A geomorphic history based on topographic map evidence

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

Introduction:

• The purpose of this essay is to use topographic map interpretation methods to explore North Fork Smith River-North Fork Musselshell River drainage divide area landform origins in the region between the Castle Mountains and the Little Belt 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 North Fork Smith River-North Fork Musselshell River drainage divide area landform evidence will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm.

North Fork Smith River-North Fork Musselshell River drainage divide area location map

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

Figure 1 provides a location map for the North Fork Smith River-North Fork Musselshell River drainage divide area. Figure 1 illustrates an area in central Montana. The North Fork Smith River and the North Fork Musselshell River both begin in the Little Belt Mountains (near the “l” and “t” in “Belt” on figure 1) as south-oriented streams. The North Fork Smith River flows southwest to White Sulphur Springs before turning to flow northwest as the Smith River to join the northeast-oriented Missouri River near Great Falls. The North Fork Musselshell River flows in a southeast direction to join the South Fork Musselshell River near Martinsdale, and then to flow east-southeast and northeast to the figure 1 east edge. East of figure 1 the Musselshell River turns to flow north to join the Missouri River. The Missouri River originates at Three Forks (located near the figure 1 south edge, west half) and flows north and northwest through Canyon Ferry Lake (a large reservoir) and the Gates of the Rocky Mountains before turning to flow northeast to Great Falls, Fort Benton, and Loma. Northeast of Loma the Missouri River turns to flow southeast and east-northeast and southeast to the figure 1 east edge. Elk Peak (located southeast of White Sulphur Springs) is the highest point in the Castle Mountains (not labeled on figure 1). The northern Castle Mountains are located along the North Fork Smith River-North Fork Musselshell River drainage divide. Essays describing regions east of the North Fork Smith River-North Fork Musselshell River drainage divide have interpreted landform evidence in the context of immense southeast-oriented flood events (essays can be found under appropriate river names on the sidebar category list). Those essays suggest mountain ranges such as the Big Snowy Mountains, Judith Mountains, and Bear Paw Mountains did not present obstacles to the flood movements and suggest the mountains 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 emerged as flood waters eroded the region. North Fork Smith River-North Fork Musselshell River drainage divide evidence is interpreted the same way. Flood waters initially moved a topographic surface where the present day Little Belt Mountains and Castle Mountains were not obstacles to southeast-oriented flood flow. Headward erosion of the deep Musselshell River valley captured southeast-oriented flood flow as the Little Belt Mountains and Castle Mountains were emerging. Headward erosion of the deep Missouri River valley then beheaded a major southeast-oriented flood flow moving flood waters to the newly eroded Musselshell River valley. Flood waters on the northwest end of that beheaded flood flow route reversed flow direction to erode the northwest and north oriented Smith River valley. Reversed flood flow probably captured yet to be beheaded southeast-oriented flood flow moving on flood flow routes further to the south. With the aid of that captured flood water a significant north and northwest oriented Smith River drainage basin evolved.

North Fork Smith River-North Fork Musselshell River drainage divide area detailed location map

