Bighorn River-Rotten Grass Creek drainage divide area landform origins in Big Horn County, Montana, USA

· Bighorn Mountains, Bighorn River, Montana
Authors


Abstract:

This essay uses topographic map evidence to interpret landform origins in the region between the Bighorn River and Rotten Grass Creek in Big Horn County, Montana. North of the Wyoming state line the Bighorn River flows in a deep northeast oriented canyon across the north end of the Bighorn Mountains and then flows in a north-northeast direction to join the northeast oriented Yellowstone River. Rotten Grass Creek is a northeast and north-northwest oriented Bighorn River tributary originating in the high Bighorn Mountains near the state line. Primary north and north-northwest oriented Bighorn River tributaries, including Soap Creek, Black Canyon Creek, and Bull Elk Creek, drain the Bighorn River-Rotten Grass Creek drainage divide area today. Rotten Grass Creek, Soap Creek, and Black Canyon Creek have northeast and east oriented headwaters in the Bighorn Mountains and near headwaters of north-northwest oriented Bull Elk Creek. North of the Bighorn Mountains shallow through valleys cross drainage divides between north oriented drainage routes and provide evidence of flood flow channels that once crossed the region. Fewer, but deeper through valleys cross drainage divides in the Bighorn Mountains and link the north oriented Bighorn 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 Bighorn Mountains and other regional mountain ranges at the same time as immense south and southeast oriented melt water floods were flowing across them. As the mountain ranges were being uplifted 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. 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. With this captured flood water the reversed flood flow channels eroded deep north oriented valleys into the rising Bighorn Mountains mass.

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-Rotten Grass Creek drainage divide area landform origins in Big Horn County, Montana, USA. Map interpretation methods can be used to unravel many geomorphic events leading up to formation of present-day drainage routes and development of other landform features. While each detailed topographic map feature provides detailed evidence to be explained, the solution must be consistent with explanations for adjacent area map evidence as well as solutions to big picture map evidence puzzles. I invite readers to improve upon my solutions 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-Rotten Grass Creek drainage divide area landform evidence in Big Horn County, Montana will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm.

Bighorn River-Rotten Grass Creek drainage divide area location map

Figure 1: Bighorn River-Rotten Grass Creek 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 –Rotten Grass Creek drainage divide area in Big Horn County, Montana. 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 across figure 1 with Montana in the north. The Yellowstone River flows in a northwest direction across the southwest corner of figure 1 and once in Montana (west of figure 1) turns to flow in a northeast direction to 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 Miles City (near northeast corner of figure 1). The Bighorn River flows in a north direction from Worland, Wyoming (near south center edge of figure 1) to the Montana border and then turns to flow in a north-northeast direction to join the Yellowstone River near Custer, Montana. The north-northwest to south-southeast oriented Bighorn Mountain range is located east of the Bighorn River in Wyoming, with the north end of the Bighorn Mountains being crossed by the north-northeast oriented Bighorn River as it flows through Bighorn Canyon just north of the Montana state line. The Bighorn Canyon National Recreation Area is where the Bighorn River crosses the Bighorn Mountain north end. Rotten Grass Creek is shown, but not labeled in figure 1 and is the northeast and north-northwest oriented tributary originating in the Bighorn Mountains near the state line and joining the Bighorn River near St. Xavier, Montana. The Bighorn River-Rotten Grass Creek drainage divide area illustrated and discussed in this essay is north of the state line, south and west of the Bighorn River, and north and east of Rotten Grass Creek and includes areas in the high Bighorn Mountains as well as regions on the plains north of the Bighorn Mountains.

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 Bighorn Mountains at approximately the same time as the deep Yellowstone River valley eroded headward from a 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 the deep “hole” 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 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 Bighorn Mountains as the immense melt water floods deeply eroded regions surrounding the rising mountain masses. Floodwaters carved deep and anastomosing south oriented valleys or flood flow channels between 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 to erode the north oriented valleys seen today. South oriented flood flow channels 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 create significant north oriented drainage systems, such as the north oriented Bighorn River drainage system.

In the case of the figure 1 headward erosion of the deep northeast oriented Yellowstone River valley beheaded and reversed south oriented flood flow channels that eroded north oriented Bighorn River and tributary valley segments, many of which captured south and southeast oriented flood flow from further to the west, which provided water volumes to required to erode the north oriented valleys. The Bighorn Mountains and other regional mountain ranges emerged as floodwaters deeply eroded regions surrounding them and as delayed ice sheet related crustal warping raised the mountain masses. Deep flood water erosion of the rising mountain masses and deposition of the eroded debris in adjacent basins may have contributed to the mountain range uplift and adjacent basin formation.

