Abstract:

This essay uses topographic map evidence to interpret landform origins in the region between the Bighorn River and Little Bighorn River in northern Wyoming. The Bighorn River flows in a north direction along the west side of the Bighorn Mountains in Wyoming and near the Montana-Wyoming state line turns to flow in a northeast across the north end of the Bighorn Mountain before turning to flow in a north-northeast direction to join the northeast oriented Yellowstone River. The Little Bighorn River originates in the northern Bighorn Mountains south and east of the Bighorn River and flows in a northeast direction to the east side of the Bighorn Mountains and then flows in a north direction to join the Bighorn River. The Bighorn River-Little Bighorn River drainage divide in northern Wyoming is located in a region of high relief and rugged topography and is crossed by through valleys or notches linking west and northwest oriented Bighorn River tributary valleys with east and northeast oriented Little Bighorn River tributary valleys. These through valleys or notches are interpreted to be remnants of valleys that were eroded into an erosion surface defined by the tops of some of the highest points on the drainage divide today. South and southeast oriented flood flow channels probably eroded the valleys at a time when the Bighorn Mountains had not emerged and/or were in the process of emerging as a mountain range. Floodwaters are interpreted to have been derived from the western margin of a thick North American ice sheet and were flowing from western Canada to and across the Bighorn Mountains region. The Bighorn Mountains are interpreted to have emerged as deep flood water erosion lowered the surrounding region and as ice sheet related crustal warping uplifted the Bighorn Mountains and created a rim surrounding the deep “hole” in which the ice sheet was located. The Bighorn Mountains region is a deeply eroded and warped segment of the deep “hole’s” southwest rim. South and southwest oriented flood flow across the emerging Bighorn Mountains was first beheaded by headward erosion of a deep south-oriented flood flow on the Bighorn River alignment. Subsequently south oriented flood flow on the Bighorn River alignment was beheaded and reversed to create the north oriented Bighorn River drainage system. The high relief in the study region is interpreted to have developed as deep valleys eroded headward into the region from different directions to capture the immense south and southeast oriented flood flow.

Preface

The following interpretation of detailed topographic map evidence is one of a series of essays describing similar evidence for all major drainage divides contained within the Missouri River drainage basin and for all major drainage divides with adjacent drainage basins. The research project is interpreting evidence in the context of a previously unexplored deep glacial erosion paradigm, which is fundamentally different from most commonly accepted North American glacial history interpretations. 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 the Bighorn River-Little Bighorn River drainage divide area landform origins in the northern Wyoming Bighorn Mountains, 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 and/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 new geomorphology 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 other Missouri River drainage basin landform origins research project essays is a thick North American ice sheet, comparable in thickness to the Antarctic ice sheet, occupied the North American region usually recognized to have been glaciated, and through its weight and erosive actions created a deep North American “hole”. The southwestern rim of that deep “hole” is today preserved in the high Rocky Mountains. The ice sheet 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 Big Horn River-Little Bighorn River drainage divide area landform evidence in the northern Wyoming Bighorn Mountains will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm.

Bighorn River-Little Bighorn River drainage divide area location map

Figure 1: Bighorn River-Little Bighorn 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 Bighorn River–Little Bighorn River drainage divide area in the northern Wyoming Bighorn Mountains and shows a region in south central Montana and in north central Wyoming. The west to east oriented Montana-Wyoming state line extends across figure 1 with Montana north of Wyoming. The Bighorn Mountains extend from just north of the state line in a south-southeast and south direction to the south center edge of figure 1. The Bighorn River flows from the south edge of figure 1 west of the Bighorn Mountains in a north direction to the state line and then in a northeast and north-northeast direction to the join the northeast oriented Yellowstone River near Custer, Montana. Bighorn Canyon in southern Montana is where the north and northeast oriented Bighorn River has eroded a deep canyon across the north end of the Bighorn Mountains. The Little Bighorn River originates in the Bighorn Mountains just south of the state line and flows in a northeast direction to Wyola, Montana. From Wyola the Little Bighorn River flows in a north and northwest direction to join the Bighorn River near Hardin, Montana. The unnamed northeast oriented tributary directly north and west of the northeast oriented Little Bighorn River headwaters and joining the Little Bighorn River near the town of Lodge Grass, Montana is Lodge Grass Creek. South of the Little Bighorn River headwaters are east-northeast headwaters of the Tongue River, which joins the Yellowstone River near the north edge of figure 1. The unnamed stream originating south of Dome Peak (south of the Tongue River headwaters) and flowing in a northwest and west-southwest direction to join the Bighorn River near Greybull, Wyoming is Shell Creek. The Bighorn River-Little Bighorn River drainage divide area investigated in this essay is located east of the Bighorn River, west of the Little Bighorn River, south of the Montana state line, and north of the west-southwest oriented Shell Creek segment.