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

Figure 2 provides a somewhat more detailed map of the North Fork Smith River-North Fork Musselshell River drainage divide area. Meagher County is located in central Montana. The county north of Meagher County in the figure 2 northeast corner area is Judith Basin County. The county line is located along the drainage divide in the Little Belt Mountains. The South Fork Judith River begins near the county line and flows east and northeast to Indian Hill in the figure 2 northeast corner. The Lost Fork (Judith River) is located south and east of Sand Point and the Middle Fork Judith River is located north of Sand Point. The Judith River flows northeast and north to join the Missouri River (see figure 1). The North Fork Musselshell River begins near the county line and flows south and southeast to Checkboard and joins the northeast-oriented South Fork Musselshell River near Martinsdale to form the southeast-oriented Musselshell River. Checkerboard Creek is a northeast-oriented North Fork Musselshell River tributary originating in the Castle Mountains. The North Fork Smith River begins in almost the same location as the North Fork Musselshell River and flows southwest to White Sulphur Springs and then to join the northwest oriented South Fork Smith River. The Smith River flows northwest and north to the figure 2 northwest corner area. The North Fork Smith River has unnamed northwest oriented tributaries originating in the Castle Mountains. The discussion in this essay begins in the Little Belt Mountains where both the North Fork Musselshell River and the North Fork Smith River originate. The discussion then moves to the region between the Little Belt Mountains and the Castle Mountains and concludes by looking at evidence in the Castle Mountain area. Note northwest and southeast orientations of many figure 2 drainage routes. The northwest-southeast oriented drainage originated when an immense southeast-oriented flood moved across the entire figure 2 map area. Flood waters initially moved a topographic surface where the Little Belt Mountains and the Castle Mountains did not interfere with flood flow. As flood waters eroded the figure 2 map region the Little Belt Mountains and Castle Mountains emerged (perhaps by a combination of deep erosion of the surrounding materials and of uplift occurring as flood waters eroded the region). The deep southeast-oriented Musselshell River valley eroded headward into the figure 2 map region first. The northwest-oriented Smith River valley and northwest-oriented Smith River tributary valleys were initiated as southeast-oriented flood flow channels. Flood flow direction in those channels was systematically beheaded from north to south by headward erosion of the deep Missouri River valley to the northwest. Flood waters on the northwest ends of the beheaded flood flow routes reversed flow direction to erode the northwest- and north-oriented Smith River drainage basin. Erosion of the Smith River drainage basin was aided by capture of yet to be beheaded flood flow from flood flow routes still south of what was then the actively eroding Missouri River valley head. The North Fork Smith River-North Fork Musselshell River drainage divide was created when headward erosion of the Missouri River valley beheaded and reversed flood flow on the Smith River alignment.

North Fork Smith River and North Fork Musselshell River headwaters area

Figure 3: North Fork Smith River and North Fork Musselshell River headwaters area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 3 illustrates the North Fork Smith River and the North Fork Musselshell River headwaters area. The dashed black line extending southeast from the figure 3 north center edge to the figure 3 east edge (south half) is the Judith Basin County-Meagher County line, and defines the Judith River drainage divide. Northeast of the county line is the Judith River drainage basin. East and southeast-oriented headwaters of the South Fork Judith River are located in the figure 3 east center area. The Lost Fork Judith River flows northeast to the figure 3 northeast corner area. East and southeast-oriented West Fork and southeast-oriented Burris Creek are the two major Lost Fork tributaries. The West Fork originates just north of Lost Fork Ridge. Immediately south of Lost Fork Ridge are headwaters of the North Fork Musselshell River, which flows south-southwest and then south and southeast to the figure 3 south center edge. Immediately west of the North Fork Musselshell River headwaters, and also immediately south of Lost Fork Ridge, are headwaters of the North Fork Smith River, which flows south, southwest, and south to the figure 3 south edge (west half). In the figure 3 northwest quadrant the red highway follows Sheep Creek, which flows south-southeast and the west-northwest to the figure 3 west edge. Sheep Creek is a Smith River tributary. Major Sheep Creek tributaries of interest in this discussion are southwest and northwest-oriented Lamb Creek and south- and southwest-oriented Deadman Creek, with its northwest-oriented South Fork. Figure 3 drainage history can be explained in the context of an immense southeast-oriented flood flowing across the region at a time when the Little Belt Mountains did not stand high above the surrounding region. Flood waters initially eroded deep south- and southeast-oriented valleys into the figure 3 map region, perhaps associated with headward erosion of the Musselshell River valley, although initially the valleys may have eroded headward from what was then the newly eroded Yellowstone River valley further to the south. Next headward erosion of the deep South Fork Judith River valley captured some of the southeast-oriented flood flow. Headward erosion of the Lost Fork Judith River valley beheaded most flood flow routes to the South Fork Judith River valley. Headward erosion of the North Fork Musselshell River valley then beheaded a flood flow route to the actively eroding Lost Fork valley head. Subsequently headward erosion of the North Fork Smith Creek valley beheaded that same flood flow route to the North Fork Musselshell River valley (Figure 4 below provides a detail map). Reversal of flood flow on the west-northwest oriented Sheep Creek valley alignment then beheaded flood flow to the North Fork Smith River.

