The Judith River-Musselshell River drainage divide area discussed here is located in the Big Snowy Mountains, Montana, USA. Although detailed topographic maps of the Big Snowy Mountains Judith River-Musselshell River drainage divide area have been available for more than fifty years detailed map evidence has not previously been used to interpret the region’s geomorphic history. The interpretation provided here is based entirely on topographic map evidence. The Big Snowy Mountains Judith River-Musselshell River drainage divide area is interpreted to have been eroded during immense southeast-oriented flood events, the first of which flowed on a topographic surface at least as high as the highest points in the present-day drainage divide area. Flood erosion across the Big Snowy Mountains drainage divide ended when headward erosion of the deep Missouri River valley and its tributary Judith River valley captured all southeast-oriented flood flow.

Introduction:

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

Big Snowy Mountains Judith River-Musselshell River drainage divide area location map

Figure 1: Big Snowy Mountains Judith River-Musselshell River drainage divide area location map (select and click on maps to enlarge). National Geographic Society map digitally presented using National Geographic Society TOPO software.

Figure 1 provides a Big Snowy Mountains Judith River-Musselshell River drainage divide area location map and illustrates a region in Montana. The Missouri River flows northwest from Three Forks (located in the figure 1 southwest quadrant) to Wolf Creek and then northeast to Great Falls, Fort Benton, and Loma (located along the figure 1 north edge). North of figure 1 the Missouri River turns to flow southeast back into the figure 1 map area and then east-northeast and southeast to Fort Peck Lake (the large reservoir located in the figure 1 northeast corner area). The Judith River flows northeast from the Little Belt Mountains area through the Judith Basin and then north to join the east-oriented Missouri River in the figure 1 north center area. The Musselshell River flows from the Little Belt Mountains area (figure 1 west center) to Martinsdale and then southeast to Harlowton, Ryegate and Lavina. From Lavina the Musselshell River flows northeast to Roundup and Melstone where it turns to flow north to join the Missouri River at Fort Peck Lake. South of the southeast- and northeast-oriented Musselshell River is the Yellowstone River, which flows through Livingston, Big Timber, Billings, and Custer. The Judith River-Musselshell River drainage divide area of concern in this essay is the drainage divide between the north-oriented Judith River and the southeast and east-oriented Musselshell River, which is located south of the Big Snowy Mountains.

• The Big Snowy Mountains today are a high isolated mountain region, and the Judith River-Musselshell River drainage divide discussed in this essay is located along the Big Snowy Mountains crest. This essay interprets that high mountain ridge to have been eroded by an immense southeast-oriented flood. At the time flood erosion occurred the Big Snowy Mountains did not stand high above the surrounding region and initially there was no Musselshell River valley to the south. And initially there was no Yellowstone River valley further to the south. The Big Snowy Mountains were not an obstacle to the southeast-oriented flood water movement perhaps because the Big Snowy Mountains were buried in easily eroded material such as easily eroded sedimentary rock and/or ice. Another possibility is the Big Snowy Mountains had not yet been uplifted and uplift occurred as flood waters eroded the region, although some combination of the two possibilities is probable.

• Regardless of why the Big Snowy Mountains did not present an obstacle to flood water movement the deep Yellowstone River valley eroded headward into the figure 1 map area to capture southeast-oriented flood waters and divert flood waters northeast. Next the deep Missouri River-Musselshell River valley eroded into the figure 1 map region to capture southeast-oriented flood flow moving to the newly eroded northeast-oriented Yellowstone River valley. The Musselshell River valley first eroded south to the Melstone area and then eroded southwest and west in the area south of the present day Big Snowy Mountains. Southeast-oriented tributaries to the east-oriented Musselshell River valley provide evidence headward erosion of the Musselshell River valley captured southeast-oriented flow. At that time the Big Snowy Mountains did not significantly interfere with flood water movements. However, as immense quantities of flood water moved to the newly eroded and deep east-oriented Musselshell River valley the Big Snowy Mountains emerged as a significant obstacle to flood flow, and southeast-oriented flood flow routes were progressively beheaded and reversed so as to flow in a northwest-oriented direction. This beheading and reversal of flood flow routes across the Big Snowy Mountains occurred as the east-oriented Missouri River and the north-oriented Judith River valley eroded deep valleys north and northwest of the Big Snowy Mountains. The Fergus County Judith River-Musselshell River drainage divide area (north of the Big Snowy Mountains essay, the Armells Creek-Box Elder Creek drainage divide area essay, Missouri River-Musselshell River drainage divide area essay, McDonald Creek-Flatwillow Creek drainage divide area essay, Flatwillow Creek-North Willow Creek and Willow Creek drainage divide area essay, and the North Willow Creek-Musselshell River drainage divide area essay describe drainage divide areas located near the Big Snowy Mountains Judith River-Musselshell River drainage divide areas discussed here and can be found under Musselshell River on the sidebar category list.

