Yellowstone River (and Tongue River)-Powder River drainage divide area landform origins, eastern Montana, USA

Authors

A geomorphic history based on topographic map evidence

Abstract:

The Yellowstone River (and Tongue River)-Powder River drainage divide area is located in eastern Montana, USA. Although detailed topographic maps of the Yellowstone River (and Tongue River)-Powder 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 Yellowstone River (and Tongue River)-Powder 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 drainage divide ended when headward erosion of the deep Yellowstone River and Tongue River valley captured all southeast-oriented flood flow.

Preface:

The following interpretation of detailed topographic map evidence is provided as evidence in the Missouri River drainage basin landform origins research project, which is compiling similar evidence for all major drainage divides contained within the Missouri River drainage basin and for all major drainage divides with and within certain adjacent drainage basins. The research project is interpreting evidence in the context of a previously unexplored geomorphology paradigm, which is briefly described in the introduction below. Project essays are listed on the sidebar category list under their appropriate Missouri River tributary drainage basin, Missouri River segment drainage basin (by state), and/or state in which the Missouri River drainage basin is located.

Introduction:

  • The purpose of this essay is to use topographic map interpretation methods to explore southeastern Montana Yellowstone River (and Tongue River)-Powder River drainage divide area landform origins. 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 Yellowstone River (and Tongue River)-Powder River drainage divide area landform evidence will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm.

Yellowstone River (and Tongue River)-Powder River drainage divide area location map

Figure 1: Yellowstone River (and Tongue River)-Powder 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 an Yellowstone River (and Tongue River)-Powder River drainage divide area location map and illustrates a region in eastern Montana. The state of Montana is located in figure 1 west of the green border line and the state of North Dakota is located in the figure 1 northeast corner and the state of South Dakota is located south of North Dakota. The Yellowstone River flows in a northeast direction from Forsyth to Miles City, Terry, and Fallon, Montana. The Tongue River flows northeast from the figure 1 southwest corner to Birney and Ashland and then south of Miles City, Montana turns northwest to join the northeast-oriented Yellowstone River as a barbed tributary. The Powder River flows northeast in the figure 1 south center to Broadus and Powderville, Montana before turning northwest to flow to the northeast-oriented Yellowstone River near the figure 1 north center. Pumpkin Creek is a major Tongue River tributary flowing to the Tongue River south of Miles City. Mizpah Creek is a major Powder River tributary flowing to the Powder River southeast of Miles City. The Tongue River-Pumpkin Creek drainage divide area essay, the Pumpkin Creek-Mizpah Creek drainage divide area essay and the Mizpah Creek-Powder River drainage divide area essay describe nearby drainage divide areas and can be found under Tongue River or Powder River on the sidebar category list (Pumpkin Creek is a Powder River tributary and Mizpah Creek is a Powder River tributary) . This essay addresses the Yellowstone River-Powder River drainage divide area and the Tongue River-Powder River drainage divide area north of Pumpkin and Mizpah Creeks. Landform evidence illustrated here is interpreted in the context of an immense southeast-oriented flood flowing across the entire figure 1 map area and which was systematically captured and diverted further and further to the northeast by headward erosion of deep valleys eroded into a topographic surface at least as high as the figure 1 region highest elevations today. In the figure 1 map region headward erosion of the northeast oriented Powder River valley captured the southeast-oriented flood flow and diverted the flood waters north to the northeast-oriented Yellowstone River valley (located north of figure 1). Headward erosion of the deep northeast-oriented Yellowstone River valley (and the north-northwest-oriented Tongue River valley) next captured the southeast-oriented flood flow and ended flood flow across the Yellowstone River (and Tongue River)-Powder River drainage divide area discussed here.

Yellowstone River (and Tongue River)-Powder River drainage divide area detailed location map