Detailed location map for Bighorn River-Rotten Grass Creek drainage divide area

Figure 2: Detailed location map Bighorn River-Rotten Grass Creek 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-Rotten Grass Creek drainage divide area in Big Horn County, Montana. The Montana-Wyoming state line extends in a west to east direction across the south half of figure 2. Green shaded areas are National Forest lands and are generally located in mountainous regions. The green shaded area straddling the south center edge of figure 2 is in the Bighorn Mountains. The brown shaded area in Montana is Crow Indian Reservation land. The brighter brown area around Bighorn Lake in Wyoming and extending north and northeast into Crow Indian Reservation lands 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 (west half) to Bighorn Lake and then in a north-northeast direction to the north center edge of figure 2.  Note how the Bighorn Mountains extend north of the state line to the Bighorn Canyon National Recreation Area in the Crow Indian Reservation. Also note how northwest oriented streams in the Montana Bighorn Mountains drain to the north-northeast oriented Bighorn River in southern Montana. The labeled northwest oriented Montana Bighorn River tributaries from south to north are Devils Canyon, Big Bull Elk Canyon, and Black Canyon Creek. East of Black Canyon Creek is northeast and north oriented Soap Creek and east of Soap Creek is northeast and north-northwest oriented Rotten Grass Creek. Rotten Grass Creek originates in the Bighorn Mountains near the state line and also near the Black Canyon Creek headwaters, which are near the Big Bull Elk Canyon headwaters. South and east of the Rotten Grass Creek headwaters is northeast oriented Lodge Grass Creek, which also originates in the Bighorn Mountains and which joins the north and north-northwest oriented Little Bighorn River near the town of Lodge Grass. The northwest, north-northwest, and north oriented Bighorn River tributary segments are located on the alignments of former south, south-southeast, and southeast oriented flood flow channels. At the time the flood flow channels were eroded the Bighorn Mountains did not stand high above surrounding regions as they do today and floodwaters were free to flow across what was a rising mountain mass. The flood flow direction was reversed as the much deeper north-northeast oriented Bighorn River valley eroded headward across the north end of the rising mountain mass. The flood flow channels were beheaded and reversed in sequence from east to west, which meant newly beheaded and reversed flood flow channels could capture floodwaters from flood flow further to the west. While today the regional topography is such that the flood flow routes and captures would be impossible, at that time elevations of the regions north and west of the Bighorn Mountain area were such that floodwaters could freely move to and across what is today the Bighorn Mountains upland surface. Evidence of flood flow captures on that Bighorn Mountain upland surface is illustrated in this essay. The present day topography emerged as the floodwaters deeply eroded regions surrounding the Bighorn Mountain at the same time as continental ice sheet related crustal warping was raising the Bighorn Mountain mass. Deep erosion of the rising Bighorn Mountains upland surface and deposition of the eroded debris in adjacent basins probably contributed to the crustal warping.

