Separation Creek-North Platte River drainage divide area landform origins along the Great Divide Basin east rim, Wyoming, USA

Authors

 

Abstract:

Topographic map evidence in the Separation Creek-North Platte River drainage divide area along the Wyoming Great Divide Basin east rim is illustrated and discussed to determine the origin of the regional erosional landforms. Today the Great Divide Basin is a large region of internal drainage located along the east-west continental divide. The North Platte River flows in a north direction just east of the Great Divide Basin and crosses mountain ranges and other geologic structures in deep water gaps before turning to flow in an east and southeast direction around the northwest end of the Laramie Mountains. Separation Creek is a north, northeast, and south oriented drainage route in the eastern Great Divide Basin. A complex of diverging and converging through valleys link the North Platte River valley with the Separation Creek valley. Some, but not all of the through valleys follow regional geologic structure orientations. The through valleys are interpreted to have been eroded by an anastomosing complex of diverging and converging south oriented flood flow channels that once crossed the region. 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 Wyoming. At that time regional mountain ranges were beginning to emerge and at first floodwaters could freely flow across what are today major mountain barriers. Mountain ranges and other geologic structures emerged as floodwaters deeply eroded surrounding basins and valleys and as ice sheet related crustal warping raised the mountain ranges, geologic structures, and the entire region. Headward erosion of the deep east and southeast oriented North Platte River valley around the Laramie Mountains northwest end beheaded south oriented flood flow channels west of the Laramie Mountains. Floodwaters on north ends of the beheaded flood flow channels reversed flow direction to flow to the much deeper east and southeast oriented North Platte River valley and created the north, east, and southeast oriented North Platte River drainage route. A similar flood flow reversal created the north, northeast, and south oriented Separation Creek drainage route. Reversal of flood flow on the North Platte River alignment also beheaded what had been diverging south oriented flood flow channels to the Separation Creek drainage route. The flood flow reversals were probably greatly aided by ice sheet related crustal warping that was raising mountain ranges and the present day North Platte River and Separation Creek headwaters areas.

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 Separation Creek-North Platte River drainage divide area landform origins along the Wyoming Great Divide Basin east rim. 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 Separation Creek–North Platte River drainage divide area landform evidence along the Wyoming Great Divide Basin east rim will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm.

Separation Creek-North Platte River drainage divide area along the Great Divide Basin east rim location map

Figure 1: Separation Creek-North Platte River drainage divide area along the Wyoming Great Divide Basin east rim 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 Separation Creek-North Platte River drainage divide area along the Wyoming Great Divide Basin east rim and illustrates a region in south central Wyoming with Colorado and Utah being located south of Wyoming (Utah is in the southwest corner of figure 1). The Great Divide Basin is labeled and is located west of the north oriented North Platte River. The Great Divide Basin is a region of internal drainage along the east-west continental divide. All drainage routes in the Great Divide Basin end in the Great Divide Basin and there is no drainage from the Great Divide Basin to external drainage routes. The North Platte River originates in Colorado south of figure 1 and flows in a north-northwest direction between the Sierra Madre and Medicine Bow Mountains into Wyoming where it makes a northwest and then a northeast jog to enter Seminoe Reservoir. From Seminoe Reservoir the North Platte River flows in a north direction to Pathfinder Reservoir and then in a north-northeast and northeast direction to Casper. Near Casper the North Platte River turns to flow around the northwest end of the Laramie Mountains and then flows in a southeast direction to the east edge of figure 1 (north of center). East of figure 1 the North Platte River flows into Nebraska with water eventually reaching the Gulf of Mexico. The Sweetwater River is an east oriented tributary flowing between the Granite Mountains and Green and the Ferris Mountains to join the North Platte River at Pathfinder Reservoir and is the major drainage route north of the Great Divide Basin. West of the Great Divide Basin is the south oriented Green River drainage basin with the Green River (not labeled in figure 1) flowing through Flaming Gorge National Recreation Area (near southwest corner of figure 1). South of figure 1 the Green River flows to the southwest oriented Colorado River with water eventually reaching the Pacific Ocean. West of the Sierra Madre Mountains is south oriented Muddy Creek, which joins the west and southwest oriented Little Snake River (not labeled in figure 1) near Baggs, Wyoming. South of figure 1 the Little Snake River joins the west oriented Yampa River, which then joins the Green River.  Separation Creek is shown in figure 1, but is not labeled, and is the north, northeast, and south-southeast oriented drainage route west and north of Rawlins. The Separation Creek-North Platte River drainage divide area along the Great Divide Basin eastern rim investigated in this essay is primarily located west of the north oriented North Platte River and east of Separation Creek in the region south of the Ferris Mountains and north of Rawlins.

