Clarks Fork Yellowstone River-Bighorn River drainage divide area landform origins near the Montana-Wyoming border, USA

Authors


Abstract:

This essay uses topographic map evidence to interpret landform origins in the region between the Clarks Fork Yellowstone River and the Bighorn River and its Shoshone River tributary near the Montana-Wyoming border. Clarks Fork of the Yellowstone River originates in the Montana Beartooth Mountains and flows in a southeast direction into Wyoming before turning to flow in a northeast and north-northeast direction between the Beartooth and Pryor Mountains to join the northeast oriented Yellowstone River near Laurel, Montana. East of the Clarks Fork is the north and north-northeast oriented Bighorn River, which originates as the southeast and north oriented Wind River before flowing across the Wyoming’s Owl Creek Mountains and Bighorn Basin to join the northeast oriented Yellowstone River north and east of the Pryor Mountains. An important Bighorn River tributary is the northeast oriented Shoshone River, which flows from the Absaroka Mountains in a northeast direction across Wyoming’s northern Bighorn Basin. Sage Creek flows in a northwest direction north of Big Pryor Mountain, and then in a south direction west of Big Pryor Mountain before turning to flow in a south-southeast direction to the Shoshone River. East of Big Pryor Mountain a through valley links the Sage Creek headwaters valley with the south-southeast oriented Crooked Creek valley, which drains to the north oriented Bighorn River as a barbed tributary. Through valleys link the Sage Creek valley and other south and southeast oriented Shoshone River tributary valleys with north-northwest oriented Clarks Fork Yellowstone River tributary valleys. Today these and similar through valleys provide evidence of what were once a maze of south oriented flood flow channels. Floodwaters were derived from a rapidly melting thick North American ice sheet and were flowing in south and southeast directions from the ice sheet’s western margin in western Canada across Montana and into Wyoming. The ice sheet was located in a deep “hole” which was formed by deep glacial erosion and by ice sheet related crustal warping. Ice sheet related crustal warping was responsible for uplift of the Pryor Mountains, Bighorn Mountains, and other regional mountain ranges. The deep northeast oriented Yellowstone River valley eroded headward across the south and southeast ice marginal melt water flood flow channels from space in the deep “hole” the rapidly melting ice sheet had formerly occupied and beheaded the south oriented flood flow channels. Floodwaters on north ends of the beheaded flood flow channels reversed flow direction and captured yet to be beheaded south oriented flood flow from flood flow channels further to the west.

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 Clarks Fork Yellowstone River-Bighorn River drainage divide area landform origins near the Montana-Wyoming border, 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 Clarks Fork Yellowstone River-Big Horn River drainage divide area landform evidence near the Montana-Wyoming border will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm.

Clarks Fork Yellowstone River-Bighorn River drainage divide area location map

Figure 1: Clarks Fork Yellowstone River-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 Clarks Fork Yellowstone River-Bighorn River drainage divide area near the Montana-Wyoming border. Yellowstone National Park is the yellow shaded area in the southwest corner of figure 1. The Montana-Wyoming state line extends in a west to east direction along the north edge of Yellowstone National Park. The Yellowstone River flows from the Yellowstone National Park area in Wyoming in a northwest direction and once in Montana turns to flow in a northeast direction to Livingston and Big Timber before turning to flow in an east-southeast direction to Columbus and then turning to flow in a northeast direction to Billings and the figure 1 northeast corner. The Absaroka Mountains extend in a south-southeast direction from southern Montana (south and east of the Yellowstone River) across the Yellowstone National Park eastern margin. East of the Absaroka Mountains, but not labeled on figure 1, are the Beartooth Mountains, which extend along the Montana-Wyoming border in an east direction from the Yellowstone National Park northeast corner to near Red Lodge, Montana. The Clarks Fork Yellowstone River originates in the Beartooth Mountains north of Cooke City, Montana (near Yellowstone National Park northeast corner) and flows in a southeast direction almost to the Beartooth Mountains east end where it then turns to flow in a northeast and north-northeast direction to join the Yellowstone River near Laurel. The Bighorn River flows in a north direction from Manderson, Wyoming (near south edge of figure 1, east of center) to the Montana state line and then turns to flow in a north-northeast direction through the Bighorn Canyon National Recreation Area to join the northeast oriented Yellowstone River near Custer. The Clarks Fork Yellowstone River-Bighorn River drainage divide area near the Montana-Wyoming border illustrated and discussed in this essay is located in the Pryor Mountains area, which is west of the Bighorn Canyon National Recreation Area and east of the northeast and north-northeast oriented Clarks Fork Yellowstone River.