Big Snowy Mountains Judith River-Musselshell River drainage divide area detailed location map

Figure 2: Big Snowy Mountains Judith River-Musselshell River drainage divide area detailed location map. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 2 illustrates a somewhat more detailed map of the Big Snowy Mountains Judith River-Musselshell River drainage divide area discussed in this essay. Judith Basin, Wheatland, Golden Valley, and Musselshell Counties are located in Montana. The unnamed county east of Judith Basin County is Fergus County and Petroleum County is located east of Fergus County. The east-oriented Missouri River is located north of the figure 2 map area. The Judith River flows northeast from the figure 2 southwest corner area through Utica and Hobson and then flows near Ross Fork and Ware near the figure 2 north edge. The Musselshell River in Wheatland County flows east to Harlowton, and then southeast to Ryegate and Lavina, before turning northeast to flow to Roundup and Delphia in Musselshell County. The Big Snowy Mountains are located near the figure 2 center area south of Lewistown and the Little Snowy Mountains are located east of the Big Snowy Mountains. Note on figure 2 southeast-oriented streams flowing from the Big Snowy Mountains to tributaries of the east-oriented Musselshell River. Also note the northwest-oriented streams flowing from the Big Snowy Mountains to the Judith River and/or to northwest oriented Judith River tributaries. This remarkable northwest-southeast alignment of drainage routes on both sides of the present day high Big Snowy Mountains Judith River-Musselshell River drainage divide provides evidence multiple southeast-oriented flood flow routes once did cross the Big Snowy Mountains. As previously mentioned, when flood waters moved southeast from northwest of the present day Big Snowy Mountains to southeast of the present day Big Snowy Mountains, the Big Snowy Mountains were not an obstacle to flood flow movement. Evidence presented here and in essays describing adjacent drainage divide areas does suggest the Big Snowy Mountains emerged as a significant obstacle as flood waters were eroding the figure 2 map area.

• The East Fork Big Spring Creek is an important Judith tributary and originates in the Big Snowy Mountains and flows north to Heath. From Heath Big Spring Creek flows through Lewistown to join the north-northwest and north-northeast oriented Judith River near the figure 2 north edge. Cottonwood Creek is another important Big Spring Creek tributary, which also originates in the high Big Snowy Mountains, and which flows northwest to join Big Spring Creek near Hanover. The North Fork McDonald Creek is an important tributary to the north-oriented Musselshell River (located east of figure 2) and originates east of Heath and flows east to Forestgrove and then east-northeast to Grassrange before joining southeast-oriented Box Elder Creek and flowing to the figure 2 east edge. Flatwillow Creek and its tributary, Half Moon Creek, are also tributaries to the north-oriented Musselshell River located east of the figure 2 map area. Half Moon Creek originates near the East Fork Big Spring Creek source area in the high Big Snowy Mountains and flows northeast to join the east-oriented North Fork Flatwillow Creek, which turns southeast to the flow to the figure 2 southeast corner. East of the figure 2 map area Flatwillow Creek turns to flow northeast and join east-oriented McDonald Creek.