Today nearly all drainage systems in the region shown by figure 1 are oriented in north directions. However, these north oriented drainage systems evolved during the reversal of immense south oriented melt water floods. The floods were derived from the western margin of a thick North American ice sheet and were flowing from western Canada to and across the region seen in figure 1. At least initially the regional mountain ranges, including the Bighorn Mountains, had not emerged as mountain ranges and floodwaters could freely flow across what are today high mountain ranges. The mountain ranges began to emerge as floodwaters eroded deep south oriented flood flow channels into the Powder River Basin east of the Bighorn Mountains and into the Bighorn Basin west of the present day Bighorn Mountains and also as ice sheet crustal warping created the southwest rim of a deep “hole” in which the ice sheet was located. The region seen in figure 1 could be considered to be a segment of the deep “hole’s” southwest rim and the Bighorn Mountains were uplifted as that rim developed. Eventually ice sheet melting began to open up space at the deep “hole’s” south end, which was drained initially by south oriented flood flow channels east of figure 1. The deep northeast oriented Yellowstone River valley eroded headward from that newly opened by space at the deep “hole’s” south end to capture the south and southeast oriented melt water flood flow moving across Montana. Headward erosion of the deep Yellowstone River valley beheaded the south and southeast oriented flood flow channels in sequence from east to west. Floodwaters on north ends of the beheaded flood flow channels reversed flow direction to create north oriented drainage systems (the Yellowstone River valley was significantly deeper than the south oriented flood flow channels). The new north oriented drainage systems often eroded headward in a northeast direction to capture south and southeast oriented flood flow channels located west of the actively eroding Yellowstone River valley head. Flood flow on the Little Bighorn River alignment east of the Bighorn Mountains north end was beheaded and reversed before flood flow on the Bighorn River alignment west of the Bighorn Mountains was beheaded and reversed. The deep north and northeast oriented Bighorn Canyon across the Bighorn Mountains north end was probably eroded headward across south and southeast flood flow channels moving floodwaters to the newly eroded Little Bighorn River valley and while most of the Bighorn Basin to the south was still being crossed by south and southeast oriented flood flow that was entering the Bighorn Basin on flood flow routes west of the Bighorn Canyon location.

Detailed location map for Bighorn River-Little Bighorn River drainage divide area

Figure 2: Detailed location map Bighorn River-Little Bighorn River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 2 provides a more detailed location map for the Bighorn River-Little Bighorn River drainage divide area in northern Wyoming. The west to east oriented Montana-Wyoming state line is located near north edge of figure 2. The green colored area in Wyoming is National Forest land and is located in the Bighorn Mountains. The Bighorn Basin is located west of the Bighorn Mountains in Wyoming. The Bighorn River flows from the south edge of figure 2 (near Greybull, Wyoming) in a north-northwest direction to the state line and then turns to flow in a north-northeast direction and north of figure 2 flows in a north and northeast direction across the north end of the Bighorn Mountains in Bighorn Canyon. The dashed county line in the green shaded area is defined by the drainage divide between streams flowing to the west side of the Bighorn Mountains and streams flowing to the east side of the Bighorn Mountains. Porcupine Creek with its Deer Creek and Trout Creek tributaries is a northwest oriented Bighorn River tributary with headwaters near the drainage divide in Wyoming and joins the Bighorn River just north of the state line. East of the Porcupine Creek headwaters are headwaters of the north and northeast oriented Little Bighorn River, which flows to the north edge of figure 2 (east half) and north of figure 2 turns to flow in a north direction to join the north-northeast oriented Bighorn River. Little Bighorn River tributaries of interest in this essay include Wagon Box Creek, Mann Creek, and Pumpkin Creek. North and west of Pumpkin Creek are headwaters of north-northeast and northeast oriented Lodge Grass Creek, which flows to the north edge of figure 2 (east of center) and which joins the Little Bighorn River north of figure 2. The west oriented Bighorn River tributary south of Porcupine Creek is Cottonwood Creek. South of Cottonwood Creek are Five Springs Creek, Crystal Creek, Dog Bear Creek, and Bear Creek. Shell Creek is the northwest oriented stream flowing from the south edge of figure 2 (just west of the Bighorn Mountains drainage divide) and then turning to flow in a west-southwest direction to the town of Shell and then to join the Bighorn River near the town of Greybull. Note how some of the southwest oriented Bighorn River and Shell Creek tributaries are aligned with northeast oriented Little Bighorn River tributaries. While today the Bighorn Mountains are a massive mountain range separating deep basins on opposites sides of the drainage divide the alignment of drainage routes is evidence of flood flow channels that existed before the Bighorn Mountains emerged. Topographic maps illustrated below provide additional evidence supporting this interpretation.