Looking at the big picture erosion history the figure 1 drainage routes developed as immense south and southeast oriented melt water floods flowed across the region and crustal warping raised the Beartooth, Pryor, and the Bighorn Mountains (in southeast corner of figure 1) at approximately the same time as the deep Yellowstone River valley eroded headward from the deep “hole” in which a large North American ice sheet was rapidly melting. The deep “hole” was located north and east of the figure 1 map area, which is located along the deep “hole’s” deeply eroded southwest wall. The east and northeast oriented Yellowstone River valley and its northeast oriented tributary valleys eroded headward from deep “hole” space being opened up by the ice sheet melting to capture immense south and southeast oriented ice marginal floods flowing from western Canada across Montana and into Wyoming. Initially mountain ranges in the figure 1 map area, including the Beartooth, Pryor, and Bighorn Mountains, did not stand high above the surrounding regions and floodwaters could freely flow across the entire figure 1 map area. Ice sheet related crustal warping raised the Beartooth, Pryor, and Bighorn Mountains as the immense melt water floods eroded regions surrounding the rising mountain masses. Floodwaters carved deep and anastomosing south oriented valleys or flood flow channels between and across the rising mountain masses. Headward erosion of the much deeper northeast oriented Yellowstone River valley from space in the deep “hole” being opened up by ice sheet melting then beheaded and reversed the south oriented flood flow channels in sequence from east to west to erode the north oriented valleys seen today. South oriented flood flow channels moving floodwaters to and across the present day Bighorn Basin were beheaded and reversed one at a time and in sequence from east to west. Newly beheaded and reversed flood flow channels captured immense quantities of yet to be beheaded south and southeast oriented flood flow from channels further to the west and with this captured flood flow were able to erode significant north oriented valleys.

In the case of the figure 1 headward erosion of the deep northeast oriented Yellowstone River valley beheaded and reversed south oriented flood flow that eroded the north oriented Bighorn River valley, which captured south and southeast oriented flood flow from further to the west. Some of the southeast oriented flood flow was moving across the present day Pryor Mountains and was captured by headward erosion of the northeast oriented Shoshone River valley (which eroded headward from the actively eroding north oriented Bighorn River valley in Wyoming). Subsequently headward erosion of the deep northeast oriented Yellowstone River valley beheaded and reversed south and southeast oriented flood flow that created the north-northeast oriented Clarks Fork Yellowstone River drainage route. Headward erosion of the actively eroding Clarks Fork Yellowstone River valley captured southeast oriented flood flow moving across the emerging Beartooth Mountains to the newly eroded north-northeast oriented Bighorn River valley in Wyoming, which accounts for the southeast oriented Clarks Fork Yellowstone River headwaters in the Beartooth Mountains. The Beartooth, Pryor, Bighorn, and other mountain ranges emerged as floodwaters deeply eroded regions surrounding the mountain ranges and as delayed ice sheet related crustal warping raised the mountain masses. Deep flood water erosion of the rising mountain masses and deposition of eroded debris in adjacent basins and valleys may have contributed to the mountain range uplift and adjacent basin formation.

Detailed location map for Clarks Fork Yellowstone River-Bighorn River drainage divide area