Wyoming, Colorado, and Utah drainage routes developed during immense melt water floods flowing from the western margin of a thick North American ice sheet from western Canada to and across the region seen in figure 1. At least initially Wyoming, Colorado, and Utah mountain ranges had not emerged and at first floodwaters could freely flow across what are today massive mountain barriers. Mountain ranges emerged as floodwaters flowed across them and as floodwaters deeply eroded surrounding basins and valleys. In addition ice sheet related crustal warping raised mountain ranges and entire regions as floodwaters flowed across them. The north oriented North Platte River drainage route west of the Laramie Mountains originated as a south oriented flood flow channel (in reality multiple flood flow channels, but for purposes of keeping this discussion brief I will refer to a single flood flow channel). The south oriented flood flow was moving to what at that time were actively eroding Colorado River headwaters and tributary valleys. Headward erosion of a much deeper southeast and east oriented valley on the present day North Platte River alignment north and east of the Laramie Mountains beheaded the south oriented flood flow channel. Floodwaters on the north end of the beheaded flood flow channel reversed flow direction to flow in a north direction to the much deeper east and southeast oriented valley and to create the north, east, and southeast oriented North Platte River drainage route. The North Platte River flow reversal west of the Laramie Mountains was greatly aided by crustal warping that raised nearby mountain ranges and the entire North Platte River headwaters area. Prior to headward erosion of the east oriented Sweetwater River valley floodwaters flowed in a south direction across the present day Great Divide Basin to what were then actively eroding Little Snake River tributary valleys. Headward erosion of the deep east oriented Sweetwater River valley beheaded all flood flow routes crossing the Great Divide Basin and diverted the south oriented flood flow to what at first was a south oriented flood flow channel on the present day north oriented North Platte River alignment (note how Sweetwater River valley turns in a south direction at Pathfinder Reservoir to join the north oriented North Platte River). The North Platte River flood flow reversal probably took place as the Sweetwater River valley was eroding headward across south oriented flood flow. Flood flow to the newly eroded Sweetwater River valley was subsequently beheaded and reversed by headward erosion of the deep northeast oriented Yellowstone River valley across Montana (north of figure 1). The deep Yellowstone River valley eroded headward from space at the south end of the deep “hole” the ice sheet had created and occupied and which was being opened up by ice sheet melting.

Detailed location map for Separation Creek-North Platte River drainage divide area

Figure 2: Detailed location map Separation Creek-North Platte 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 Separation Creek-North Platte River drainage divide area. The North Platte River flows in a north-northwest direction from the south edge of figure 2 (east half) to Fort Steele and then in a northwest direction along the southwest side of Cedar Ridge before turning in a northeast direction to enter Seminoe Reservoir where the flooded North Platte River is oriented in a north direction. Seminoe Dam is located where the North Platte River flows in a deep water gap across the Seminoe Mountains and before entering the south end of Pathfinder Reservoir along the north edge of figure 2 (east half). North of figure 2 the North Platte River turns to flow in a north-northeast, northeast, east, and southeast direction around the Laramie Mountains northwest end with water eventually reaching the Gulf of Mexico. North Platte River tributaries seen in figure 2 are oriented in both north and south directions. Southwest and southeast oriented tributaries join the north oriented North Platte River as barbed tributaries and provide evidence that the North Platte River flow direction has been reversed. Cheyenne Ridge is west of Seminoe Reservoir and Coal Creek is an east-northeast oriented North Platte River tributary south of Cheyenne Ridge. Sugar Creek is a northeast, east, and northeast oriented North Platte River tributary flowing near Rawlins. The Great Divide Basin east rim is shown with a labeled dashed line located west of the North Platte River drainage basin. Near the north edge of figure 2 the Great Divide Basin rim is located south of the Ferris Mountains (”Mountains” in figure 2) and near the south edge of figure 2 the Great Divide Basin rim extends in a southwest direction around the Sugar Creek drainage basin. Separation Creek is located west of the Great Divide Basin east rim and flows in a north-northeast direction from the south edge of figure 2 (west half) to Daleys Ranch and then in a north direction on the west side of Separation Flats to near Mahoney Lakes. Just north of the Mahoney Lakes area Separation Creek turns in an east-northeast and then south-southeast direction to end at Separation Lake. South of Separation Lake is Dry Lake. North of Separation Lake is southwest oriented Stone Creek, which appears to end in the middle of nowhere, but on more detailed maps ends in an intermittent lake. Separation Point is located west of that lake. South of figure 2 Separation Creek headwaters are linked by through valleys with the west and south oriented Muddy Creek valley, which drains to the west and southwest oriented Little Snake River with water eventually reaching the Pacific Ocean.

