the pre-glacial surface, which no longer exists. The Musselshell River drainage basin location was probably located south and west of ice sheet’s southwest margin, although evidence for the ice sheet’s southwest margin has probably been removed by deep melt water flood erosion. The pre-glacial surface under the ice sheet was completely destroyed by deep glacial erosion and the pre-glacial surface adjacent to the ice sheet and elsewhere on the North American continent was deeply eroded by deep melt water flood erosion and was also probably significantly altered by crustal warping caused by the thick North American ice sheet presence.

• Events important to Musselshell River drainage basin history began as the ice sheet was rapidly melting and had melted to the point it no longer stood high above the surrounding non-glaciated surface, which had probably already been significantly lowered by deep melt water erosion. Immense melt water floods were flowing in a southeast direction along the ice sheet’s southwest margin and were deeply eroding the region between the Rocky Mountains and the ice sheet’s southwest margin, which at that time was located north and east of today’s Musselshell River drainage basin. Also at that time immense southeast and south-oriented supra-glacial melt water rivers were flowing across the ice sheet’s surface to the ice sheet’s southern margin and carving giant ice-walled and ice-floored (later bedrock floored) canyons into the decaying ice sheet surface. A huge southeast and south-oriented ice-walled and ice-floored (later bedrock-floored) canyon was carved north and east of the ice sheet southwest margin and the Missouri Escarpment is today what remains of that giant canyon’s west and southwest wall. The ice floor of that giant southeast and south-oriented ice-walled canyon was significantly lower in elevation than the bedrock surface south and west of the decaying ice sheet margin and the huge melt water river flowing in that southeast and south-oriented ice-walled canyon represented the region’s major drainage route, which captured the immense southeast-oriented ice-marginal floods by eroding deep northeast-oriented tributary valleys headward across the ice sheet’s southwest margin and then headward into the adjacent bedrock surface. These deep northeast-oriented valleys diverted the immense southeast-oriented ice-marginal floods into space the ice sheet had once occupied.

• Melting of what had been the thick ice sheet roots, which may have extended hundreds of meters deep (perhaps one or more kilometers deep) below the surrounding bedrock surface, progressively lowered both the ice sheet surface, the ice-walled canyon floor, and the surrounding bedrock surface, creating a situation where new and even deeper northeast-oriented valleys repeatedly eroded headward to capture immense southeast-oriented ice-marginal melt water floods. The deep northeast-oriented Yellowstone River, Missouri River, and Musselshell River valleys (and other northeast-oriented valleys) we see today were probably eroded very late in the ice sheet melt down history and were probably preceded by several earlier, but similar northeast-oriented valleys which diverted massive southeast-oriented ice-marginal floods onto the decaying ice sheet surface. How much the region between the high Rocky Mountain outlier mountain ranges and the former ice sheet margin was lowered by melt water erosion is difficult to determine, especially from topographic map evidence alone. However, topographic map evidence presented in this Musselshell River drainage basin landform origins overview essay and its related detailed essays does demonstrate the immense ice-marginal melt water floods at one time did flow across what are today high Rocky Mountain outlier mountain ranges. Either the melt water floods removed whatever material was located between those mountains or the mountains were uplifted as the ice-marginal floods were underway. If flood water erosion removed bedrock material between the mountain ranges the amount of bedrock material removed would have been tremendous, suggesting some of the removed material may have been stagnant ice, which simply melted as flood waters flowed across the region.

Figure 4: North Fork Musselshell River and North Fork Smith River headwaters on south side of Little Belt Mountains and the through valley linking the two rivers, which today flow in opposite directions. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Topographic map evidence demonstrating flood waters flowed across the present day high Rock Mountain outlier mountain ranges is abundant and is illustrated and described here. For example, figure 4 above illustrates the through valley linking the south and southeast-oriented North Fork Musselshell River headwaters with the south and southwest-oriented North Fork Smith River headwaters. Figure 5 below provides a detailed topographic map of the headwaters of both rivers showing they both begin in almost the same place along the same ridge in the Little Belt Mountains and are also linked by a through valley at their headwaters. Today the North Fork Smith River flows in a south and southwest direction before  joining the South Fork Smith River to form the northwest-oriented Smith River, which joins the northeast-oriented Missouri River west of the Little Belt Mountains (see figure 1). The North Fork Musselshell River today flows in a south and southeast direction before joining the South Fork to form the Musselshell River, which then flows in east-southeast, northeast, north-northwest, and north direction to join the Missouri River far east of the Little Belt Mountains (see figure 1). Figure 4 evidence suggests the through valley south of the Little Belt Mountains, which links the west-oriented North Fork Smith River valley with the east-oriented North Fork Musselshell River valley (in which the west to east oriented highway is located) is at least to some extent a water eroded valley. South of the figure 4 map area are the Castle Mountains so the valley may also to some extent be a tectonically formed valley.  Also south of the Castle Mountain is a similar through valley between the Castle Mountains and the Crazy Mountains, which also links the north-oriented South Fork Smith River valley with the northeast-oriented South Fork Musselshell River valley. The deep through valleys north and south of the Castle Mountains are easy to see and were eroded by southeast-oriented flood waters moving to what was once the actively eroding Musselshell River valley prior to headward erosion of the deep northeast-oriented Missouri River valley to the northwest of the figure 4 map area (see figure 1). Headward erosion of the deep northeast-oriented Missouri River valley beheaded the southeast-oriented flood flow to what had been the actively eroding Musselshell River valley and flood waters on northwest and west ends of the beheaded flood flow routes reversed flow direction to erode the northwest-oriented Smith River valley and its tributary valleys. Most likely headward erosion of deep northeast-oriented Missouri River valley and the reversal of flood flow was aided by tectonic uplift in the Little Belt, Castle, and Crazy Mountains region, which must have occurred as the massive southeast-oriented floods were flowing across the region. Such tectonic uplift could have been a delayed response to ice sheet caused crustal warping (think about what placing an Antarctic-sized ice sheet on part of the North American continent would do to adjacent and other continental areas).

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.