North end Bighorn River-Rotten Grass Creek drainage divide area

Figure 3: North end Bighorn River-Rotten Grass 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 north end of the Bighorn River-Rotten Grass Creek drainage divide area and is located north of the Bighorn Mountains.  The Bighorn River flows in a north-northeast direction from the west edge of figure 3 (south half) to the north edge of figure 3 (west half). Rotten Grass Creek is the north-northwest oriented stream flowing from the south edge of figure 3 (east half) to the north center edge of figure 3 and which joins the Bighorn River a short distance north of figure 3. Soap Creek is the longer north oriented Bighorn River tributary in the southwest quadrant of figure 3 while Mountain Pocket Creek is the north oriented stream west of Soap Creek. Note how Bighorn River tributaries from the northwest are oriented in southeast directions and join the north-northeast oriented Bighorn River as barbed tributaries. Also note the north and northwest oriented tributaries to not only the Bighorn River, but also to Rotten Grass Creek and Soap Creek. Rotten Grass Creek does have a few barbed tributaries from the east, with Ox Yoke Coulee near the north edge of figure 3 being the most obvious. However, Rotten Grass Creek tributaries from the west are not barbed and are oriented in northeast directions. Follow the Bighorn River-Rotten Grass Creek and Soap Creek-Rotten Grass Creek drainage divides and note how the northwest-oriented Soap Creek and Bighorn River tributary valleys are linked by shallow through valleys with the northeast oriented Rotten Grass tributary valleys. The map contour interval for figure 3 is 20 meters and the through valleys are generally defined by two contour lines on a side. The through valleys are evidence of flood flow channels that once crossed the Bighorn River (and Soap Creek)-Rotten Grass Creek drainage divide. At the time the flood flow channels were eroded the deep Bighorn River valley did not exist and floodwaters were flowing to what was then a newly beheaded and reversed flood flow channel that was eroding the deep north-northwest oriented Rotten Grass Creek valley headward. Floodwaters were flowing in a southeast direction to approximately the location of the present day Bighorn River-Rotten Grass Creek drainage divide and then were turning to flow in a northeast direction to the actively eroding north-northwest oriented Rotten Grass Creek valley. Headward erosion of the deep north-northeast oriented Bighorn River valley next beheaded the southeast oriented flood flow channels in sequence (from the northeast to the southwest) and floodwaters on the northwest ends of the beheaded flood flow channels reversed flow direction to erode northwest oriented Bighorn River tributary valleys.

Detailed map of Bighorn River-Dipping Vat Coulee drainage divide area

Figure 4: Detailed map of Bighorn River-Dipping Vat Coulee 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 Bighorn River-Dipping Vat Coulee drainage divide area seen in less detail in figure 3. The Bighorn River-Rotten Grass Creek drainage divide extends in a north to south direction across the center of figure 4. Northeast oriented streams, including labeled West Dipping Vat Coulee, east of the drainage divide flow to north-northwest Rotten Grass Creek while northwest oriented streams west of the drainage divide flow to the northeast oriented Bighorn River. Note how northeast oriented Rotten Grass Creek tributary valleys are linked by through valleys with the northwest oriented Bighorn River tributary valleys. For example, near the center of figure 4 in section 25 the northeast oriented West Dipping Vat Coulee valley is linked by a through valley with a northwest oriented Bighorn River tributary valley. The map contour interval for figure 4 is 20 feet and the through valley floor elevation at the drainage divide is between 3560 and 3580 feet. The hill in section 25 immediately to the north is shown as rising to 3737 feet and the hill in the northeast quadrant of section 36 to the south rises to 3826 feet. These elevations suggest the through valley is at least 150 feet deep. Similar through valleys can be seen elsewhere along the drainage divide. The through valleys provide evidence that multiples flood flow channels once crossed what is now the Bighorn River-Rotten Grass Creek drainage divide. At the time floodwaters flowed across the present day drainage divide there was no Bighorn River valley to the west and consequently there was no drainage divide. Floodwaters were flowing in a southeast direction from the northwest and were being captured by the actively eroding and much deeper north-northwest oriented Rotten Grass Creek valley. The capture of the floodwaters resulted in headward erosion of the deep northeast oriented Rotten Grass tributary valleys. Headward erosion of the deeper northeast oriented Bighorn River valley next beheaded the southeast oriented flood flow channels in sequence from the northeast to the southwest. Floodwaters on northwest ends of the beheaded flood flow channels reversed flow direction to erode the northwest oriented Bighorn River tributary valleys. Because the flood flow channels were interconnected reversed flood flow on a newly beheaded flood flow channel could capture flood flow from yet to be beheaded flood flow channels further to the southwest. These captures provided the water volumes that eroded the deep northwest oriented Bighorn River tributary valleys.