• Evidence in detailed maps below illustrates flood waters originally flowed on a topographic surface at least as high as the highest Big Snowy Mountains elevations today. Flood waters of concern in this essay originally were moving to what was then a newly eroded and deep northeast-oriented Yellowstone River valley (see figure 1). Headward erosion of the deep east and northeast-oriented Musselshell River valley at some point captured the southeast-oriented flood flow routes and diverted the flood waters east, northeast and north, although flood waters moving across the figure 2 map area continued to flow on the same southeast-oriented flood flow routes. As flood waters flowed across the figure 2 map area the Big Snowy Mountains and Little Snowy Mountains emerged as major obstacles, which forced flood flow to move around them. Emergence of these isolated mountain areas occurred as flood waters deeply eroded the region.

Area north and west of Big Snowy Mountains Judith River-Musselshell River drainage divide

Figure 3: Area north and west of Big Snowy Mountains Judith River-Musselshell River drainage divide. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 3 illustrates the region north and west of the Big Snowy Mountains Judith River-Musselshell River drainage divide. Ross Fork (of the Judith River) flows north from Garnell (located in the figure 3 southwest corner area near the south edge) to the figure 3 northwest corner area and then turns north to join the northeast-oriented Judith River north of the figure 3 map area. The northeast-oriented Judith River is located northwest of the northeast-oriented Ross Fork valley segment in the figure 3 northwest corner. The Big Snowy Mountains are located in the figure 3 south center area and southeast quadrant along the figure 3 south edge. West of the figure 3 southwest corner are the Little Belt Mountains and the north-south highway and railroad in the figure 3 southwest corner area are located in a large north-south through valley known as Judith Gap. The Judith Gap through valley links the north-oriented Judith River valley, which drains to the Missouri River, with the east-oriented Musselshell River valley south of figure 3. North-oriented Ross Fork originates south of Judith Gap and has south-oriented tributaries which flow from the west end of the Big Snowy Mountains.

• The Big Snowy Mountains drainage divide of concern is located along the high ridge representing Big Snowy Mountains’ highest points. Knife Blade Ridge (located just south of the gap between “Snowy” and “Mountains” in the figure 3 southeast quadrant) forms the central section of the drainage divide discussed here. North of Knife Blade Ridge the north oriented East and West Forks have eroded deep north oriented valleys from  Knife Blade Ridge  that converge like an inverted “Y” to form north and northwest oriented Cottonwood Creek. Cottonwood Creek flows to the figure 3 north edge and joins the Judith River north of the figure 3 map area. East of the deep north-oriented East Fork Cottonwood Creek valley, the East Fork Big Spring Creek has eroded a deep north-northeast  and north oriented valley from the high Big Snowy Mountains drainage divide area to Heath in the figure 3 northeast quadrant. At Heath, the East Fork Big Spring Creek turns to flow northwest, west-southwest to join Big Spring Creek, which then flows northwest to the figure 3 north edge. Big Spring Creek continues to flow northwest and joins the Judith River north of the figure 3 map area. East Fork Big Spring Creek is the easternmost Judith River tributary draining the high Big Snowy Mountains area.

• The large northeast-oriented valley (in the figure 3 southeast corner area) leading from the high Big Snowy Mountain drainage divide to the large east-oriented valley at the Big Snowy Mountain base is the Half Moon Creek valley. Half Moon Creek flows northeast from the high Big Snowy Mountains and on reaching the Big Snowy Mountains base turns east to become the North Fork of Flatwillow Creek. Flatwillow Creek east of figure 3 flows northeast, southeast, and northeast, and then southeast to reach the north-oriented Musselshell River east of figure 3. North of North Fork Flatwillow Creek are other east-oriented valleys draining eventually to Flatwillow Creek and the north-oriented Musselshell River. Note how at the Big Snowy Mountains base, at the point where northeast-oriented Half Moon Creek turns east to become the North Fork Flatwillow Creek, there is a large through valley linking the Half Moon Creek and North Fork Flatwillow Creek valleys with the north-oriented East Fork Big Spring Creek valley. That through valley provides evidence water once flowed between the modern-day Judith River drainage basin and the present day Flatwillow Creek drainage basin.