Bear Creek-Shell Creek drainage divide area

Figure 3: Bear Creek-Shell Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 3 provides a topographic map of Bear Creek-Shell Creek drainage divide area and is located west of the Bighorn Mountains in the Bighorn Basin and begins this essay in the Bighorn Basin south and west of the Bighorn River-Little Bighorn River drainage divide, which is located in the high Bighorn Mountains. The Bighorn River flows in a north-northwest and north direction from the south edge of figure 3 (just west of center) to the north edge of figure 3 (west half). Shell Creek is the west oriented Bighorn River flowing near the south edge of figure 2 from the east edge of figure 2. Sheep Mountain extends in a southeast direction from the northwest corner of figure 3 and appears to be a doubly plunging anticline.  The Bighorn River has eroded a deep canyon across Sheep Mountain even though much lower elevations are seen today at either end of Sheep Mountain. For example at the southeast end of Sheep Mountain there are south-southeast oriented tributaries to west oriented Shell Creek. A through valley links those Shell Creek tributaries with north and north-northwest oriented tributaries to Bear Creek, which then flows in a northwest direction to join the Bighorn River on the northeast side of Sheep Mountain. The map contour interval for figure 3 is 20 meters and the through valley floor elevation is between 1260 and 1280 meters. Sheep Mountain elevations on both sides of the Bighorn River rise to more than 1400 meters and the Bighorn River canyon across Sheep Mountain is more than 300 meters deep. Why did the Bighorn River erode a 300-meter deep canyon across Sheep Mountain when much lower routes are available at each end of Sheep Mountain? Sheep Mountain in a sense could be considered a small-scale model of the much larger Bighorn Mountains to the east and north. Depending on how the Bighorn Mountains are defined Bighorn Canyon where the Bighorn River north of figure 3 crosses the Bighorn Mountains north end could be considered to be almost 1500 meters deep or just a few hundred meters deep. In either case, much more logical routes the Bighorn River could have used exist so something has happened with the Bighorn Mountains and the adjacent Bighorn Basin that deserves an explanation. In brief, at the time Bighorn River route was established the Sheep Mountain and the Bighorn Mountains did not exist and immense south oriented floods could flow across what are today major topographic barriers. The floodwaters were derived from a thick North American ice sheet that was responsible for crustal warping that probably uplifted Sheep Mountain and the Bighorn Mountains as south and southeast oriented floodwaters flowed across them. The uplift occurred as the huge ice sheet created the southwest rim of the deep “hole” in which the ice sheet was located. The south oriented floodwaters eroded the Bighorn Basin surrounding Sheep Mountain as Sheep Mountain emerged and as floodwaters stripped bedrock material from the Bighorn Basin surface. The south oriented floodwaters also eroded a slightly deeper south oriented flood flow channel across the emerging Sheep Mountain structure. Subsequently flood flow in the Bighorn Basin was beheaded and reversed when the deep northeast oriented Yellowstone River valley eroded headward from the space in the deep “hole” being opened up by ice sheet melting. Because the flood flow across the emerging Sheep Mountain structure was slightly moving in a slightly deeper channel after the flood flow reversal that deeper channel became the major north oriented flow route and the adjacent flood flow channels were abandoned.