Dry Soap Creek-Rotten Grass Creek drainage divide area

Figure 5: Dry Soap Creek-Rotten Grass Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 5 illustrates the Dry Soap Creek-Rotten Grass Creek drainage divide area just south of figure 3, but does not includes an overlap area with figure 3. East-northeast dipping hogbacks along the Bighorn Mountains east-northeast flank can be seen in the southwest quadrant of figure 5. Soap Creek flows in a northeast direction in Soap Creek Canyon, which has been eroded into these dipping strata and then gradually turns to flow in a northwest and north direction to the north edge of figure 5 (west of center) and joins the Bighorn River north of figure 5. Dry Soap Creek is a north and north-northwest oriented Soap Creek tributary originating in the south center region of figure 5. War Man Creek is the north oriented Bighorn River tributary near the northwest corner of figure 5. Rotten Grass Creek flows in a northeast direction across the southeast quadrant of figure 5 and then turns to flow in a north direction near the east margin of the northeast quadrant of figure 5.  North oriented Lodge Grass Creek can just barely be seen in the southeast corner of figure 5. Note how in the south center region of figure 5 the north oriented Soap Creek drainage basin is linked by through valleys with the northeast oriented Rotten Grass Creek drainage basin. For example, in section 6 (just north of the south center edge of figure 6) a deep through valley links the north oriented Dry Soap Creek headwaters valley with the east oriented Rotten Grass Creek valley. The map contour interval for figure 5 is 20 meters and the through valley floor elevation at the drainage divide is between 1300 and 1320 meters. The hill immediately to the east rises to at least 1380 meters suggesting the through valley is at least 60 meters deep. The through valley may have been eroded by south oriented flood flow moving to the newly eroded Rotten Grass Creek valley prior to headward erosion of the deep Bighorn River valley north of figure 5. Headward erosion of the deep Bighorn River valley beheaded and reversed the south oriented flood flow to create the north oriented Soap Creek drainage system including the north and north-northwest oriented Dry Soap Creek valley. Another, although less likely, possibility is the through valley was eroded as one of several diverging and converging north oriented flood flow routes, when large volumes of floodwaters from south and west of figure 5 (which is in the high Bighorn Mountains today) flowed into the region toward the actively eroding Bighorn River valley and its actively eroding north oriented tributary valleys.

Detailed map of Dry Soap Creek-Rotten Grass Creek drainage divide area

Figure 6: Detailed map of Dry Soap Creek-Rotten Grass Creek 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 Dry Soap Creek-Rotten Grass Creek drainage divide area seen in less detail in figure 5. Rotten Grass Creek flows in a northeast direction in section 12 from the south edge of figure 6 and then turns in section 5 to flow in an east-southeast direction before turning to flow in a northeast direction to the east center edge of figure 6. Dry Soap Creek originates in the north half of section 6 (just north of Rotten Grass Creek) and flows in a north direction to the north center edge of figure 6. Note in section 6 the through valley linking the north oriented Dry Soap Creek valley with the east oriented Rotten Grass Creek valley. The map contour interval for figure 6 is 20 feet and the through valley floor elevation at the drainage divide is between 4300 and 4320 feet. The ridge to the east in the northwest quadrant of section 5 rises to 4503 feet while the hogback ridge crossing the section 6 and section 1 boundary to the west rises to 4628 feet. These elevations suggest the through valley is at least 180 feet deep. The through valley is a water eroded feature and as mentioned in the figure 5 discussion was probably eroded by south oriented flood flow moving on the yet to be beheaded and reversed Dry Soap Creek alignment to the reversed flood flow on the Rotten Grass Creek alignment, which is east of the Soap Creek alignment and which would have been beheaded and reversed before flood flow on the Soap Creek alignment would have been beheaded and reversed. While other interpretations are possible, this interpretation is consistent with headward erosion of the deep northeast oriented Bighorn River valley across multiple south and southeast oriented flood flow channels. What makes the interpretation somewhat complicated is the presence of the Bighorn Mountains to the south and west of figure 5. While based on present day topography southeast oriented flood flow might be possible along the Bighorn Mountains northeastern flank, assuming some modest crustal warping, evidence illustrated in the remaining figures below suggest floodwaters crossed what are today much higher Bighorn Mountains regions. If so, significant crustal warping took place as floodwaters flowed across region and at least initially the Bighorn Mountains did not present a barrier to the south and southeast oriented flood flow.