Riddle Creek-Rankin Creek drainage divide area

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

Figure 3 provides a topographic map of Riddle Creek-Rankin Creek drainage divide area. The map contour interval for figure 3 is 20 meters. A small area of Seminoe Reservoir can be seen in the southeast corner of figure 3 and more of Seminoe Reservoir can be seen near the east center edge of figure 3. Seminoe Dam is located north and west of the east center edge of figure 3 near where a northwest oriented North Platte River segment in Seminoe Canyon turns to flow in a northeast direction. Seminoe Canyon is the deep water gap where the North Platte River crosses the Seminoe Mountains. Southeast and east-southeast oriented Morgan Creek joins the North Platte River at the elbow of capture (where the North Platte River direction of flow changes). Further west Hurt Creek originates near the center of figure 3 and flows in a southeast and east-southeast direction to the east edge of figure 3 (south half) and joins the north oriented North Platte River as a barbed tributary east of figure 1. North of the Hurt Creek headwaters and east of Bradley Peak are headwaters of north-northwest and north-northeast oriented Deweese Creek (not labeled in figure 3). North of figure 3 Deweese Creek flows in a north-northeast and northeast direction to join the north oriented North Platte River. A through valley east of Bradley Peak links the north-northwest oriented Deweese Creek headwaters valley with the southeast oriented Hurt Creek headwaters valley. The through valley floor elevation is between 2400 and 2420 meters. Elevations east of the through valley rise to 2564 meters and Bradley Peak to the west rises to more than 2700 meters. These elevations suggest the through valley may be as much as 144 meters deep. The through valley was eroded by south oriented flood flow that had been moving on the present day north oriented Deweese Creek alignment. The Deweese Creek-Hurt Creek alignment was probably used by a flood flow channel that diverged from the present day North Platte River alignment north of figure 3 and that then converged with the North Platte River alignment east of figure 3. Headward erosion of a deeper valley on the North Platte River alignment, which may have occurred as flood flow was reversed on the North Platte River alignment, beheaded and reversed flood flow on the Deweese Creek alignment and ended south oriented flood flow to the Hurt Creek valley. Cheyenne Ridge is probably a major hogback ridge in the south half of figure 3 and Windy Ridge is west of Cheyenne Ridge. Rankin Creek originates near the north end of Windy Ridge and flows in a southeast, east-southeast, and east direction north of Cheyenne Ridge to the east edge of figure 3 (south half) and joins the North Platte River east of figure 3. West of the Rankin Creek headwaters, but also near the north end of Windy Ridge, are headwaters of west oriented Riddle Creek, which flows to the west edge of figure 3 (south of center). West of figure 3 Riddle Creek flows to southwest oriented Stone Creek, which then flows to an intermittent lake and ends. Northwest to southeast oriented through valleys between Windy Ridge to the southwest and the Seminoe Mountains to the northeast link the Rankin Creek drainage basin to the east with the Great Divide Basin to the west. The through valley linking Riddle Creek with Rankin Creek has a floor elevation of between 2200 and 2220 meters. Windy Ridge elevations rise to more than 2400 meters and Seminoe Mountains elevations are even higher suggesting the through valleys are as much as 180 meters deep. The through valleys were eroded by southeast and east oriented flood flow moving from the Great Divide Basin to what at that time was a south oriented flood flow channel on the North Platte River alignment. However, after flood flow on the North Platte River alignment was reversed to flow in a north direction south oriented flood flow probably still moved into the Great Divide Basin and probably moved in a southeast and east direction to the newly reversed North Platte River drainage route.