Figure 2: Detailed location map Clarks Fork Yellowstone River-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 Clarks Fork Yellowstone River-Bighorn River drainage divide area near the Montana-Wyoming border. The Montana-Wyoming state line extends in a west to east direction across the center of figure 2. Green shaded areas are National Forest lands and are generally located in mountainous regions. The green shaded area straddling the west edge of figure 2 is mostly in the Beartooth Mountains with some Absaroka Mountain regions near the southwest corner of figure 2. Clarks Fork Yellowstone River flows from the west edge of figure 2 (just south of the state line) in a southeast direction almost to the edge of the green shaded area and then turns to flow in a northeast and north-northeast direction to the north center edge of figure 2. Note how the north-northeast oriented Clarks Fork segment has several north and northwest oriented tributaries including (from north to south) Jack Creek, Cottonwood Creek, Silver Tip Creek, Big Sand Coulee, and Pat O’Hara Creek. The green shaded area along the east edge of figure 2 in Wyoming is located in the Bighorn Mountains. The smaller green shaded area in Montana labeled Custer National Forest is in the Pryor Mountains. The brown shaded area in northeast quadrant of figure 2 (in Montana) is Crow Indian Reservation land. The brighter brown area around Bighorn Lake in Wyoming and extending north and northeast into Montana is the Bighorn Canyon National Recreation Area and is located adjacent to the Bighorn River. The Bighorn River flows in a north direction from the south edge of figure 2 to Bighorn Lake and then in a north-northeast direction to the northeast corner of figure 2.  The Shoshone River flows in a northeast and east-northeast direction from the south edge of figure 2 (west of center) through the town of Lovell to Bighorn Lake. Sage Creek is an important Shoshone River tributary in the following essay and begins as a west-northwest oriented stream in the Pryor Mountains and flows to the southwest corner of the Crow Indian Reservation (west of the Bighorn River) and then turns to flow in a south and south-southeast direction to join the Shoshone River near Lovell. Crooked Creek originates in the Custer National Forest area east of the Sage Creek headwaters and flows in a south and southeast direction to join the north oriented Bighorn River as a barbed tributary (near north end of Bighorn Lake). North of the Crooked Creek headwaters are headwaters of north, northeast, and east oriented Dry Head Creek, which flows to the Bighorn River. Also note the location of Polecat Bench north of Powell, Wyoming, which is located on the Clarks Fork Yellowstone River-Shoshone River drainage divide and which is seen in topographic maps below. In addition, note the location of Heart Mountain east of the green shaded near the south edge of the southwest quadrant of figure 2. Cottonwood Creek is a southeast oriented Shoshone River tributary south of Heart Mountain and Pat O’Hara Creek is a north oriented Clarks Fork Yellowstone River tributary north of Heart Mountain.

Sage Creek-Crooked Creek drainage divide area

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

Figure 3 provides a topographic map of the Sage Creek-Crooked Creek drainage divide area at Big Pryor Mountain. Big Pryor Mountain is the large mountain mass in the southwest quadrant of figure 3. East Pryor Mountain is located east of Big Pryor Mountain and is the east edge of figure 3. Sage Creek originates near the center of figure 3 and flows in a west-northwest direction to the west edge of figure 3 (north half). Crooked Creek originates a short distance south and east of the Sage Creek headwaters and flows in a south-southeast direction along the Big Pryor Mountain east flank to the south edge of figure 3 (east half). Note how a deep through valley located between Big Pryor Mountain and East Pryor Mountain links the Sage Creek valley with the Crooked Creek valley. The map contour interval for figure 3 is 50 meters and the through valley floor elevation at the drainage divide is between 2150 and 2200 meters. The spot elevation on the top of East Pryor Mountain reads 2645 meters while the spot elevation on Big Pryor Mountain reads 2676 meters. These elevations suggest the through valley is at least 450 meters deep. The through valley is a water eroded feature and was eroded by southeast oriented flood flow moving to what at that time was a south oriented flood flow channel on the alignment of the present day north oriented Bighorn River. Also note a short distance east of the North Fork Sage Creek headwaters the northeast oriented headwaters of Dry Head Creek, with Dry Head Creek flowing in a northeast and east direction to the east edge of figure 3 (north half). A through valley links the North Fork Sage Creek valley with the northeast oriented Dry Head Creek valley. The elevation of this second through valley at the drainage divide is deeper and is between 2000 and 2050 meters. While not seen in figure 3 elevations north of through valley rise to 2238 meters. Headward erosion of the deep northeast oriented Dry Head Creek valley captured the southeast oriented flood flow, which was eroding the south-southeast oriented Crooked Creek valley and diverted floodwaters in a northeast direction, probably to eventually reach the actively eroding Yellowstone River valley, which was eroding headward from space in a deep “hole” being opened up by the melting ice sheet, which had occupied that space. The flood flow capture was probably aided by uplift of the Pryor Mountains, which was occurring as floodwaters were flowing across the area. The reversal of flood flow in the west-northwest oriented Sage Creek valley segment is discussed under figures 5 and 6 below.