Black Canyon-Rotten Grass Creek drainage divide area

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

Figure 7 illustrates the Black Canyon-Rotten Grass Creek drainage divide area south and west of figure 5 and includes an overlap area with figure 5. Black Canyon Creek is the north-northwest oriented stream flowing in Black Canyon from the south of figure 7 (just west of center) to the north edge of figure 7 (west half). The map contour interval for figure 7 is 20 meters and Black Canyon is places is more than 600 meters deep. Rotten Grass Creek flows in Rotten Grass Canyon from the figure 7 south edge (just east of center) in a northeast and east direction to the east center edge of figure 7. Note how the North Fork of Rotten Grass Creek originates near the Black Canyon rim in the south center area of figure 7) and flows in an east-northeast direction to join Rotten Grass Creek.  There is a deep through valley linking a west oriented Black Canyon Creek tributary valley with the east and east-northeast oriented North Fork Rotten Grass Creek valley. The through valley floor elevation at the road intersection is given as 2014 meters. Point Lookout to the north rises to more than 2200 meters while elevations to the south rise even higher. In other words, the through valley is approximately 200 meters deep and could have been deeper when eroded. While today it is difficult to imagine what eroded the through valley, the through valley is a water eroded feature and was eroded by south and southeast oriented flood flow moving to the Rotten Grass Creek valley at a time when the deep Black Canyon valley did not exist. At that time flood flow was moving in a south-southeast direction on what is today the north-northwest oriented Black Canyon alignment. At that time the deep northeast oriented Bighorn River Canyon to the northwest did not exist, the Bighorn Mountains had not yet emerged as a barrier to south and southeast oriented flood flow, and floodwaters were freely moving from the northwest into the region shown by figure 7. Bighorn Mountains emergence as a major mountain range was probably accomplished by a combination of crustal warping as floodwaters crossed the region and by deep flood water erosion of the surrounding region. Headward erosion of the deep northeast oriented Bighorn River valley north and west of figure 7 beheaded and reversed flood flow on the Black Canyon Creek alignment. Floodwaters from west and south of figure 7 were captured by reversed flood flow on the Black Canyon Creek alignment and provided the water volumes required to erode the deep north-northwest oriented Black Canyon valley.

Detailed map of Black Canyon-Rotten Grass Creek drainage divide area

Figure 8: Detailed map of Black Canyon-Rotten Grass 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 Black Canyon-Rotten Grass Creek drainage divide seen in less detail in figure 7 above. Black Canyon Creek flows in a north-northwest direction from the south edge of figure 8 (just west of center) to the north edge of figure 8 (east of northwest corner). An unnamed west oriented tributary joins Black Canyon Creek south of Point Lookout (in northwest quadrant of figure 8). Rotten Grass Creek flows in a north-northeast direction from the south edge of figure 8 (east half) to the east edge of figure 8 (north half). The North Fork Rotten Grass Creek flows in a north and east-northeast direction from the center region of figure 8 to join Rotten Grass Creek near the east edge of figure 8. The map contour interval for figure 8 is 40 feet and Black Canyon near Point Lookout is more than 2600 feet deep with Point Lookout having an elevation of 7245 feet. Note the through valley linking the west oriented Black Canyon tributary valley with the east-northeast oriented North Fork Rotten Grass Creek valley. The through valley floor elevation at the drainage divide is between 6560 and 6600 feet, which means the through valley is at least 600 feet deep. While a 600-foot depth pales when compared with the much deeper Black Canyon depth, the through valley is a water-eroded feature and was eroded prior to erosion of the deep Black Canyon valley. Water that eroded the through valley was coming from the northwest and was flowing to what was at that time the actively eroding Rotten Grass Creek valley and was deeply eroding the Bighorn Mountains flank, which is seen in the northeast quadrant of figure 8. A major flood flow reversal, probably triggered by headward erosion of the deep northeast oriented Bighorn River valley north and west of figure 8 and probably aided by Bighorn Mountains uplift, which was occurring as floodwaters flowed across the region, beheaded the flood flow to the actively eroding Rotten Grass Creek drainage basin and began to erode the deep north-northwest oriented Black Canyon Creek valley. Significant flood flow from west of the actively eroding Bighorn River valley head continued to flow onto what is now the Bighorn Mountains upland surface and provided water volumes required to erode the deep north-northwest oriented Black Canyon valley.