Detailed map of Riddle Creek-Rankin Creek drainage divide area

Figure 4: Detailed map of Riddle Creek-Rankin 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 Riddle Creek-Rankin Creek drainage divide area seen in less detail in figure 3. The map contour interval for figure 4 is 20 feet. Rankin Creek originates near the northwest corner of section 14 and flows in a south and south-southeast direction to the south edge of figure 4 (east half). South of figure 4 Rankin Creek flows in a southeast, east-southeast, and east direction to join the north oriented North Platte River. Fieldhouse Cut is a narrow water gap eroded by Rankin Creek across Wind Ridge and is almost 400 feet deep. Riddle Creek originates near the northeast corner of section 15 (just west of the Rankin Creek headwaters) and flows in a southwest and west direction to the west edge of figure 4 (south of center) and west of figure 4 Riddle Creek flows in a west direction to join southwest oriented Stone Creek, which flows to an intermittent lake in the Great Divide Basin and ends. A through valley near the corner of sections 10,11, 14, and 15 links the southwest oriented Riddle Creek valley with the south-southeast oriented Rankin Creek valley (sections 10 and 11 are north of sections 15 and 14). The through valley floor elevation is between 7280 and 7300 feet. Elevations in section 11 just north of figure 4 rise to 7774 feet and Windy Ridge elevations in section 21 rise to 7924 feet. These elevations suggest the Riddle Creek-Rankin Creek through valley is at least 474 feet deep. A northwest to southeast oriented through valley in the southeast corner of section 10 (north of section 15) links the Riddle Creek and Rankin Creek headwaters area with lower elevation Great Divide Basin regions to the northwest of figure 4. This northwest to southeast oriented through valley has a floor elevation of between 7360 and 7380 feet. Elevations west of the through valley rise to more than 7500 feet suggesting this northwest to southeast oriented through valley is at least 120 feet deep. The probable figure 4 drainage history began with a southeast oriented flood flow on the northwest to southeast oriented through valley alignment splitting into two diverging flow channels near the corner of sections 10, 11, 14, and 15, with one diverging flood flow channel moving floodwaters in a south-southeast and southeast direction probably to a south oriented flood flow channel on the present day north oriented North Platte River alignment and the other flood flow channel moving floodwaters in a southwest, west, and southwest direction to a south oriented flood flow channel on the present day north oriented Separation Creek alignment with floodwaters eventually reaching what were then the actively eroding west and south oriented Muddy Creek valley, which had eroded headward from the Little Snake River valley. Both flood flow channels eventually converged in the Colorado River valley after flowing along very different routes.