Detailed map of Sheep Canyon water gap area

Figure 4: Detailed map of Sheep Canyon water gap area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 4 provides a detailed topographic map of the Sheep Canyon water gap area seen in less detail in figure 3. Before heading into the Bighorn Mountains a close look at Sheep Canyon through Sheep Mountain is merited. The Bighorn River flows in a north direction from the south center edge of figure 4 to the north center edge of figure 4. Sheep Mountain extends in a northwest to southeast direction across figure 4. The map contour interval for figure 4 is 20 feet. The high point on Sheep Mountain located north and west of the Bighorn River is located in section 27 and is 5087 feet. The high point on Sheep Mountain south and east of Sheep Canyon and seen in figure 4 is between 4660 and 4680 feet although south and east of figure 4 Sheep Mountain reaches elevations greater than 5000 feet. The Bighorn River crosses the 3720-foot contour line in Sheep Canyon, which suggests Sheep Canyon is approximately 1280 feet deep. Based on the discussion in figure 3 this 1280-foot deep canyon was initiated as a south oriented flood flow channel while south oriented flood flow on either side of the emerging Sheep Mountain stripped the surface of great thicknesses of bedrock material. Flood flow on the Sheep Canyon alignment was apparently much more concentrated than flood flow on the surrounding flood flow routes and was able to erode a deeper flood flow channel. When flood flow in the Bighorn Basin was reversed this deeper flood flow channel was able to capture the reversed flood flow and became the dominant north oriented flood flow channel, which became the north oriented Bighorn River drainage route.

Porcupine Creek-Little Bighorn River drainage divide area

Figure 5: Porcupine Creek-Little Bighorn River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 5 illustrates the Porcupine Creek-Little Bighorn River drainage divide area north and east of figure 3 and does not include an overlap area with figure 3. Figure 5 shows a region in the Bighorn Mountains and the dashed line extending from near the north center edge of figure 5 to the south edge of figure 5 (east of center) is a county line, which is defined by the Bighorn River-Little Bighorn River drainage divide. West of that line streams flow down the Bighorn Mountains steep west slope to join the north oriented Bighorn River. East of that line drainage is to the northeast oriented Little Bighorn River, which flows to the northeast corner of figure 5 and to the northeast side of the Bighorn Mountains. The map contour interval for figure 5 is 20 meters and today the region is a region of high relief and it is difficult to imagine floodwaters flowing across the Bighorn River-Little Bighorn River drainage divide. Yet follow that drainage divide from the north edge of figure 5 to the south edge of figure 5 and note how the drainage divide is crossed by notches or through valleys linking west oriented Bighorn River tributary valleys with east oriented Little Bighorn River tributary valleys. Near the north edge of figure 5 what appears to be a shallow through valley links the west oriented Bucking Mule Creek valley with the east oriented Mann Creek valley. Further south and just north of Duncum Mountain a somewhat deeper through valley linking the west oriented Big Tepee Creek valley with the east oriented Wagon Box Creek valley. South of Duncum Mountain a much deeper through valley links the northwest oriented Porcupine Creek valley with the northeast oriented Duncum Creek valley. This deeper through valley has a floor elevation of between 2720 and 2740 meters. Duncum Mountain to the north rises to 2996 meters. Bald Mountain to the south rises to more than 3040 meters. These elevations suggest the through valley is approximately 250 meters deep. The 250-meter deep through valley is a significant landform and deserves an explanation. The through valley was eroded by water flowing across what is today a high mountain range. Obviously at the time water eroded the through valley the Bighorn Mountains did not stand high above the surrounding regions as it does today. Orientations of the present day drainage routes provide some clues as to directions of water may have been moving at the time the through valley was eroded, although based on figure 5 evidence alone these clues only provide speculative interpretations. Briefly my speculative interpretation is southeast oriented flood flow on the present day northwest oriented Porcupine Creek alignment converged with southwest oriented flood flow on the present day northeast oriented Duncum Creek alignment and then flowed in a southwest direction toward the southwest corner of figure 5. This southwest oriented flood flow eroded southwest-facing escarpment seen in the southwest corner region of figure 5 before a reversal of flood flow on the Little Bighorn River drainage route captured the southeast oriented flood flow and diverted the floodwaters in a northeast direction. Next headward erosion of the deep north and northeast oriented Bighorn Canyon (north and west of figure 5) beheaded and reversed flood flow on the Porcupine Creek alignment. This speculative interpretation requires very deep valleys to erode headward into the region from different directions and these very deep valleys then competed with each other as they systematically captured flood flow from flood flow channels in a large-scale southeast oriented anastomosing channel complex crossing the emerging Bighorn Mountains.