Figure 2: Yellowstone River (and Tongue River)-Powder 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 Yellowstone River (and Tongue River)-Powder River drainage divide area discussed here. Custer County is located in Montana. The county north of Custer County is Prairie County. The Yellowstone River (and Tongue River)-Powder River drainage divide area discussed here is located in Custer County and southwest Prairie County. The Tongue River flows northeast and northwest from the figure 2 south edge to join the northeast oriented Yellowstone River at Miles City. The Powder River flows from the figure 2 south edge (east half) in a northwest, north, and northwest direction to join the northeast oriented Yellowstone River. The Powder River is also a barbed tributary. Figure 2 shows numerous southeast and northwest  oriented Yellowstone River tributaries, southeast oriented and northwest oriented Powder River tributaries, and some southeast and northwest oriented Tongue River tributaries. This northwest-southeast drainage alignment is evidence the northeast oriented Yellowstone River valley eroded southwest to capture multiple southeast-oriented flood flow routes such as might be found in a southeast-oriented anastomosing channel complex. Subsequently the northwest- and north-oriented Powder River valley eroded south across the same southeast-oriented flood to capture flood waters and to divert flood waters north. Further, the drainage alignment is evidence the northeast- and northwest-oriented Tongue River valley subsequently eroded south and southwest to capture the some of the same southeast-oriented flood flow. The southeast-oriented tributary valleys were eroded by southeast-oriented flood flow moving into the newly eroded and deep Yellowstone River, Powder River, and Tongue River valleys. The northwest-oriented tributary valleys were eroded by reversed flood flow on the northwest ends of beheaded southeast-oriented flood flow routes. Because flood waters move in and erode anastomosing (or interconnected) channels reversed flood flow on a beheaded flood flow route could capture flood flow from yet to be beheaded flood flow routes. Such captures of yet to be beheaded flood flow could enable the reversed flood flow routes to erode much deeper and larger northwest-oriented valleys than might otherwise be possible. Often evidence for such flow reversals and captures can be found on detailed topographic maps. Detailed maps below start with a look at the Yellowstone River-Powder River drainage divide area located south and southwest of where the Powder River joins the northeast-oriented Yellowstone River. Subsequent detailed maps illustrate evidence along the Tongue River-Powder River drainage divide north of the Pumpkin Creek-Mizpah Creek drainage divide area.

North end of Yellowstone River-Powder River drainage divide area

Figure 3: North end of Yellowstone River-Powder River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 3 illustrates the north end of the Yellowstone River-Powder River drainage divide area. The northeast-oriented Yellowstone River flows from the figure 3 southwest corner to the figure 3 north center. The north and northwest-oriented Powder River flows from the figure 3 southeast quadrant to join the Yellowstone River in the figure 3 north center. Note how the Powder River joins the Yellowstone River as a barbed tributary, just like the other Yellowstone River tributaries seen in figure 3. Yellowstone River tributaries from the west are southeast oriented and from the east are northwest-oriented. This southeast and northwest-orientation of tributary valleys is evidence the deep Yellowstone River valley eroded headward across multiple southeast-oriented flood flow channels to capture the southeast-oriented flood flow and to divert the flood waters northeast. The northwest-oriented tributary valleys, including the northwest-oriented Powder River valley segment seen here, were initially eroded by reversed flood flow on the northwest ends of beheaded flood flow routes. Because the southeast-oriented flood flow was moving in anastomosing (or interconnected channels) it was easy for reversed flood flow channels to capture yet to be beheaded flood flow (and also reversed flood flow) from adjacent channels. Note how northwest-oriented Camp Creek has captured reversed flood flow from multiple channels. The Camp Creek success in capturing reversed flood flow was probably related to the success of the deep Powder River valley in capturing both reversed flood flow and yet to be beheaded flood flow routes, especially south of the reversed flood flow region. Note the Yellowstone River-Powder River drainage divide in this figure 3 map area is an asymmetric drainage divide with a much steeper slope on the Powder River side. This asymmetric drainage divide suggests the deep Powder River valley may have eroded south across reversed flood flow routes that had been eroding the northwest-oriented Camp Creek valley system. The Powder River-O’Fallon Creek drainage divide essay describes regions east of the Powder River valley and how northwest-oriented Powder River valley segments were eroded when the deep northeast-oriented Yellowstone River valley beheaded a major southeast oriented flood flow route.