Detailed map of Sage Creek-Crooked Creek drainage divide area

Figure 4: Detailed map of Sage Creek-Crooked Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 4 provides a detailed topographic map of the Sage Creek-Crooked Creek drainage divide area seen in less detail in figure 3. Big Pryor Mountain is located near the southwest corner of figure 4. Sage Creek originates in northwest corner of section 32 east of Big Pryor Mountain and flows in a west-northwest and northwest direction to the northwest corner of figure 4. Stevens Draw is a north-northwest oriented tributary originating on The Flat and joining Sage Creek in the southeast corner of section 25. East of The Flat in section 5 are headwaters of south oriented Crooked Creek. The map contour interval for figure 4 is 40 feet and the elevation of the drainage divide at The Flat is between 7080 and 7120 feet. The spot elevation labeled “Ice Cave” on Big Pryor Mountain reads 8786 feet. The west flank of East Pryor Mountain is located in the southeast quadrant of figure 4, although the highest elevations are east of figure 4. The top of East Pryor Mountain has an elevation remarkably similar to the elevation of Big Pryor Mountain, with the spot elevation reading 8776 feet. These elevations suggest the through valley linking the Sage Creek valley with the Crooked Creek valley is approximately 1650 feet deep. Also note Roberts Bench just north and east of the Sage Creek headwaters. Elevations on Roberts Bench are remarkably similar to elevations on The Flat, suggesting both were eroded at approximately the same time. East of Roberts Bench are headwaters of Dry Head Creek. North of Roberts Bench, along the north edge of figure 4, is the through valley linking the west oriented North Fork of Sage Creek valley with the Dry Head Creek headwaters valley. The through valley elevation at the north edge of figure 4 (which is the through valley’s deepest point) is between 6600 and 6640 feet or at least 440 feet lower than the elevation of the through valley at The Flat. This deeper through valley provides evidence that headward erosion of the northeast oriented Dry Head Creek captured the southeast oriented flood flow moving to the Crooked Creek valley and diverted the floodwaters in a northeast direction toward the developing deep “hole” from which the deep northeast oriented Yellowstone River valley was eroding. At the same time as this flood flow capture was taking place the Pryor Mountain region and the entire Bighorn Basin region to the south were probably being uplifted, which contributed to the massive flood flow reversal taking place.

Bluewater Creek-Sage Creek drainage divide area

Figure 5: Bluewater Creek-Sage Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 5 illustrates the Bluewater Creek-Sage Creek drainage divide area west of figure 3 and includes an overlap area with figure 3.  The north oriented Clarks Fork Yellowstone River can just barely be seen in the northwest corner of figure 5. The west end of Big Pryor Mountain can be seen in the southeast corner of figure 5. Sage Creek flows in a northwest direction from the east center edge of figure 5 and then turns to flow in a south direction just west of Big Pryor Mountain to the figure 5 south edge (east of center). The Sage Creek elbow of capture was formed when headward erosion of the deeper south oriented Sage Creek valley (west of Big Pryor Mountain) beheaded and reversed the southeast oriented flow channel, which had been moving flood water to the Dry Head Creek valley and previously to the Crooked Creek valley seen in figures 3 and 4. Note Summit Creek in the northeast corner area of figure 5. Summit Creek flows in a northwest direction to enter a southwest to northeast oriented through valley known as Pryor Gap (see figures 9 and 10 in the Clarks Fork-Pryor Creek drainage divide essay). What makes Summit Creek particularly interesting is on entering the Pryor Gap through valley Summit Creek splits with some of its flow going in a northeast direction to north oriented Pryor Creek and the remaining flow going in a southwest direction to join south oriented Sage Creek. Bowler Flats is the flat floored basin directly northwest of Big Pryor Mountain and is drained in the east by south oriented Sage Creek and in the west and north by the north oriented South Fork Bluewater Creek, which flows to north-northwest oriented Bluewater Creek, which in turn flows to the Clarks Fork Yellowstone River. North-northwest oriented Clarks Fork tributaries flowing to the west edge of figure 5 include Cottonwood Creek, Jack Creek, and Bridger Creek. Note how these streams are flowing in a north-northwest direction while just a short to the east Sage Creek is flowing at a somewhat higher elevation in a south-southeast direction. The Sage Creek flow direction is a relic of the south oriented flood flow that once crossed the region before the massive flood flow reversal that eroded the deeper valleys used by the present day north oriented drainage routes. The map contour interval for figure 5 is 50 meters and note there are no contour lines in the Bowler Flats area between the south oriented Sage Creek valley and the north oriented South Fork Bluewater Creek valley. The Bowler Flats drainage divide was formed as headward erosion of the deeper Clarks Fork valley (from the deep northeast oriented Yellowstone River valley, which had eroded headward from the deep “hole” the melting ice sheet had occupied) captured and reversed what had been south oriented flood flow in the Pryor Mountains region and the landscape has changed little since.