Detailed map of Porcupine Creek-Little Bighorn River drainage divide area

Figure 6: Detailed map of Porcupine Creek-Little Bighorn River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 6 provides a detailed topographic map of the Porcupine Creek-Little Bighorn River drainage divide area seen in less detail in figure 5. The dashed county line marks the Bighorn River-Little Bighorn River drainage divide and extends from Bald Mountain just south of figure 6 (west of center) to the north edge of figure 6 (west half) and to Duncum Mountain just north of figure 6. Porcupine Creek originates in the northeast corner of section 29 and flows in a northeast direction into the southwest corner of section 21 where it turns to flow in northwest direction to the west edge of figure 6 (near northwest corner) and then flows to join the north-northeast oriented Bighorn River see figure 10). Just south of the Porcupine Creek headwaters in section 29 and in the east half of section 29 are headwaters of a northeast and east oriented tributary to the north-northeast oriented Little Bighorn River, which flows to the northeast corner of figure 6. The map contour interval for figure 6 is 40 feet and the lowest elevations between the northwest oriented Porcupine Creek valley and the east oriented Little Bighorn River tributary valley are between 9080 and 9120 feet. An even lower through valley crossing the drainage divide can be found along the border between section 8 and section 17 further to the north. This northern through valley links the northwest oriented Porcupine Creek valley with the northeast oriented Duncum Creek valley and the elevation of this northern through valley floor is shown as 8951 feet. These two distinct through valleys are separated by Rooster Hill, which rises to 9420 feet. Bald Mountain is just south of figure 6 and rises to 10,042 feet and Duncum Mountain is just north of figure 6 and rises to 9830 feet. These elevations suggest the through valley between Duncum Mountain and Bald Mountain is approximately 900 feet deep. The through valleys provide evidence that water once crossed the high Bighorn Mountains drainage divide. Further the two adjacent, yet independent through valleys provide evidence of diverging and converging flood flow channels that once crossed the high Bighorn Mountains drainage divide. Note how north and east of Rooster Hill a 400-foot deep through valley links the northeast oriented Duncum Creek valley with the east oriented Half Ounce Creek valley, which drains to the north-northeast oriented Little Bighorn River valley. These through valleys, notches, passes, or gaps crossing present day drainage divides are evidence of former valleys and drainage routes that once crossed the region.

Deer Creek-Lodge Grass Creek drainage divide area

Figure 7: Deer Creek-Lodge Grass Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 7 illustrates the Deer Creek-Lodge Grass Creek drainage divide area north of figure 5 and includes an overlap area with figure 5. The west to east oriented Montana-Wyoming state line is located near the north edge of figure 7. The Sheridan County-Big Horn County line in Wyoming is shown with a dashed line that extends from the state line (west half) in a south direction to the south edge of figure 7 (west of center) and is the Bighorn River-Little Bighorn River drainage divide. Porcupine Creek flows in a northwest direction across the southwest corner of figure 7 and west of figure 7 joins the north-northeast oriented Bighorn River. Sheep Mountain is located on the Sheridan-Big Horn County line and Deer Creek originates west of Sheep Mountain and flows in a northwest direction to the west edge of figure 7 (north half) and west of figure 7 joins northwest oriented Porcupine Creek. North of Deer Creek is west-northwest oriented Trout Creek, which flows from the Cookstove Basin to join Porcupine Creek west of figure 7. East of the high Bighorn Mountains drainage divide the Little Bighorn River flows in a north-northeast direction across the southeast corner of figure 7. Wagon Box Creek is an east oriented Little Bighorn River tributary near the south edge of figure 7. North of Wagon Box Creek three streams (from south to north, Mann Creek, Cub Creek, and Pumpkin Creek) make up the headwaters of the north-northeast oriented West Fork Little Bighorn River, which flows to the east edge of figure 7 and which joins the Little Bighorn River east of figure 7. Between the headwaters of these east and northeast oriented West Fork Little Bighorn River tributaries and the north to south oriented county line (or high Bighorn Mountains drainage divide) are north oriented headwaters of Lodge Grass Creek, which turn to flow in a northeast direction to the north edge of figure 7 (east half) and north and east of figure 7 flow to join the Little Bighorn River. Today the high Bighorn Mountains drainage divide is a relatively narrow ridge separating deep valleys on either side, the ridge is marked by notches or high level through valleys linking deep valleys on either side. The map contour interval for figure 7 is 20 meters and Sheep Mountain reaches an elevation of 2991 meters. Duncum Mountain just south of figure 7 reaches an elevation 2996 meters (note the similarity to the Sheep Mountain elevation, which suggest that at one time the entire region was an erosion surface at a similar elevation, although the region may have been uplifted since that time). Between Sheep Mountain and Duncum Mountain elevations along the drainage divide drop and are less than 2880 meters between west oriented Bucking Mule Creek (a Porcupine Creek tributary in southwest quadrant of figure 7) and Mann Creek and also between Big Tepee Creek and Wagon Box Creek near the south edge of figure 7. These notches or through valleys are more than 100 meters deep and represent evidence of deeper channels on the floor of what was once a broader valley eroded into the former high level erosion defined by the tops of  Sheep Mountain and Duncum Mountain. North of Sheep Mountain elevations along the high Bighorn Mountains drainage divide gradually decrease, although similar through valleys (or notches) crossing the drainage divide can be found. Flood flow movements responsible for eroding the deep valleys on either side of the high drainage divide were complex as floodwaters were being reversed and the Bighorn Mountains were emerging when the deep valleys were being eroded, but immense quantities of flood water eroded those deep valleys.