Area south and east of Big Snowy Mountains Judith River-Musselshell River drainage divide

Figure 4: Area south and east of Big Snowy Mountains Judith River-Musselshell River drainage divide. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 4 illustrates the area south and east of the high Big Snowy Mountains Judith River-Musselshell River drainage divide. The high east-southeast oriented Big Snowy Mountains drainage divide is located in the figure 4 northwest quadrant. In the figure 4 northeast quadrant are the Little Snowy Mountains, which were discussed in the Flatwillow Creek-North Willow and Willow Creek drainage divide area essay. In that essay evidence suggested southeast-oriented flood water once flowed south across the Little Snowy Mountains to Willow Creek and the northeast-oriented Musselshell River. A north-south oriented valley near Red Hill, located between the Big and Little Snowy Mountains, provides evidence of one south-oriented flood flow route (see southeast corner area of figure 6 for a more detailed map of the Red Hill through valley). The Big Snowy Mountains are much higher than the Little Snowy Mountains, and unlike the Little Snowy Mountains, there are no deep through valleys eroded across the high Big Snowy Mountains drainage divide ridge.

• Note the south and southeast-orientation of drainage routes on the Musselshell River side of the high Big Snowy Mountains drainage divide and how that orientation continues to the figure 4 south edge. Further, note how deep valleys extend headward all the way up to the high drainage divide ridge. Some of the south-facing valley heads are large escarpment-rimmed basins, which have the appearance of glacial cirques. Figures 6 and 7 below illustrate those cirque-like basins in more detail. While it is possible those cirque-like basins are really cirques, and were eroded by glacial ice, an alternate hypothesis is those basins are abandoned headcuts eroded by immense southeast-oriented floods moving into what was then the newly eroded east-oriented Musselshell River valley. If so, the Big Snowy Mountains at that time did not stand high like they do today. Evidence presented in this essay suggests the Big Snowy Mountains were being uplifted as the immense southeast-oriented flood crossed the present day Big Snowy Mountains drainage divide.

• Note also the linear Big Snowy Mountains southwest flank, which suggests the Big Snowy Mountains may be an uplifted fault block tilting to the north, although the linear south flank may be related to steeply dipping beds of a large anticlinal structure. In other words, figure 4 provides evidence the Big Snowy Mountains were not always high like they are today, but were uplifted at some point in geologic history. 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, such as the Big Snowy Mountains, 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.

Musselshell River valley south of Big Snowy Mountains Judith River-Musselshell River drainage divide

Figure 5: Musselshell River valley south of Big Snowy Mountains Judith River-Musselshell River drainage divide. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 5 illustrates the east-oriented Musselshell River valley south of the figure 4 map area and includes overlap areas with figure 4. The Musselshell River is located near the west to east oriented highway near the figure 5 south edge and flows in an east direction across the figure 5 map area. The region south of the figure 5 map area has been described in the Fish Creek-Big Coulee Creek drainage divide area and Painted Robe Creek-Yellowstone River drainage divide area essays, among others. Evidence presented in those essays suggests immense southeast oriented floods moved across the present day Big Snowy Mountains drainage divide area to what was then the newly eroded and deep Yellowstone River valley. Ridges in the figure 5 map area, especially surrounding the Womans Pocket area, are probably hogbacks and suggest the presence of eroded structures. Note how many south and southeast oriented streams coming from the high Big Snowy Mountains drainage divide area do not directly enter the Musselshell River, but join southeast and east-southeast oriented Musselshell River tributaries to indirectly flow to the present day Musselshell River. Probably, the initial east-oriented Musselshell River valley, which eroded headward into the figure 5 map area to capture southeast- and south oriented flood flow, was a shallower valley than exists today. Drainage routes from the present day high Big Snowy Mountains drainage divide area were probably established in reference to that shallower Musselshell River valley. Uplift of the Big Snowy Mountains must have blocked further south and southeast-oriented flood flow into the figure 5 map area while headward erosion of the Musselshell River valley west of the figure 5 map area was still capturing large flood water volumes, and those captured flood waters were still moving east in what was then the newly eroded east-oriented Musselshell River valley. Flood waters from the west further eroded and altered the Musselshell River valley.