North end of Cottonwood Creek-Powder River drainage divide area

Figure 4: North end of Cottonwood Creek-Powder River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 4 illustrates the Yellowstone River-Powder River drainage divide area south and slightly west of the figure 3 map area and includes overlap areas with figure 3. The northeast-oriented Yellowstone River is located in the west and northwest areas of figure 4. The north and north-northwest oriented Powder River is located along the figure 4 east edge. North-northwest oriented Cottonwood Creek flows from the figure 4 south center to join the Yellowstone River in the figure 4 northwest quadrant and illustrates on a small-scale how the deeper and larger north-northwest oriented Powder River valley eroded headward to capture multiple flood flow routes that had been reversed to flow northwest into the newly eroded and deep northeast-oriented Yellowstone River valley. Note how Yellowstone River tributaries west of the north-northwest oriented Cottonwood Creek valley are northwest-oriented and are linked by shallow through valleys across the Yellowstone River-Cottonwood Creek drainage divide to shorter east-oriented tributaries to north-northwest oriented Cottonwood Creek. Also note how Cottonwood Creek tributaries from the west are northwest-oriented and are linked by shallow through valleys across the asymmetric Cottonwood Creek-Powder River drainage divide to east-oriented tributaries to the north- and north-northwest oriented Powder River. The northwest-oriented tributary valleys on either side of the Cottonwood Creek valley provide evidence the north-northwest-oriented Cottonwood Creek valley may have eroded headward across multiple reversed flood channels to capture the reversed flood flow and to divert the flood waters more directly to the newly eroded northeast-oriented Yellowstone River valley. The Powder River valley may have been eroded headward to capture reversed flood flow also. If so it provides evidence that in this figure 4 map area large quantities of flood waters had been reversed and were flowing northwest. While evidence in this essay does not show where the water was coming from other Missouri River drainage basin landform origins research project drainage divide area essay (published on this website)illustrate how the deep north-northwest oriented Powder River valley was probably eroded headward along a reversed flood flow route that prior to being beheaded and reversed had been a major southeast-oriented flood flow route responsible for headward erosion of the northeast-oriented Little Beaver Creek valley (which drains to the Little Missouri River). Further these other essays (which can be found Powder River on the sidebar category list)  have shown how headward erosion of the north-northwest oriented Powder River valley probably captured the northeast-oriented Little Beaver Creek headwaters valley that was being supplied by flood flow that had not yet been beheaded by Yellowstone River valley headward erosion.

South end of Cottonwood Creek-Powder River drainage divide area

Figure 5: South end of Cottonwood Creek-Powder River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 5 illustrates the south end of the Cottonwood Creek-Powder River drainage divide area south of the figure 4 map area and there is a small gap between figure 4 and figure 5. The north-oriented Powder River is located along the figure 5 east edge. Cottonwood Creek flows northeast from the figure 5 west center to the east end of the figure 5 northwest quadrant. Little Cottonwood Creek is the major Cottonwood Creek tributary flowing from the figure 5 center area to join northeast-oriented Cottonwood Creek in the east end of the figure 5 northwest quadrant. Note how the northwest-oriented Little Cottonwood Creek valley is linked by multiple through valleys with east and southeast-oriented Powder River tributaries. The through valleys provide evidence that multiple channels of southeast-oriented flood water once crossed the Cottonwood Creek-Powder River drainage divide, such as would be expected in a southeast-oriented anastomosing channel complex. Figures 6 and 7 below are detailed maps to illustrate the Schlosser Creek-Wolf Creek drainage divide area near Blue Mountain, which is located along the figure 5 north edge (west end of east half), and the Little Cottonwood Creek-North Fork Smith Creek drainage divide area, which is located where the highway crosses the Cottonwood Creek-Powder River drainage divide. Because the Cottonwood Creek valley is northeast-oriented and the Powder River valley is north-oriented much of the figure 5 evidence probably originated as southeast-oriented flood water flowed across the figure 5 map area. Headward erosion of the deep Powder River valley was before headward erosion of the Cottonwood Creek valley and headward erosion of the Yellowstone River valley (west and northwest of figure 5). Following headward erosion of the deep north-oriented Powder River valley headward erosion of the northeast-oriented Cottonwood Creek valley next captured the southeast-oriented flood flow and reversed flood flow on the northwest ends of the flood flow routes it beheaded. The reversed flood flow eroded the northwest-oriented Cottonwood Creek tributary valleys and created the present day Cottonwood Creek-Powder River drainage divide. Subsequently headward erosion of the deep Yellowstone River beheaded the southeast-oriented flood flow routes moving flood waters to the Cottonwood Creek valley. Reversed flood waters on the northwest ends of the beheaded flood flow routes then eroded northwest-oriented Yellowstone River tributary valleys and created the present day Yellowstone River-Cottonwood Creek drainage divide.