Detailed map of Bucking Mule Creek-Mann Creek drainage divide area

Figure 8: Detailed map of Bucking Mule Creek-Mann Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 8 provides a detailed topographic map of the Bucking Mule Creek-Mann Creek drainage divide seen in less detail in figure 7. The Sheridan County-Big Horn County boundary is shown with a labeled dashed line and extends from the north edge of figure 8 (west half) to the south edge of figure 8 (slightly west of center) and is also the Bighorn River-Little Bighorn River drainage divide. Wagon Box Creek is an east oriented Little Bighorn River tributary in the southeast quadrant of figure 8. Mann Creek is an east oriented West Fork Little Bighorn River tributary in sections 15, 14, and 13 to the north of Wagon Box Creek. Cub Creek is a northeast oriented West Fork Little Bighorn River tributary flowing to the north edge of figure 8 (near northeast corner). Big Tepee Creek is a north-northwest and west oriented Porcupine Creek tributary in sections 29, 25, and 26 (southwest quadrant of figure 8). Bucking Mule Creek is a south and southwest oriented Porcupine Creek tributary in sections 17, 24, and 23 to the north of Big Tepee Creek. In the southeast corner of section 16 a notch or through valley links the Mann Creek valley with the Bucking Mule Creek valley. The map contour interval for figure 8 is 40 feet and the through valley floor elevation is between 8400 and 8440 feet. North of figure 8 elevations on the drainage divide rise to 9813 feet on Sheep Mountain. South of figure 8 elevations on the drainage divide rise to 9831 feet on Duncum Mountain. Again, note the similarity of elevations at the tops of these two mountains, which suggests the possibility of a former erosion surface at that level. These elevations suggest the Bucking Mule Creek-Mann Creek through valley may be as much 1400 feet deep and may be a channel in the floor of what was once a much broader valley eroded into the high level erosion surface defined by the tops of Sheep and Duncum Mountains. The broader valley and the deep channel were eroded into that high level erosion surface before the deep Bighorn River valley to the north and west existed and at a time when the Bighorn Mountains did not stand high above the surrounding region. Probably the through valley and the deeper channels carved into its floor were eroded by southeast oriented floodwaters that had been captured by reversed flood flow on the Little Bighorn River alignment, although evidence in figure 8 is not adequate to say for sure.