Detailed map of North Fork Smith River and North Fork Musselshell River headwaters area

Figure 4: Detailed map of North Fork Smith River and North Fork Musselshell River headwaters area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 4 provides a detailed map of the North Fork Smith River-North Fork Musselshell River drainage divide area seen in less detail in figure 3 above. Lost Fork Ridge is located near the figure 4 north edge. The Musselshell Ranger District in Meagher County is located in the North Fork Musselshell River drainage basin. The Kings Hill Ranger District is located in the Smith River drainage basin (the figure 4 northwest corner shows a small area in the Sheep Creek drainage basin, otherwise the figure 4 Kings Hill Ranger District area  is located in North Fork Smith River drainage basin). The Judith Ranger District is  located in the Judith River drainage basin. Note how both the North Fork Smith River and North Fork Musselshell River begin along Lost Fork Ridge as southeast or south-southeast oriented streams. Also note the west to east oriented through valley in the figure 4 center (at Ant Park) linking the North Fork Smith River valley with the North Fork Musselshell River valley. Further, note how that through valley extends east to the Lost Fork Judith River valley located in the figure 4 east center edge area. In addition, note how a southeast-oriented North Fork Smith River tributary valley is linked by a high level through valley with a northwest-oriented valley draining to Sheep Creek in the figure 4 northwest corner. These linked through valleys tell an important story. Southeast-oriented flood flow originally moved from the Sheep Creek drainage basin area in the figure 4 northwest corner across the present day North Fork Smith River valley and the present day North Fork Musselshell River valley to what was then the actively eroding Lost Fork Judith River valley. Headward erosion of the North Fork Musselshell River valley then beheaded the east-oriented flood flow route to the Lost Fork Judith River valley. Next headward erosion of the North Fork Smith River valley captured east-oriented flood flow moving across the Ant Park through valley to the newly eroded North Fork Musselshell River valley. Finally a reversal of flood flow in the Sheep Creek drainage basin created the present day Sheep Creek-North Fork Smith Creek drainage divide. There are many other through valleys in the figure 4 map area, some rather subtle. To fully reconstruct the figure 4 drainage history it is necessary to start with flood waters moving on a topographic surface at least as high as the highest present day figure 4 elevations.

North Fork Smith River-North Fork Musselshell River drainage divide area south of Little Belt Mountains

Figure 5: North Fork Smith River-North Fork Musselshell River drainage divide area south of Little Belt Mountains. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 5 illustrates the North Fork Smith River-North Fork Musselshell River drainage divide area south of the Little Belt Mountains and is located south of figure 3 and includes overlap areas with figure 3. The North Fork Musselshell River is located in the figure 5 east half and flows from the figure 5 north edge in a south-southeast direction to Bair Reservoir, Checkerboard, and the figure 5 southeast corner. The North Fork Smith River flows southwest and south from the figure 5 north center edge area to Lake Sutherlin and then southwest and west to the figure 5 west edge. Note Eightmile Creek, which begins in the figure 5 north center area and which flows east of Volcano Butte in a southwest direction to reach Lake Sutherlin and the North Fork Smith River. Also, note southeast-oriented Copper Creek, which begins near Eightmile Creek and then flows to the North Fork Musselshell River. The Eightmile Creek-Copper Creek drainage divide area is illustrated in more detail in figure 6 below. Also note the South Fork Eightmile Creek, which flows near the red highway to reach Lake Sutherlin and northeast and east-oriented Checkerboard Creek, which joins the North Fork Musselshell River at Checkerboard in the figure 5 southeast corner. The South Fork Eightmile Creek-Checkerboard Creek drainage divide area is illustrated in more detail in figure 7 below. In addition to the detailed drainage divide areas illustrated below figure 5 illustrates a large west to east oriented through valley linking the present day west and northwest-oriented Smith River drainage basin with the present day southeast-oriented Musselshell River drainage basin. This through valley is located between the Little Belt Mountains to the north and the Castle Mountains to the south (see figure 8 below for region south of figure 5). The through valley was at least in part eroded by immense volumes of southeast-oriented flood water moving from what is today the Smith River drainage basin to what is now the Musselshell River drainage basin, although it is probable the Musselshell River valley eroded headward into the figure 5 map area late in the flood history. Prior to headward erosion of the Musselshell River valley flood waters were probably moving to what was then the actively eroding northeast-oriented Yellowstone River valley, located further to the southeast.