East end of Big Snowy Mountains Judith River-Musselshell River drainage divide area

Figure 6: East end of Big Snowy Mountains Judith River-Musselshell River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 6 illustrates the east end of the high Big Snowy Mountains drainage divide region seen in less detail in figures 3 and 4 above. East and northeast oriented North Fork Flatwillow Creek is located in the figure 6 northeast corner area and drains eventually to the north-oriented Musselshell River east of the figure 6 map area (see figures 1 and 2). Half Moon Creek flows north-northeast from the Greathouse Peak area (located in the figure 6 center) to join east and northeast oriented North Fork Flatwillow Creek. Note the through valley near the figure 6 north edge linking the north-northeast oriented Half Moon Creek valley and the east-oriented North Fork Flatwillow Creek valley with the north-oriented Dry Canyon valley and the northeast, northwest, and northeast oriented valley located west of the Dry Canyon valley. That northeast, northwest and northeast oriented valley is the East Fork Big Spring Creek valley seen in figure 3 above, which flows north and then northwest to join the Judith River. The through valley provides evidence south-oriented flood water in the East Fork Big Spring drainage basin once flowed to the Flatwillow Creek drainage basin. The East Fork Big Spring Creek valley and the Half Moon Creek valley both originate in the high Big Snowy Mountains near Greathouse Peak. Those high level valley heads provide evidence headward erosion of both the East Fork Big Spring Creek valley and the Half Moon Creek valley reached into what is today the highest level Big Snowy Mountains erosion surface. Figure 7 below provides a more detailed map of the Greathouse Peak area.

• Northeast-oriented South Fork Flatwillow Creek is located in the figure 6 east center edge area and joins the North Fork east of the figure 6 map area and then flows as Flatwillow Creek to join the north-oriented Musselshell River further to the east. Maynard Coulee, Horsethief Canyon, and Hay Coulee are the named drainage routes flowing to the northeast-oriented South Fork Flatwillow Creek valley. Note how Horsethief Canyon begins as a southeast oriented drainage route southeast of Old Baldy and then turns to flow northeast to northeast-oriented South Fork Flatwillow Creek. Also note southeast and south oriented Musselshell River tributary valleys located immediately south and southeast of the Horsethief Canyon elbow of capture. Those valleys provide evidence headward erosion of the South Fork Flatwillow Creek (Half Moon Creek valley) into  the east end of the Big Snowy Mountains beheaded southeast and south oriented flood flow routes. Further, note, how a northwest-southeast oriented through valley links the North Fork Flatwillow Creek valley with the South Fork Flatwillow Creek. That through valley provides evidence southeast-oriented flood flow to the South Fork Flatwillow Creek valley was beheaded by headward erosion of the deep North Fork Flatwillow Creek valley. Another prominent through valley is located further south at Red Hill, which is a north-south pass between the South Fork Flatwillow Creek valley and south and southeast oriented Musselshell River tributary valleys to the south. The Red Hill Pass through valley provides evidence headward erosion of the South Fork Flatwillow Creek valley beheaded south-oriented flood flow to the Musselshell River valley.

• Continuing west along the high Big Snowy Mountains drainage divide there are two large south-facing cirque-like basins eroded into the high level Big Snowy Mountain erosion surface. One is south of Half Moon Pass and the other is south of Knife Blade Ridge. While it is possible these are indeed glacially eroded cirques, the hypothesis proposed here is they are large abandoned headcuts eroded by south and southeast oriented flood waters probably moving to what was then the newly eroded Yellowstone River valley and later to the east-oriented Musselshell River valley. If this hypothesis is correct uplift of the Big Snowy Mountains was probably just beginning as immense quantities of south oriented flood water began to erode the large headcuts into the rising Big Snowy Mountains. Flood waters continued to erode these large headcuts into the Big Snowy Mountains south face until uplift of the Big Snowy Mountains caused the southeast and south oriented flood flow routes to be captured and diverted to flow in other directions. The Painted Robe Creek-Yellowstone River drainage divide area essay documents immense floods moved southeast from this eastern Big Snowy Mountains drainage divide area to what was then the newly eroded Yellowstone River valley.