Porcupine Creek-Cottonwood Creek drainage divide area

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

Figure 9 illustrates the Porcupine Creek-Cottonwood Creek drainage divide area north and west of figure 5 and south and west of figure 7 and includes overlap areas with figures 5 and 7. Bighorn Lake, along the west edge of figure 9, floods the north and northwest oriented Bighorn River valley. West and north of figure 9 the Bighorn River turns to flow in a north and then northeast direction across the north end of the Bighorn Mountains and then turns to flow in a north-northeast direction to join the northeast oriented Yellowstone River (see figure 1). Cottonwood Creek is the northwest and west oriented Bighorn River tributary near the south edge of figure 9. Porcupine Creek flows in a north-northwest, west, and north-northwest direction from near the southeast corner of figure 9 to the north edge of figure 9 (west of center) and north of figure 9 turns to flow in a west-northwest direction to join the north oriented Bighorn River (see figure 10). Railroad Springs Creek is a north-northeast and north-northwest oriented Porcupine Creek tributary located in the southeast quadrant of figure 9 and is located east of Mexican Hill. The map contour interval for figure 9 is 20 meters and the pool level of Bighorn Lake is 1109 meters. Little Mountain rises to more than 1920, which means it is 900 meters higher than Bighorn Lake. Elevations in the southeast corner and along the east edge of figure 9 are almost 1000 meters higher than the top of Little Mountain and still higher elevations are found further to the southeast. The steep cliffs and relatively smooth surface on the southwest flank of Little Mountain suggest the region is underlain by gradually dipping resistant sedimentary strata. Note also the steep west-facing escarpment extending from near the north center edge of figure 9 to the south edge of figure 9 (west of center), which also suggests the region is underlain by gradually dipping sedimentary strata. Depending on where and how the west-facing escarpment is measured it could be considered to be anywhere from 200 to 400 meters high. Something was responsible for stripping the sedimentary strata off the Little Mountain surface (west of the west-facing escarpment) and for stripping the strata even deeper at the northeast end of Little Mountain. This is the area where the Bighorn River has eroded a deep valley across the north end of the Bighorn Mountains. Immense south oriented floods flowing into what was then the emerging Bighorn Basin stripped the sedimentary strata stripped from the surfaces seen in figure 9. Figure 9 provides some idea of the depth of the south oriented flood flow channels that eroded headward in the Bighorn Basin west of the emerging Bighorn Mountains prior to the reversal of flood flow. Flood flow in the Bighorn Basin was reversed to create the north oriented Bighorn River drainage route when headward erosion of the much deeper northeast oriented Yellowstone River valley and north-northeast oriented Bighorn River valley beheaded the southwest and south oriented flood flow channel that had eroded the deep Bighorn Canyon valley across the north end of the Bighorn Mountains.

South Bighorn Canyon area

Figure 10: South Bighorn Canyon area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 10 provides a topographic map of the South Bighorn Canyon area north and west of figure 9 and includes an overlap area with figure 9. The Montana-Wyoming state line extends in a west to east direction across the center of figure 10. The map contour interval for figure 10 is 20 meters in Wyoming and 50 meters in Montana. The Bighorn River flows in a north direction from near the south center edge of figure 10 to the north center edge of figure 10. Mountains north and west of the Bighorn River in Montana are known as the Pryor Mountains while the mountains east of the Bighorn River are the Bighorn Mountains. North and east of figure 10 the Bighorn River turns in a northeast direction and then flows across the north end of the Bighorn Mountains in a deeper canyon than the Bighorn Canyon seen in figure 10. Porcupine Creek flows in a north-northwest and west-northwest direction from the east edge of figure 10 (south half) to join the Bighorn River north of the state line. The elevation of Bighorn Lake (or Yellowtail Reservoir), which floods the Bighorn River valley in figure 10 (and also in the northern Bighorn Canyon section located north and east of figure 10), is 1109 meters. Elevations in the Pryor Mountains to the north and west of figure 10 rise to more than 2600 meters. Elevations in the Bighorn Mountains to the south and east of figure 10 rise to more than 3000 meters suggesting the Bighorn River valley seen in figure 10 may be as much as 1500 meters deep. While some of the valley depth may be related to Pryor Mountain and Bighorn Mountains structures a significant component of that depth was achieved by water erosion. The broad and deep valley was eroded initially and primarily by immense volumes of south oriented floodwaters flowing into what at that time was the emerging Bighorn Basin west of what was at that time the emerging Bighorn Mountains. Headward erosion of this deep south-oriented flood flow channel beheaded and reversed southeast oriented flood flow channels crossing what is now the high Bighorn Mountains upland surface and created this huge valley between the Pryor and Bighorn Mountains. The present day Bighorn Canyon alignment was probably established by the south oriented flood flow before headward erosion of the deep Yellowstone River valley beheaded and reversed the flood flow to create the north oriented Bighorn River drainage route seen in figure 10.

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.