• The Castle Mountains and the Little Belt Mountains emerged as significant flood flow obstacles as flood waters eroded the figure 5 map 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, such as the Castle Mountains and the Little Belt 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.

Eightmile Creek-Copper Creek drainage divide area

Figure 6: Eightmile Creek-Copper Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 6 illustrates the Eightmile Creek-Copper Creek drainage divide area seen in less detail in figure 5 above. The North Fork Smith River is flowing southwest in the figure 6 northwest corner area. Eightmile Creek originates in section 30 (figure 6 northwest quadrant) and flows southeast before turning to flow southwest across section 31 and then to section 1 in the figure 6 southwest corner. The North Fork Musselshell River flows south-southeast and south-southwest in sections 28 and 33 and then turns to flow southeast in sections 4 and 3 in the figure 6 southeast corner. Copper Creek originates as a west-oriented stream in the north of section 5 (in the figure 6 south center) and then in section 6 turns to flow southeast to the figure 6 south edge. Note how multiple through valleys link all of these drainage routes. The through valleys provide evidence the present day valleys eroded headward across multiple previous valleys or channels, such as might be expected in an anastomosing channel complex. For example, multiple through valleys link the southwest-oriented North Fork Smith River valley with the southwest-oriented Eightmile Creek valley. The through valleys suggest the Eightmile Creek valley eroded head headward to capture multiple southeast-oriented flood flow channels and to divert the water southwest. Through valleys linking the Eightmile Creek valley with the Copper Creek valley and the North Fork Musselshell River valley suggest the multiple southeast-oriented flood flow channels were captured by headward erosion of the Eightmile Creek valley (and subsequently headward erosion of the North Fork Smith River valley). Prior to being captured flood waters were moving to what was then the actively eroding southeast-oriented Musselshell River valley. Note the presence of such through valleys across all drainage divides shown in figure 6. The presence of such through valleys across high drainage divides provides evidence flood waters originally flowed on a topographic surface at least as high as the highest present day figure 6 elevations. Based on that evidence, flood waters deeply eroded the figure 6 landscape.

South Fork Eightmile Creek-Checkerboard Creek drainage divide area

Figure 7: South Fork Eightmile Creek-Checkerboard Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 7 illustrates the South Fork Eightmile Creek-Checkerboard Creek drainage divide area shown in less detail in figure 5 above. The South Fork Eightmile Creek flows north-northeast from section 36 in the figure 7 southwest quadrant to the figure 7 northwest quadrant and then turns to flow northwest to the figure 7 north edge. Checkerboard Creek flows northeast across the figure 7 southeast corner area. A through valley in the figure 7 southwest quadrant links the north-northeast-oriented South Fork Eightmile Creek headwaters with headwaters of a southeast-oriented Checkerboard Creek tributary located in section 1. Another, higher level northwest-southeast oriented through valley has been eroded across the hill in the figure 7 center area and links a northwest-oriented South Fork Eightmile Creek tributary valley with a southeast-oriented Checkerboard Creek tributary valley. These two through valleys are examples of multiple through valleys linking the present day North Fork Smith River drainage basin with the North Fork Musselshell River drainage basin. The through valleys provide evidence of multiple flood flow channels that once moved large volumes of flood waters from what is today the North Fork Smith River drainage basin to what is today the North Fork Musselshell River drainage basin. Flood waters initially may have been moving to what was then the actively eroding Yellowstone River valley, although at some point the Musselshell River valley eroded headward into the region to capture the southeast-oriented flood flow. Headward erosion of the North Fork Musselshell River valley and its tributary Checkerboard Creek valley next captured the southeast-oriented flood flow. Southeast-oriented flood flow on the northwest-oriented South Fork Eightmile Creek valley alignment was beheaded and reversed by headward erosion of the southwest-oriented North Fork Smith River valley. Flood waters on the northwest end of the beheaded flood flow route reversed flow direction to flow northwest to the newly eroded North Fork Smith River valley. The reversed flood waters eroded the northwest-oriented South Fork Eightmile Creek valley and also created the present day South Fork Eightmile Creek-Checkerboard Creek drainage divide.