Jack Creek-Sage Creek drainage divide area

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

Figure 6 provides a topographic map of the Jack Creek-Sage Creek drainage divide area south and east of figure 5 and includes an overlap area with figure 5. Big Pryor Mountain is the mountain seen in the northeast quadrant of figure 6. Sage Creek flows in a south and south-southeast direction from the north edge of figure 6 (just west of center) to the town of Warren and then to the south edge of figure 6 (just east of center) and south of figure 6 joins the east-northeast oriented Shoshone River. Jack Creek originates a short distance northwest of Warren and flows in a northwest direction along the highway and railroad to the northwest corner of figure 6 and north and west of figure 6 joins the north-northeast oriented Clarks Fork Yellowstone River. Cottonwood Creek flows in a north and northwest direction from the south edge of figure 6 (west half) to the west edge of figure 6 (just north of center), and west of figure 6 joins the Clarks Fork Yellowstone River. The map contour interval for figure 6 is 50 meters. Note how near the northwest corner of figure 6 northwest oriented Jack Creek crosses the 1250-meter contour line. Just a few kilometers to the east south oriented Sage Creek crosses the 1400-meter contour line. There is no significant topographic barrier between the south oriented Sage Creek valley and the northwest oriented Jack Creek valley. The Sage Creek valley is simply located on a higher-level erosion surface. The Jack Creek and Cottonwood Creek alignments were initiated as south oriented flood flow channels that once converged with the south oriented flood flow channel on the Sage Creek alignment. The deeper northwest oriented Jack Creek and Cottonwood Creek valleys were eroded during the reversal of flood flow caused by headward erosion of the deeper north oriented Clarks Fork valley, which had eroded headward from the deep northeast oriented Yellowstone River valley, which had eroded headward from the deep “hole” the melting ice sheet had occupied. Note how tributaries to south oriented Sage Creek from the Big Pryor Mountain are oriented in south directions indicating that when the Sage Creek valley was eroded the drainage across the rising Big Pryor Mountain mass was in a south direction, which is consistent with the south-oriented flood flow hypothesis being presented here.

Cottonwood Creek-Sage Creek drainage divide area

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

Figure 7 illustrates the Cottonwood Creek-Sage Creek drainage divide area south of figure 6 and includes an overlap area with figure 6. The Montana-Wyoming state line extends in a west to east direction across figure 7 (a short distance south of center). Warren is located in the north center area of figure 7 and Frannie is located a short distance east of the south center area of figure 7. Sage Creek flows in a south-southeast direction from the north edge of figure 7 through Warren and Frannie to the south edge of figure 7 and joins the Shoshone River south of figure 7. Cottonwood Creek flows in a northeast direction from the southwest corner of figure 7 and then turns to flow in a north-northwest direction to the northwest corner of figure 7 and then joins the north-northeast oriented Clarks Fork Yellowstone River. Note several south-southeast oriented tributaries near the west edge of the southwest quadrant of figure 7 flowing to the northeast oriented Cottonwood Creek segment. Those south-southeast oriented tributaries provide evidence that reversed flood flow on the north-northwest oriented Cottonwood Creek segment alignment captured south-southeast oriented flood flow from flood flow routes further to the west. Note the southeast-facing escarpment east of the northeast oriented Cottonwood Creek segment in Wyoming. The erosion surface between that escarpment and Cottonwood Creek is the northeast end of Polecat Bench, which serves as the Clarks Yellowstone River-Shoshone River drainage divide. South-southeast oriented streams drain the southeast-facing escarpment-surrounded basin to east and southeast oriented Polecat Creek, which flows to Sage Creek, which then flows to the Shoshone River. The escarpments ringing the escarpment-surrounded basin originated as a giant southeast oriented headcut,which was eroded by south-southeast oriented flood flow moving to what was then the newly eroded Shoshone River valley. The U-turn made by south-southeast oriented floodwaters that were captured by reversed flood flow on the north-northwest oriented Cottonwood Creek valley alignment is documented by the Cottonwood Creek drainage system seen near the west margin of figure 7. The reversal of flood flow occurred when headward erosion of the deeper north-northeast oriented Clarks Fork Yellowstone River valley beheaded south-southeast oriented flood flow on the north-northwest oriented Cottonwood Creek segment alignment.