• North of the western cirque-like basin are the north-oriented East and West Fork valleys of Cottonwood Creek, which form an inverted “Y” where they meet in the figure 6 northwest quadrant. A close look reveals linkages between these valley and also with adjacent valleys. These linkages suggest the valleys originated as channels in an anastomosing channel complex. The hypothesis is proposed here is the anastomosing channel complex was eroded by south and southeast-oriented flood waters that eroded the south-facing headcuts located south of Knife Blade Ridge. Then as the Big Snowy Mountains were uplifted the south-oriented flood flow channels were progressively beheaded and reversed so flood waters would flow north. Based on evidence from east of Greathouse Peak the beheading and reversal of flood flow channels probably occurred one channel at a time, beginning in the east and progressing to the west. Because channels were anastomosing (meaning they were interconnected) reversed flood flow in a newly beheaded channel could capture south-oriented flood flow from yet to be beheaded channels. Capture of flood waters from yet to be beheaded channels enabled the newly reversed flood flow to erode the deep north-oriented valleys now seen on the Big Snowy Mountains north slope. Figure 8 below provides a more detailed map of the anastomosing valleys.

Big Snowy Mountains Judith River-Musselshell River drainage divide area near Half Moon Pass

Figure 7: Big Snowy Mountains Judith River-Musselshell River drainage divide area near Half Moon Pass. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 7 provides a detailed map of the Big Snowy Mountains Judith River-Musselshell River drainage divide area near Half Moon Pass. Half Moon Creek originates near Greathouse Peak and flows east and northeast in a deep valley to the figure 7 north edge, and then to the North Fork Flatwillow Creek, Flatwillow Creek, and the north-oriented Musselshell River. North of Greathouse Peak are north-northeast headwaters of the East Fork Big Spring Creek, which flows north and northwest from the figure 7 map area to eventually reach the north-oriented Judith River. Remember, figure 6 illustrated linkages between the East Fork Big Spring Creek valley and the Half Moon Creek valley further to the north. West of Greathouse Peak are northwest-oriented headwaters of the East Fork of Cottonwood Creek. Cottonwood Creek as seen in figure 3 flows northwest to eventually reach the north-oriented Judith River. South of Greathouse Peak are headwaters of southeast-oriented Swimming Woman Creek, which is a southeast-oriented Musselshell River tributary. The Swimming Woman Creek headwaters are located in the easternmost of the south-facing cirque-like basins seen in figure 6. Note the saddle between Knife Blade Ridge along the figure 7 west edge and Greathouse Peak. That saddle is what remains of a channel that once carried southeast-oriented flood waters moving on the East Fork Cottonwood Creek alignment to the Swimming Woman Creek valley.

• Note also Half Moon Pass, which represents what remains of the valley that eroded headward along the Half Moon Creek valley alignment to capture southeast oriented flood flow moving to the actively eroding southeast oriented Swimming Woman Creek valley (which means at that time the Swimming Woman Creek valley head had not yet reached the figure 7 map area). Apparently there was enough flood water moving across this figure 7 map area to erode the deep northeast-oriented Half Moon Creek valley and to enable the actively eroding Swimming Woman Creek valley head to erode a large headcut into the figure 7 map area and in the process behead the flood flow route to the northeast-oriented Half Moon Creek valley. Southeast of Old Baldy (in the figure 7 southeast quadrant) are southeast oriented headwaters of southeast and northeast oriented Horsethief Canyon. East and northeast of Old Baldy are northeast oriented headwaters of Horsethief Canyon tributaries (figure 6 provides a big picture view of the Horsethief Canyon drainage system). Horsethief Canyon drains to the South Fork Flatwillow Creek, Flatwillow Creek, and the north-oriented Musselshell River.