Bull Elk Canyon-Black Canyon drainage divide area

Figure 9: Bull Elk Canyon-Black Canyon drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 9 illustrates the Bull Elk Canyon-Black Canyon drainage divide area south and west of figure 7 and includes an overlap area with figure 7. The Montana-Wyoming state line is the west to east oriented dashed line near the south edge of figure 9. Bull Elk Ridge is labeled and extends in a south-southeast direction from the northwest corner of figure 9. Just east of Bull Elk Ridge is north-northwest oriented Bull Elk Canyon and east of Bull Elk Canyon is the Bighorn Mountains crest ridge. Black Canyon Creek headwaters are located east of the Bighorn Mountains crest ridge in the south center region of figure 9 with northeast oriented headwaters between Long Ridge and Telephone Ridge and between Telephone Ridge and Buffalo Horn Ridge. East of the Black Canyon Creek headwaters, in the southeast corner of figure 9, are east oriented headwaters of Lodge Grass Creek. Further north are northeast oriented headwaters of Rotten Grass Creek, with Rotten Grass Creek flowing to the east edge of figure 9 (near northeast corner). Through valleys eroded across the Bighorn Mountains crest ridge between Bull Elk Canyon and Black Canyon provide evidence of channels south and southeast oriented floodwaters used to reach the newly beheaded and reversed flood flow on the Black Canyon Creek alignment, which in time eroded the deep Black Canyon valley. The two most obvious through valleys link headwaters of north-northwest Bull Elk Canyon with headwaters of the northeast oriented Black Canyon tributaries located between Long Ridge and Telephone Ridge. The map contour interval for figure 9 is 20 meters and floors of the through valleys at the drainage divide have elevations of between 2620 and 2640 meters. Elevations on the Bighorn Mountains crest ridge to the north rise to more than 2740 meters and to the south elevations rise even higher. These elevations suggest the two through valleys are at least 120 meters deep. Today these two through valleys appear inconsequential when compared with the much deeper canyons on either side, but the through valleys are water eroded features and were eroded by water crossing the present day drainage divide. At the time the through valleys were eroded a south-southeast oriented flood flow channel on the Black Canyon alignment had been beheaded by headward erosion of the much deeper northeast oriented Bighorn River valley, but the Bighorn River valley had not yet beheaded the south-southeast oriented flood flow channel on the Bull Elk Canyon alignment. As a result floodwaters were flowing in a north-northwest direction on the Black Canyon alignment to the much deeper Bighorn River valley and were capturing floodwaters from the south-southeast oriented flood flow channel on the Bull Elk Canyon alignment. Prior to the reversal of Black Canyon flood flow and headward erosion of the deep Black Canyon valley floodwaters on the Bull Elk Canyon alignment had been captured by headward erosion of the deep Lodge Grass Creek and Rotten Grass Creek valleys, which are seen east of Black Canyon. While difficult to visualize with present day topography, immense south and southeast oriented floods once flowed across the Bighorn Mountains crest ridge in figure 9.

Detailed map of Bull Elk Canyon-Black Canyon drainage divide area

Figure 10: Detailed map of Bull Elk Canyon-Black Canyon 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 Bull Elk Canyon-Black Canyon drainage divide area seen in less detail in figure 9. Bull Elk Ridge extends in a northwest to southeast direction across the southwest quadrant of figure 10. Bull Elk Creek flows in a northwest direction in Bull Elk Canyon to the west of figure 10 (north half and north and east of Bull Elk Ridge). Black Canyon is the north oriented canyon located near the east edge of the northeast quadrant of figure 10. A northeast oriented Black Canyon tributary is located just north and west of Telephone Ridge and originates near the south center edge of figure 10 and is joined by an east oriented tributary, which is located south of Long Ridge. Note the two through valleys linking that east oriented tributary valley with the northwest oriented Bull Elk Canyon valley. The map contour interval for figure 10 is 40 feet and the through valley floor elevations at the drainage divides are remarkably similar and are both between 8600 and 8640 feet. The elevation at the Benchmark labeled “Spruce” to the north is 8973 feet and the elevation at Windy Point Lookout (near south edge of figure 10) is 9132 feet. In other words both through valleys are at least 330 feet deep. Other similar, but less obvious through valleys can be seen crossing other drainage divides in figure 10. These through valleys are water-eroded features and were eroded at a time when floodwaters flowed freely across what is now the high Bighorn Mountains upland surface. At that time the Bighorn Mountains were just beginning to emerge as a high mountain range. Emergence of the high Bighorn Mountain range occurred as ice sheet crustal warping raised the mountain mass while immense south and southeast oriented melt water floods deeply eroded the surrounding region. In addition, the south and southeast oriented floodwaters deeply eroded the rising mountain core while at same time the floodwaters were depositing debris in the adjacent valleys and basins, which contributed to the crustal warping. Further complicating the situation the deep northeast oriented Yellowstone River and Bighorn River valleys were eroding headward from the deep “hole” the melting ice sheet had occupied and were beheading and reversing the south and southeast oriented flood flow channels.

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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