Detailed map of Cottonwood Creek–Sage Creek drainage divide area

Figure 8: Detailed map of Cottonwood Creek-Sage 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 Cottonwood Creek-Sage Creek drainage divide seen in less detail in figure 7 above. Cottonwood Creek flows in a north-northeast and north-northwest direction from the southwest corner of figure 8 to the northwest corner of figure 8. Polecat Bench is the flat-topped erosion surface labeled “BENCH” just east of the north-northeast oriented Cottonwood Creek segment. North oriented Jack Creek headwaters flow across section 32 near the north center edge of figure 8. The southeast-facing escarpment on the Polecat Bench east margin is the abandoned headcut, which was eroded headward by south-southeast oriented flood flow from the newly eroded Shoshone River valley. Through valleys link the north-northwest oriented Cottonwood Creek drainage basin and north and north-northwest oriented Jack Creek drainage basin with the southeast oriented Sage Creek drainage basin and can be seen in sections 21 and 28 just north and west of the center of figure 8. The map contour interval for figure 8 is 20 feet and the through valley linking the Jack Creek drainage basin with the Sage Creek drainage basin has an elevation of between 4620 and 4640 feet at the drainage divide while the through valley in section 21 linking the Cottonwood Creek drainage basin and the Sage Creek drainage basin has an elevation at the drainage divide of between 4660 and 4680 feet and the through valley in section 28 (northwest corner) has an elevation of between 4680 and 4700 feet. Polecat Bench elevations gradually rise to the southwest and at the southwest end (seen in figure 9) the elevations exceed 5200 feet. As seen in earlier figures Big Pryor Mountain is located east and north of figure 8 and reaches elevations much greater than 5200 feet. In other words, figure 8 is located on the floor of a much larger north-northwest to south-southeast oriented through valley linking the north-northeast oriented Clarks Fork Yellowstone River valley with the northeast oriented Shoshone River valley. This through valley is more than 500 feet deep and is several miles across. The through valley was eroded by south-southeast oriented flood flow moving to what was then the newly eroded Shoshone River valley at a time when the deep north-northeast oriented Clarks Fork Yellowstone River valley did not exist. Headward erosion of the deep north-northeast oriented Clarks Fork Yellowstone River valley (from the deep northeast oriented Yellowstone River valley, which eroded headward from space in the deep “hole” the melting ice sheet had occupied) captured the south-southeast oriented flood flow and reversed flood flow on the north-northwest ends of beheaded flood flow channels to create the present day north-northwest oriented Cottonwood Creek and Jack Creek drainage basins.