• Figure 7 evidence describes the following drainage history. First, an immense southeast-oriented flood flowing across the figure 7 map area, perhaps to an actively eroding Swimming Woman Creek valley head located southeast of the figure 7 map area, was partially captured by headward erosion of the South Fork Flatwillow Creek-Horsethief Canyon valley. Second, headward erosion of the North Fork Flatwillow Creek-Half Moon Creek valley partially captured the southeast-oriented flood flow to the actively eroding Swimming Woman Creek valley head and beheaded flood flow routes to what was then the actively eroding Horsethief Canyon valley system. Third, the south-oriented flood flow route using the present day East Fork Big Spring Creek valley alignment was beheaded and reversed, causing flood waters in the East Fork Big Spring Creek valley to flow north and to partially capture southeast-oriented flood flow to the actively eroding Swimming Woman Creek valley head (and perhaps the actively eroding Half Moon Creek valley). After the large southeast-oriented Swimming Woman Creek headcut eroded into the figure 7 map area, southeast-oriented flood flow using the East Fork Cottonwood Creek alignment was beheaded and reversed. Ongoing Big Snowy Mountains region uplift  may have been a major contributing factor to these flood capture events.

Anastomosing valley complex on Big Snowy Mountains north slope

Figure 8 illustrates an anastomosing valley complex located on the Big Snowy Mountains north slope. Figure 6 provides a big picture view of the figure 8 map area. The East Fork Big Spring Creek valley is located in figure 8 southeast corner and is north-northeast and northeast oriented and east of the figure 8 map area becomes northwest-oriented and finally north northeast-oriented along the figure 8 east edge. West of the East Fork Big Spring Creek valley and draining to the figure 8 north edge is north and northwest-oriented Lyon Canyon. North of the figure 8 map area Lyon Canyon is linked by through valleys with both the north oriented East Fork Big Spring Creek valley and the northwest-oriented Cottonwood Creek valley (that linkage is illustrated in figure 10 of the Fergus County Judith River-Musselshell River drainage divide area essay). Flowing north from the figure 8 south center edge area and then northwest to the figure 8 northwest corner area is the East Fork Cottonwood Creek. In the figure 8 northwest corner area it joins north-northwest and north-northeast oriented West Fork Cottonwood Creek to form north- and northwest-oriented Cottonwood Creek. An unnamed northwest-oriented West Fork Cottonwood Creek tributary is linked by a deep through valley with the north-oriented East Fork Cottonwood Creek valley. Also note the north- and west-oriented East Fork Cottonwood Creek tributary valley in the figure 8 center east area linking the Lyon Canyon valley with the much deeper East Fork Cottonwood Creek valley. Those valleys and other similar, but less obvious through valleys, provide evidence of an anastomosing valley complex on the Big Snowy Mountains north slope. The anastomosing valley complex probably originated as southeast-oriented flood waters eroded deep channels into what was then the rising Big Snowy Mountains. For example, the deep unnamed northwest-oriented West Fork Cottonwood Creek tributary valley is on the same alignment as the East Fork Cottonwood Creek headwaters seen in figure 7 above and was initiated as a southeast-oriented valley. Later when flood flow routes were beheaded and reversed to flow north, reversed flood flow in the East Fork Cottonwood Creek valley somehow captured large volumes of flood water from flood flow routes further to the west and some of that water flowed northwest from the East Fork valley to the West Fork valley, while most of the water flowed north and northwest in the present day East Fork valley. Linkages between the Lyon Canyon valley in figure 8 and also further north as illustrated in the essay cited above, provide further evidence of anastomosing valleys.

West end of Big Snowy Mountains Judith River-Musselshell River drainage divide area

Figure 9: West end of Big Snowy Mountains Judith River-Musselshell River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 9 illustrates the west end of the high Big Snowy Mountains drainage divide area discussed in this essay. The figure 9 map area is located west of the figure 6 map area and includes overlap areas with figure 6. The west end of the Big Snowy Mountains north slope is drained by north-northwest oriented Big Rock Creek, located north of Mount Harlow (located east of Crystal Lake). North and west of figure 9 Big Rock Creek flows northwest and north-northwest to join the Judith River (see figure 3). South and southwest-oriented drainage in the figure 9 extreme southwest corner flows to north-oriented Ross Fork Creek and to the Judith River (see figure 3). The U-turn made by these south-oriented streams provides evidence of a major flood flow reversal in the deep Judith Gap through valley located west of the Big Snowy Mountains. That flood flow reversal occurred when headward erosion of the deep east-oriented Missouri River and its north-oriented tributary Judith River valley beheaded what must have been a major south-oriented flood route eroding the deep Judith Gap valley. South-oriented valleys eroded into the western Big Snowy Mountains are not as deep or as large as south-facing valleys in the Knife Blade Ridge area and tend to be southwest-oriented. The southwest-orientation may be related to a south-southeast oriented valley that was beginning to erode headward into the Judith Gap area. Initially that valley may have eroded headward from what was then the newly eroded Yellowstone River valley across the present day Lake Basin area illustrated and described in the Painted Robe Creek-Yellowstone River drainage divide area essay. Headward erosion of the east-oriented Musselshell River valley subsequently captured that south-southeast oriented flood flow route, although Judith River valley headward erosion beheaded and reversed flood flow along that flood flow route soon thereafter.