Schlosser Creek-Wolf Creek drainage divide area

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

Figure 6 illustrates a detailed map of the Schlosser Creek-Wolf Creek drainage divide area seen in less detail in figure 5 above. Northwest-oriented Schlosser Creek originates west of Blue Mountain and flows to the figure 6 northwest corner and then to north-oriented Cottonwood Creek. Southeast-oriented Wolf Creek headwaters are located southeast of Blue Mountain. The east-oriented North Fork Wolf Creek originates southeast of Green Mountain. Wolf Creek flows to the north-oriented Powder River. Note the various saddles or through valleys eroded across the present day Cottonwood Creek-Powder River drainage divide. In particular note the valley just northeast of Blue Mountain on the northwest-oriented Schlosser Creek alignment that provides a link to an unnamed southeast oriented Wolf Creek tributary on that same northwest-southeast alignment. That through valley and others like it provides evidence that southeast oriented flood waters once flowed across the figure 6 map area on a topographic surface at least as high as the highest figure 6 elevations today. Headward erosion of the deep north-oriented Powder River valley then captured the southeast oriented flood flow and southeast- and east-oriented Powder River valley tributary valleys then began to erode northwest and west along the incoming southeast-oriented flood flow routes. However, before those Powder River tributary valleys had time to erode deeply into the newly eroded Powder River valley west wall headward erosion of the Cottonwood Creek valley beheaded the southeast-oriented flood flow routes and diverted the flood waters north. Flood waters on the northwest ends of the beheaded flood flow routes reversed flow direction to flow northwest to the newly eroded Cottonwood Creek valley and in the process eroded the northwest-oriented Schlosser Creek valley and created the Schlosser Creek-Wolf Creek drainage divide.

Little Cottonwood Creek-North Fork Smith Creek drainage divide area

Figure 7: Little Cottonwood Creek-North Fork Smith Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 7 illustrates a detailed map of the Little Cottonwood Creek-North Fork Smith Creek drainage divide area seen in less detail in figure 5 above. Little Cottonwood Creek flows northeast along the highway in the figure 7 southwest quadrant and then makes an abrupt turn to flow northwest toward the figure 7 northwest corner area and northeast-oriented Cottonwood Creek (located northwest of the figure 7 map area). West-oriented Trough Creek is the major Little Cottonwood Creek tributary shown in figure 7 and northwest-oriented Cedar Canyon Creek is the major Trough Creek tributary. East of the Cottonwood Creek-Powder River drainage divide southeast-oriented North Fork Smith Creek flows from the figure 7 east center area to the figure 7 east edge (near the highway). Note the multiple through valleys linking the northwest-oriented Little Cottonwood Creek drainage basin with the north-oriented Powder River drainage basin. Also note the through valleys are located adjacent to the present day hills or “mountains” located along the Cottonwood Creek-Powder River drainage divide. The through valleys and the hills provide evidence southeast-oriented flood water once flowed across the figure 7 map area on a topographic surface at least as high as the highest figure 7 elevations today. The multiple through valleys provide evidence flood waters flowed in multiple channels such as would be expected in a southeast-oriented anastomosing channel complex. Headward erosion of the deep north-oriented Powder River valley first captured the southeast-oriented flood flow and diverted the flood waters north to what was then the actively eroding northeast-oriented and deep Yellowstone River valley head. Shortly after (before Powder River tributaries had time to erode deep valleys into the newly eroded Powder River valley west wall) headward erosion of the Cottonwood Creek valley captured the flood flow and diverted the flood waters north to what was then the actively eroding and deep Yellowstone River valley head. Flood waters on the northwest ends of the beheaded flood flow routes, such the flood flow route using the present day northwest-oriented Little Cottonwood Creek valley alignment) reversed flow direction to flow northwest to the newly eroded Cottonwood Creek valley, to erode northwest-oriented Cottonwood Creek tributary valleys, and to create the present day Cottonwood Creek-Powder River drainage divide.

Yellowstone River-Cottonwood Creek drainage divide area

Figure 8: Yellowstone River-Cottonwood Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 8 illustrates the Yellowstone River-Cottonwood Creek drainage divide area northwest of Miles City and west of figure 5 and includes overlap areas with figure 5. The Yellowstone River flows northeast from the Miles City area to the figure 8 north center edge. Northeast-oriented Cottonwood Creek is located in the figure 8 southeast corner. Note northwest-oriented Yellowstone River tributaries. For example note northwest-oriented Jones Creek, which begins in an upland area just northwest of where northeast Cottonwood Creek begins. Through valleys link the Jones Creek headwaters area with the Cottonwood Creek headwaters area. The through valleys provide evidence southeast-oriented flood flow using the Jones Creek alignment was captured by headward erosion of the northeast-oriented Cottonwood Creek valley. Subsequently headward erosion of the deep Yellowstone River valley beheaded the southeast-oriented Jones Creek flood flow channel and caused a reversal of flood flow that eroded the northwest-oriented Jones Creek valley and also created the present day Jones Creek-Cottonwood Creek drainage divide. Cottonwood Creek tributaries are east and southeast-oriented and are also linked with northwest-oriented Yellowstone River tributaries. Figure 8 evidence suggests southeast-oriented flood waters flowed across the figure 8 region on a topographic surface at least as high as the highest figure 8 elevations today. Headward erosion of what was then a deep northeast-oriented Cottonwood Creek valley then captured the flood flow and diverted the flood waters northeast and north to what was then the actively eroding and deep Yellowstone River valley headcut face. Subsequently headward erosion of the deep northeast-oriented Yellowstone River valley beheaded and reversed the southeast-oriented flood flow routes supplying water to the newly eroded Cotton Creek valley. Reversal of flood flow on the northwest ends of he beheaded flood flow routes eroded the northwest-oriented Yellowstone River tributary valleys and also created the present day Yellowstone River-Cottonwood Creek drainage divide.