Clarks Fork Yellowstone River-Shoshone River drainage divide area

Figure 9: Clarks Fork Yellowstone River-Shoshone River drainage divide are at Pryor Gap. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 9 illustrates the Clarks Fork Yellowstone River-Shoshone River drainage divide area and is located south and west of figure 7 and does not include overlap areas with previous figures. The Figure 9 map has been reduced in size to show a larger region. The Shoshone River flows in a northeast direction in the southeast corner of figure 9. The southwest end of Polecat Bench can be seen along the east edge of figure 9 (north of Ralston Flats). Alkali Creek is the southeast oriented tributary flowing just north of the words “Ralston Flats” and joins the Shoshone River east of figure 9. Note how Alkali Creek headwaters are oriented in a northeast direction and flow near the crest of a northwest-facing escarpment, with the north oriented Little Sand Coulee valley and northeast oriented West Fork Big Sand Coulee valley being located at the escarpment base (Big Sand Coulee drains in a north direction in the northeast quadrant of figure 9 and north of figure 9 joins the Clarks Fork Yellowstone River). The Clarks Fork Yellowstone River flows in a southeast, east-northeast, and north direction near the northwest corner of figure 9.  Paint Creek is the north-northeast oriented Clarks Fork tributary west of Kimball Bench and Pat O’Hara Creek is the Clarks Fork tributary between Kimball Bench and Chapman Bench with Little Sand Coulee being the Clarks Fork tributary east of Chapman Bench. The map contour interval for figure 9 is 50 meters and the elevation where Paint Creek joins the Clarks Fork (near northwest of figure 9) is between 1300 and 1350 meters. The northeast oriented Alkali Creek valley has an elevation of more than 1500 meters and crosses the 1500-meter contour line near where Alkali Creek turns to flow in a southeast direction. The Shoshone River in the southeast corner of figure 9 has an elevation of between 1350 and 1400 meters. In other words the Clarks Fork is located in the deeper valley. Note the through valleys in the Badland Hills area linking the Little Sand Coulee valley with the Big Sand Coulee valley.  These through valleys were eroded as anastomosing flood flow channels during the flood flow reversal that captured southeast oriented flood flow moving to what was then the newly eroded Shoshone River valley. The previously mentioned northwest-facing escarpment was eroded by south oriented flood flow that was captured and diverted to flow in a northeast direction to the north oriented Big Sand Coulee valley (which north of figure 9 turns to drain in a northwest direction to the north oriented Clarks Fork valley). This evidence suggests south and southeast oriented flood flow on the Big Sand Coulee alignment was beheaded and reversed before south and southeast oriented flood flow on the Little Sand Coulee valley alignment was beheaded and reversed. Also note Heart Mountain, which straddles the south edge of figure 9 (just west of center). Figure 10 below provides a better view of the Clarks Fork-Shoshone River drainage divide area west of Heart Mountain.

Skull Creek-Cottonwood Creek drainage divide area

Figure 10: Skull Creek-Cottonwood Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 10 provides a topographic map of the Skull Creek-Cottonwood Creek drainage divide area west and south of figure 9 and includes overlap areas with figure 9. Heart Mountain is the forested high area just east of the center of figure 10. The Heart Mountain Irrigation Canal drains in a north direction near the east edge of figure 10 and obtains its water from the Shoshone River, which is south of figure 10. The East and West Forks of Alkali Creek meet north of Heart Mountain and form northeast oriented Alkali Creek, which flows to the north edge of figure 10 (east half). Pat O’Hara Mountain straddles the west center edge of figure 10 and Pat O’Hara Creek flows in an east and northeast direction from the west edge of figure 10 along the Pat O’Hara Mountain south flank and the turns to flow in a north and north-northeast direction just east of Pat O’Hara Mountain to the north edge of figure 10 (west half). Skull Creek originates a short distance southeast of the Pat O’Hara Creek elbow of capture (at east end of Pat O’Hara Mountain) and flows in n east-southeast, northeast, and north-northwest direction to join Pat O’Hara Creek just north of figure 10. North of figure 10 Pat O’Hara Creek joins the Clarks Fork Yellowstone River. Cottonwood Creek originates as an east-southeast oriented stream and then turns to flow in a south-southeast direction along the highway to the south edge of figure 10. South of figure 10 Cottonwood Creek joins the northeast oriented Shoshone River. Note how a deep through valley links the north-northwest oriented Skull Creek valley segment with the south-southeast oriented Cottonwood Creek valley segment. The map contour interval for figure 10 is 50 meters and the through valley elevation where the highway crosses the drainage divide is between 1800 and 1850 meters. Heart Mountain elevations rise to 2476 meters and Pat O’Hara Mountain elevations rise even higher suggesting the through valley may be as much as 600 meters deep. Also note how higher level through valleys link the east and northeast oriented Pat O’Hara Creek segment (along the Pat Hara Mountain south flank) with the east-southeast oriented Skull Creek headwaters valley segment and also with the east-southeast oriented Cottonwood Creek headwaters valley segment. These through valleys suggest prior to the reversal of flood flow that created the north oriented Clarks Fork drainage system floodwaters from what were then emerging mountains west of figure 10 flowed in a southeast direction to what was then the newly eroded Shoshone River valley.

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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