Seminoe Canyon area

Figure 5: Seminoe Canyon area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 5 illustrates the Seminoe Canyon area east and slightly south of figure 3 and there is an overlap area with figure 3. The map contour interval for figure 5 is 20 meters. Before looking at Separation Creek the Seminoe Canyon water gap across the Seminoe Mountains deserves more attention. Seminoe Reservoir floods the north oriented North Platte River valley from the south edge of figure 5 to Seminoe Dam, which is located in the north center area of figure 5. Before reaching the Seminoe Dam site the North Platte River turns in a northwest direction to be joined by southeast and east-southeast oriented Morgan Creek and then the North Platte River turns in a northeast direction to flow to the north edge of figure 5 (slightly east of center). Seminoe Canyon is more than 400 meters deep and cuts across the Seminoe Mountains and deserves an explanation. First it is necessary to imagine the region being crossed by south oriented flood flow with elevations north of the Seminoe Mountains being at least as high as the Seminoe Mountains. Evidence for the former south oriented flood flow can be found in the south oriented tributaries flowing to the North Platte River on both sides of Seminoe Reservoir. Hurt Creek on the west side has already been discussed. Hurt Gulch and Sips Creek are barbed tributaries located on the east side. South-southwest oriented Hurt Gulch is linked by a through valley with a north-northeast oriented Hurt Gulch (flowing to west-northwest oriented Lost Creek north of figure 5) and the south-southwest oriented Sips Creek valley is linked by a through valley with a north-northeast oriented stream (also flowing to Lost Creek). North of figure 5 Lost Creek flows to the north-northwest oriented North Platte River segment there. The northwest oriented North Platte River valley segment seen in figure 5 was initiated as a southeast oriented flood flow channel flowing on the Morgan Creek alignment, which captured southwest and south-southwest oriented flood flow channels as it eroded headward into the Seminoe Mountains mass. The southeast and east-southeast oriented Morgan Creek valley is linked by a northeast to southwest oriented through valley with the northeast oriented Hamilton Creek valley providing evidence of what began as still another southwest oriented flood flow channel. The southwest oriented flood flow channel on the present day northeast oriented Seminoe Canyon alignment became the primary flood flow channel and eroded a deeper valley than the adjacent diverging and converging flood flow channels and began to behead those diverging flood flow channels north of the emerging Seminoe Mountains. The the much larger-scale reversal of flood flow on the North Platte River alignment then created the north oriented North Platte River drainage route, which captured south oriented drainage routes that had formed on the south ends of the former converging flood flow channels.

Separation Creek-Coal Creek drainage divide area

Figure 6: Separation Creek-Coal 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 Separation Creek-Coal Creek drainage divide area south and west of figure 3 and includes an overlap area with figure 3. The map contour interval for figure 6 is 20 meters. Cheyenne Ridge is located near the northeast corner of figure 6. Boggy Meadows is located near the northwest corner of figure 6. Stone Creek flows in a south-southwest direction from the north edge of figure 6 (west half) to an intermittent lake in the Boggy Meadows area (Boggy Meadows Lake is west of figure 6). Remember west oriented Riddle Creek seen in figures 3 and 4 flowed to southwest oriented Stone Creek north of figure 6. Rendle Hill is a high hogback ridge in the southwest corner of figure 6. East of Rendle Hill is Separation Lake. Separation Creek flows in a north direction on the west side of Rendle Hill (west of figure 6-see figure 7) and then turns to flow in a northeast and east direction across the west edge of figure 6 (south of center) before turning again to flow in a south and southeast direction to Separation Lake where Separation Creek ends. The low relief area between Boggy Meadows and Separation Creek is a major through valley linking the south-southwest oriented Stone Creek drainage route with the north, east, and south oriented Separation Creek drainage route. At one time large volumes of south oriented flood flow diverged in the region north of Rendle Hill with one flood flow channel continuing in a south direction between Rendle Hill and the Haystack Mountains in the Separation Lake valley to the North Platte River alignment (see figure 8) and the other flood flow channel continuing in a south direction on the present day north oriented Separation Creek alignment (west of figure 6 and seen in figure 7). Little Shoe Creek originates on Haystack Mountain and flows in a northeast direction across the southeast corner of figure 6 and east of figure 6 joins northeast oriented Corral Creek, which then joins east oriented Coal Creek to flow to the north oriented North Platte River. These northeast and east oriented streams have eroded canyons across the eastern Haystack Mountains hogback ridges suggesting the streams once had a source of water west of the Haystack Mountains. The Haystack Mountain crest ridge is notched suggesting water came from the west, although all evidence of the drainage routes west of the Haystack Mountains is now lost.