Tongue River tributaries from the east

Figure 9: Tongue River tributaries from the east. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 9 illustrates Tongue River tributaries east of the Tongue River valley south of figures 5 and 8 and includes overlap areas with figure 5. Miles City is located just to the northwest of figure 9. The north-northwest oriented Tongue River is located along the figure 9 west edge. Northwest-oriented Pumpkin Creek joins the Tongue River just north of the figure 9 south edge. Figure 9 illustrates east-oriented Tongue River tributary valleys west of the Tongue River-Powder River drainage divide, which is illustrated in figure 10 below. Tributary valley orientations often provide useful clues about a region’s drainage history. For example, Mill Creek is a major Tongue River tributary seen in figure 9 and originates in the figure 9 north center with southeast, south, northwest, and west oriented tributaries and headwaters and flows southwest, west and northwest to join the north-northwest oriented Tongue River. Whitney Creek is a major Mill Creek tributary and originates as a northwest-oriented stream near the figure 9 north center edge and makes a U-turn to flow south-southeast to southwest, west and northwest oriented Mill Creek. Cowles Creek in the figure 9 northwest corner area  also makes some interesting turns and flows southeast before making a U-turn when it joins the northwest-oriented Tongue River. South of Mill Creek west-oriented Squaw Creek has northwest-oriented tributaries and headwaters and a number of southeast-oriented tributaries. The southeast and northwest orientations of tributaries and headwaters of these Tongue River tributaries provides evidence these Tongue River tributary valleys eroded headward to capture southeast-oriented flood flow and divert the flood flow to the Tongue River valley. However, the northwest-orientation of the Tongue River valley suggests it was initially a southeast-oriented flood flow route that was beheaded and reversed by headward erosion of the deep northeast-oriented Yellowstone River valley. If so, it is possible some of the west-oriented Tongue River tributary valleys were initiated as tributary valleys to a southeast-oriented flood flow channel, which was subsequently reversed and deepened when the deep Yellowstone River valley beheaded the southeast-oriented flood flow route.

Tongue River-Powder River drainage divide area

Figure 10: Tongue River-Powder River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 10 illustrates the Tongue River-Powder River drainage divide area east of figure 9 map area and includes overlap areas with figure 9. The north-northwest oriented Powder River is located along the figure 10 east edge. West-oriented streams flowing to the figure 10 west edge are Tongue River tributaries, which can be seen west of the figure 10 map area in figure 9 above. Note how many of Powder River tributaries from the west are southeast-oriented and enter the north-northwest oriented Powder River as barbed tributaries. Also note how headwaters of these southeast-oriented Powder River tributaries are linked to northwest-oriented tributaries (or headwaters) of west-oriented Tongue River tributaries. Through valleys cross the Tongue River-Powder River drainage divide at these linkage points and elsewhere. The through valleys and the orientations of tributary valleys on either side of the through valleys provide evidence that multiple channels of southeast-oriented flood flow once moved across what is today the Tongue River-Powder River drainage divide, probably as channels eroded as components of a southeast-oriented anastomosing channel complex. Flood flow at that time was on a topographic surface at least as high as the highest figure 10 elevations today. Headward erosion of the deep Powder River valley into the figure 10 map area then captured the southeast-oriented flood flow and diverted the flood waters north to what was then the actively eroding and deep northeast-oriented Yellowstone River valley. Subsequently headward erosion of the deep Tongue River valley (and associated Yellowstone River valley and various Tongue River tributary valleys) beheaded the southeast-oriented flood flow routes to the newly eroded Powder River valley and diverted the flood waters to what was then the actively eroding Yellowstone River valley head at its new location near present day Miles City. Flood flow reversals on northwest ends of the beheaded flood flow routes reversed flow direction to flow northwest and to erode northwest-oriented valleys and to create the present day Tongue River-Powder River drainage divide.

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.

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