North Fork Smith River-North Fork Musselshell River drainage divide area north of Castle Mountains

Figure 8:North Fork Smith River-North Fork Musselshell River drainage divide area north of Castle Mountains. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 8 illustrates the North Fork Smith River-North Fork Musselshell River drainage divide area north of the Castle Mountains and south of figure 5. Figure 8 includes overlap areas with figure 5. The North Fork Smith River is located in the figure 8 northwest quadrant and flows southwest from Lake Sutherlin to the figure 8 west edge. Major North Fork Smith River tributaries are northwest-oriented South Fork Eightmile Creek, northwest-oriented Fivemile Creek,and northwest-oriented Fourmile Creek, which originates as a northeast-oriented stream in the high Castle Mountains. Southwest of Fourmile Creek is northwest-oriented Willow Creek, which originates as a northwest-oriented stream in the high Castle Mountains and then turns to flow north, before turning northwest to join the southwest-oriented North Fork Smith River (which west of figure 8 joins the northwest-oriented South Fork Smith River to form the northwest-oriented Smith River-see figures 1 and 2). The North Fork Musselshell River is located in the figure 8 northeast quadrant and flows southeast to Bair Reservoir and Checkerboard and then to the figure 8 east edge. One major North Fork Musselshell River tributary is northeast-oriented Checkerboard Creek and an important Checkerboard Creek tributary is north-northeast and east-southeast oriented Hall Creek. Figures 9 and 10 below provide detailed maps of the Fourmile Creek-Hall Creek drainage divide area and Fourmile Creek-West Fork Checkerboard Creek drainage divide areas. Another North Fork Musselshell River tributary is Flagstaff Creek. The West Fork Flagstaff Creek originates as a southeast-oriented stream in the figure 8 southeast corner and then turns northeast to join northeast-oriented Dry Fork and to flow to join the North Fork Musselshell River east of the figure 8. South of southeast-oriented West Fork Flagstaff Creek headwaters (near Judge Mine) are headwaters southeast-oriented North and South Forks of Bonanza Creek, which flows to the South Fork Musselshell River. Figure 8 evidence suggests southeast-oriented flood waters originally flowed from the North Fork Smith River valley area along what is today the northwest-oriented Fourmile Creek valley alignment to what was then the actively eroding southeast-oriented Bonanza Creek valley. Next headward erosion of the West Fork Checkerboard Creek valley captured the southeast-oriented flood flow and diverted flood waters northeast to what was the newly eroded North Fork Musselshell River valley. Subsequently headward erosion of the North Fork Smith River valley beheaded the southeast-oriented flood flow route. Flood waters on the northwest end of the beheaded flood flow route reversed flow direction to erode the northwest-oriented Fourmile Creek valley. Apparently the reversed flood flow also captured significant yet to be beheaded southeast-oriented flood flow further to the south. That yet to be beheaded flood flow eroded the Fourmile Creek valley. Obviously, such a capture sequence would not be possible with present day topography. The Castle Mountains were probably emerging as significant topographic features as flood waters eroded the region.