Separation Lake-Dry Lake drainage divide area

Figure 7: Separation Lake-Dry Lake drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 7 illustrates the Separation Lake-Dry Lake drainage divide area south and west of figure 6 and includes an overlap area with figure 6. The map contour interval for figure 7 is 20 meters. Separation Creek flows in a north direction from the south edge of figure 7 (near southwest corner) into the northwest quadrant of figure 7 and then turns in a northeast direction to almost reach the north edge of figure 7. After flowing for a short distance in an east direction near the north edge of figure 7 Separation Creek turns to flow in a south and southeast direction to Separation Lake where Separation Creek ends. Shamrock Draw and Cold Spring Draw are north oriented Separation Creek tributaries east of the north oriented Separation Creek segment. Rendle Point is the high point on Rendle Hill west of Separation Lake and is at the north end of an east-facing escarpment bounding the north-northwest to south-southeast oriented Separation Lake-Dry Lake through valley that crosses the east edge of figure 7. The escarpment is located along a hogback ridge, as is the north to south oriented ridge west of the highway and east of Cold Spring Draw. Likewise the Shamrock Hills between Separation Creek and Shamrock Draw appear to be located along a hogback ridge. While many of the valley orientations follow underlying geologic structure orientations the valleys are water-eroded valleys and were eroded by south oriented floodwaters, which originally flowed on an erosion surface as high, if not higher, than Rendle Point. Rendle Point today has an elevation of between 2320 and 2340 meters. Elevations in the Separation Lake basin are less than 1960 meters and elevations in the Separation Creek valley near the southwest corner of figure 7 are between 2000 and 2020 meters. Floodwaters flowing in a south direction from the Boggy Meadows area north of figure 7 split into two diverging flood flow channels north of Rendle Hill. One of the diverging flood flow channels moved floodwaters in a south-southeast in the present day Separation Lake-Dry Lake through valley to a south oriented flood flow channel on the present day north oriented North Platte River alignment (see figure 8). The other flood flow channel moved floodwaters in a southwest and south direction on the present day north oriented Separation Creek alignment. Diverging south oriented flood flow channels from this southwest and south oriented flood flow channel also followed the present day north oriented Shamrock Draw and Cold Spring Draw alignments and south of figure 7 at least some of those floodwaters turned in a west direction to converge with the south oriented flood flow channel on the present day north oriented Separation Creek alignment. The present day north oriented Separation Creek and Shamrock Draw and Cold Spring Draw drainage routes were created by a reversal of flood flow that may have been triggered by headward erosion of a deeper south-southeast oriented flood flow channel on the present day Separation Creek-Dry Lake through valley alignment. Crustal warping that was raising the region and probably the regional geologic structures probably also contributed to the flood flow reversal.

Dry Lake-North Platte River drainage divide area

Figure 8: Dry Lake-North Platte River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 8 provides a topographic map of the Dry Lake-North Platte River drainage divide south and east of figure 7 and includes an overlap area with figure 7. The map contour interval for figure 8 is 20 meters. Rawlins, Wyoming is located south of the southwest corner of figure 8. Dry Lake is located near the northwest corner of figure 8 and the Separation Lake-Dry Lake through valley seen in figure 7 extends from Dry Lake in a south-southeast and southeast direction to the south edge of figure 8. The North Platte River flows in a north-northwest direction from the south edge of figure 8 (east half) along the through valley northeast wall and then flows across the Haystack Mountains (or Fort Steele Breaks) structure before turning in an east direction to the east edge of figure 8 (south of center). Elevations in the through valley from the Separation Lake Basin all the way to the North Platte River valley are less than 2000 meters. Rendle Hill seen in figure 7 rises to more than 2320 meters and elevations at Rawlins Peak north and west Rawlins (not seen in figure 8, but south of the southwest corner of figure 8) exceed 2360 meters. Elevations on the Haystack Mountains also exceed 2360 meters. These elevations suggest the through valley is 360 meters deep. The through valley orientation is controlled by underlying geologic structure orientation, however the through valley was also eroded by south-southeast and southeast oriented flood flow moving to a south oriented flood flow channel on the present day north-northwest and northwest oriented North Platte River alignment south of figure 8. The North Platte River water gap crossing the Haystack Mountains south end or the Fort Steele Breaks northwest end is more than 300 meters deep and was probably initially eroded by south oriented flood flow from east of the Haystack Mountains. The south oriented flood flow from east of the Haystack Mountains converged with the south-southeast oriented flood flow channel from west of the Haystack Mountains in the region south of the water gap. Headward erosion of a somewhat deeper east oriented valley north of the water gap beheaded the south oriented flood flow channel. Floodwaters on the north end of the beheaded flood flow channel reversed flow direction to create the north and east oriented North Platte River segments seen in figure 8. Ice sheet related crustal warping that was raising the region and areas to the south of figure 8 contributed to the flood flow reversal, which after floodwaters had drained from the region ended flood flow in the through valley west of the Haystack Mountains.