• West of the Knife Blade Ridge area the drainage divide is somewhat lower and becomes progressively lower to the west. The western drainage divide is marked by saddles located between various peaks. The saddles provide evidence of multiple channels of south-oriented flood flow that once moved across the present day western Big Snowy Mountains drainage divide. The multiple channels suggests flood waters were moving in anastomosing channels from what is today the Big Rock Creek drainage basin to the present day Musselshell River drainage. The multiple north oriented Big Rock Creek tributary valleys probably originated as channels eroded into the rising Big Snowy Mountains by this south-oriented anastomosing channel complex. This south-oriented channel complex probably captured flood water from an earlier southeast-oriented channel complex. For example, note how the northwest-oriented East Fork of Green Pole Canyon (a Big Rock Creek tributary located in the figure 9 north center area) is on the same northwest-southeast alignment as the unnamed West Fork Cottonwood Creek tributary linking the East and West Forks of Cottonwood Creek (discussed in figure 8) and the East Fork Cottonwood headwaters illustrated in figure 7. Uplift of the Big Snowy Mountains combined with the progressive beheading of southeast-oriented flood flow routes as the deep north oriented Judith River valley head eroded south and southwest systematically reversed south-oriented flood flow channels to create the north and northwest oriented Big Rock Creek drainage system seen today.

Judith River-Musselshell River drainage divide area at Judith Gap

Figure 10 illustrates the Judith River-Musselshell River drainage divide area at Judith Gap and is located west and south of the figure 9 map area. Figure 10 includes overlap areas with figure 9. West Peak at the west end of the high Judith River-Musselshell River drainage divide in the Big Snowy Mountains is located in the figure 10 northeast corner (see figure 9 for West Peak location along the high Big Snowy Mountains drainage divide). The eastern tip of the Little Belt Mountains is located along the figure 10 west edge just north of the Wheatland County line. Ross Fork Creek is a north-oriented Judith River tributary and originates on the south side of the Big Snowy Mountains. Blake Creek, which drains the Big Snowy Mountains south-facing slope south of West Peak is the westernmost Musselshell River tributary draining the high Big Snowy Mountains. West of Blake Creek the Judith River-Musselshell River drainage divide is located between south-southeast, west, and north-northwest oriented Ross Fork Creek and south-oriented streams flowing to the figure 10 south edge.

• Judith Gap is a remarkable through valley, eroded by south-oriented flood water moving to what was then the newly eroded Yellowstone River valley. The route flood waters flowed on, south and southeast from the present day Musselshell River valley, is described in the Fish Creek-Big Coulee Creek and Painted Robe Creek-Yellowstone River drainage divide area essay. Evidence presented in those essays describes immense volumes of flood water moving across what is today the Lake Basin to what was then the newly eroded Yellowstone River valley. Flood flow to the Yellowstone River valley ended when headward erosion of the Musselshell River valley captured the south-oriented flood waters. Subsequently, headward erosion of the east-oriented Missouri River valley to the north, and its tributary northeast and north-oriented Judith River valley beheaded the south-oriented flood flow route moving flood waters through Judith Gap to what was then the newly eroded Musselshell River valley. Flood waters on the north end of the beheaded flood flow route reversed flow direction to flow north to the newly eroded Judith River valley. The reversed flood flow eroded the north-oriented Ross Fork Creek valley and created the Judith River-Musselshell River drainage divide seen here.

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.