Fourmile Creek-Hall Creek drainage divide area

Figure 9: Fourmile Creek-Hall Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 9 provides a detailed map of the Fourmile Creek-Hall Creek drainage divide area seen in less detail in figure 8 above. Fourmile Creek flows northwest from the figure 9 south edge (west half) through sections 21 and 16 to the figure 9 west edge. Hall Creek originates in section 23 in the figure 9 south center and flows north-northeast through sections 14, 11 and 12 to reach the figure 9 north edge (east half). Northeast-oriented West Fork Checkerboard Creek joins the north-northeast oriented East Fork in section 24 (Figure 9 southeast corner area) to form north-northeast oriented Checkerboard Creek, which flows to the figure 9 northeast corner. Note the large through valley linking the northwest-oriented Fourmile Creek valley with the north-northeast oriented Hall Creek valley and the higher level through valleys linking the Hall Creek valley with the northeast-oriented West Fork Checkerboard Creek valley. The through valleys provide evidence southeast-oriented flood water once moved along what is today the northwest-oriented Fourmile Creek valley alignment to the northeast-oriented West Fork Checkerboard Creek valley, although on a topographic surface much higher than prevails today. Headward erosion of the north-northeast oriented Hall Creek valley then captured the southeast-oriented flood flow and diverted the flood water north-northeast. Next, a reversal of flow direction in the Fourmile Creek valley created the Fourmile Creek-Hall Creek drainage divide. The Fourmile Creek valley was then eroded deeper by capture of yet to be beheaded southeast-oriented flood flow from areas further to the south (which are today in the high Castle Mountains). Capture events described by figure 9  suggest Castle Mountain uplift may have played a significant role in enabling headward erosion of deep valleys into the figure 9 map area and in the sequence of stream capture event which took place.

Fourmile Creek-West Fork Checkerboard Creek drainage divide area

Figure 10: Fourmile Creek-West Fork Checkerboard Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 10 illustrates the Fourmile Creek-West Fork Checkerboard Creek drainage divide area south of the figure 9 map area and includes overlap areas with figure 9. The figure 10 map area was shown in less detail in figure 8 above. Fourmile Creek flows northeast from the figure 10 southwest corner through sections 4 and 33 into section 27 where it turns to flow north-northwest to the figure 10 north edge. The West Fork Checkerboard Creek originates in sections 34 and 35 and flows northeast through section 26 to the figure 10 northeast corner. The East Fork Checkerboard Creek originates in section 36 (near the figure 10 east edge) and flows north-northeast to the figure 10 northeast corner. Note high level through valleys linking various valleys. For example, in the north of section 34 a high level through valley links the Fourmile Creek valley with the West Fork Checkerboard Creek headwaters. In the northeast corner of section 35 is a high level through valley linking the West Fork Checkerboard Creek valley with the East Fork Checkerboard Creek valley. In the southeast corner of section 36 a through valley links the East Fork Checkerboard Creek with southeast-oriented headwaters of southeast-oriented North and South Forks of Bonanza Creek, which flows to the northeast-oriented South Fork Musselshell River (see figure 8). Near the corner of sections 3, 2 and 34 is a high level through valley linking the West Fork Checkerboard Creek valley with headwaters of southeast-oriented Hensley Creek, which also flows to the South Fork Musselshell River. And in section 3 is a high level through valley linking the Fourmile Creek valley with the South Fork Bonanza Creek valley. These through valleys were eroded by water at a time when the present day drainage system was just beginning to evolve. The multiple through valleys suggest multiple flow channels, such as might be found in anastomosing channel complexes. While reconstructions are difficult, the multiple channels probably initially carried flood waters to what was then the actively eroding southeast-oriented Bonanza Creek valley system. Headward erosion of the deep northeast-oriented East and West Checkerboard Creek valleys then captured much of the flood flow and diverted the flood waters northeast. The northeast-oriented Fourmile Creek valley in figure 10 may have originated (at a higher topographic level) as a southwest extension of the north-northeast Hall Creek valley and then was captured by the reversal of flood flow on the northwest end of the beheaded southeast-oriented flood flow route moving on what is today the northwest-oriented Fourmile Creek valley alignment.

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.