Dry Ditch-North Platte River drainage divide area

Figure 9: Dry Ditch-North Platte River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 9 illustrates the Dry Ditch-North Platte River drainage divide area east and north of figure 8 and there is an overlap area with figure 8. The map contour interval for figure 9 is 20 meters. The North Platte River flows in a north direction through the water gap across the northwest end of the Fort Steele Breaks in the southwest corner of figure 9 and then turns to flow in an east, east-northeast, and north direction to the north edge of figure 9 (east half). Seminoe Reservoir has flooded much of the North Platte River valley seen in figure 9. Reddish brown areas along the east edge of figure 9 are strip mines. The Haystack Mountains are located along the west edge of figure 9. Dirtyman Draw is an east and south oriented North Platte River tributary originating in the Haystack Mountains (north of center) and joining the North Platte River in the southwest quadrant of figure 9. The highway makes use of a north to south oriented through valley linking the south oriented Dirtyman Draw valley with northeast oriented North Platte River tributaries flowing to the north edge of figure 9.  The through valley floor elevation is between 2040 and 2060 meters. East of the south oriented Dirtyman Draw valley elevations rise to more than 2080 meters suggesting the through valley is at least 20 meters deep. While not deep the through valley provides evidence of what was once a south oriented flood flow channel. A slightly deeper through valley is found east of the south oriented Dirtyman Draw valley. Dry Ditch is an east-northeast oriented tributary joining the north oriented North Platte River near the north edge of figure 9. South of the Dry Ditch headwaters is a north to south oriented through valley linking the Dry Ditch valley with the North Platte River valley to the south. The through valley floor elevation is between 2000 and 2020 meters. Elevations on both sides of the through valley rise to more than 2060 meters suggesting the through valley is more than 40 meters deep. The through valley continues in a north direction north of the Dry Ditch valley and north of figure 9 is linked to a northeast oriented North Platte River tributary valley. These through valleys provide evidence of diverging and converging south oriented flood flow channels east of the Haystack Mountains prior to the reversal of flood flow that created the north oriented North Platte River drainage route. The reversal of flood flow was probably responsible for headward erosion of the deep east-northeast oriented North Platte River valley segment that resulted in the reversal of flood flow south of the water gap seen in figure 8.

Detailed map of Dry Ditch-North Platte River drainage divide area

Figure 10: Detailed map of Dry Ditch-North Platte River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 10 provides a detailed topographic map of the Dry Ditch-North Platte River drainage divide area seen in less detail in figure 9. The map contour interval for figure 10 is 20 feet. Seminoe Reservoir floods the east-northeast and north oriented North Platte River valley in the east half of figure 10. Dry Ditch is the east-northeast oriented drainage route crossing the northwest quadrant of figure 10. A north to south oriented through valley is located in sections 18 and 19 and links the east-northeast Dry Ditch valley in section 7 with the east-northeast oriented North Platte River valley south of figure 10. The through valley floor elevation is between 6560 and 6580 feet. Elevations in the northeast corner of section 20 to the east rise to 6800 feet and elevations in section 23 to the west rise to more than 6800 feet. These elevations suggest the north to south oriented through valley is at least 220 feet deep. The through valley is a water-eroded valley and was eroded by south oriented flood flow moving to the North Platte River valley south of figure 10. At that time the North Platte River valley was just one of many diverging and converging south oriented flood flow channels crossing the region. The present day North Platte River alignment probably developed in segments on flood flow channels that were able to erode deeper valleys than the adjacent diverging and converging flood flow channels. The deeper flood flow channels beheaded the shallower diverging flood flow channels and floodwaters on north ends of the beheaded flood flow channels reversed flow direction to flow to the deeper flood flow channel. Headward erosion of the much deeper east and southeast oriented North Platte River valley around the northwest end of the Laramie Mountains (north of figure 10 and of this essay’s study area) eventually beheaded and reversed south oriented flood flow on the North Platte River alignment. Flood flow was beheaded and reversed in multiple steps and flood flow may have been beheaded and reversed in the figures 9 and 10 regions before flood flow was reversed in the figure 8 region (although based on evidence seen and discussed in this essay arguments could also be made for quite different sequences).

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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