Abstract:

This essay uses topographic map evidence to interpret landform origins in the region between Pryor Creek and the Bighorn River in Yellowstone and Big Horn Counties, Montana. Pryor Creek is a northwest, north, and north-northeast oriented Yellowstone River tributary originating in the Pryor Mountains, which are located south of Billings, Montana. East of Pryor Creek is the north and north-northeast oriented Bighorn River, which originates as the southeast and north oriented Wind River in Wyoming before flowing across the Wyoming’s Bighorn Basin to join the northeast oriented Yellowstone River north and east of the Pryor Mountains. While the Pryor Creek-Bighorn River drainage divide area is dominated today by north oriented drainage routes, those drainage routes are located in valleys which were initiated as anastomosing southeast oriented flood flow channels prior to headward erosion of the deep northeast oriented Yellowstone River valley, which beheaded the southeast oriented flood flow channels in sequence from east to west. Today a maze of through valleys links the north oriented drainage routes and provides evidence of the former southeast 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 routes from space in the deep “hole” the rapidly melting ice sheet had formerly occupied. Floodwaters on northwest ends of the beheaded flood flow channels reversed flow direction and captured yet to be beheaded southeast oriented flood flow from flood flow channels further to the west.  The flood flow reversals, which eroded the north oriented Pryor Creek and Bighorn River valleys, were aided by uplift of regional mountain ranges and other areas in Wyoming as floodwaters flowed across the region.

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 Pryor Creek-Bighorn River drainage divide area landform origins in Yellowstone and Big Horn Counties, 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 Pryor Creek-Big Horn River drainage divide area landform evidence in Yellowstone and Big Horn Counties, Montana will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm.

Pryor Creek-Bighorn River drainage divide area location map

Figure 1: Pryor Creek-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 Pryor Creek-Bighorn River drainage divide area in Yellowstone and Big Horn Counties, 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 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, Montana before turning to flow in an east-southeast direction to Columbus and then turning to flow in a northeast direction to Billings, Custer, Forsyth, and the figure 1 east edge (north half). Pryor Creek is located south of Billings and originates in the Pryor Mountains and flows in a north-northeast direction through Pryor, Montana to join the Yellowstone River near Huntley. The Bighorn River flows in a north direction from Greybull, Wyoming (near south center edge of figure 1) 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, Montana. The Pryor Creek-Bighorn River drainage divide area illustrated and discussed in this essay is located south of Interstate highway 90, west of the Bighorn River, and east of Pryor Creek.

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 Pryor Mountains and the Bighorn Mountains (in southeast corner of figure 1) 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 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 Pryor Mountains and Bighorn Mountains as the immense melt water floods flowed across the region. As the Pryor and Bighorn Mountains were being uplifted south and southeast oriented floodwaters flowing across the region carved deep valleys or flood flow channels into and between the rising mountain masses. Headward erosion of the much deep northeast oriented Yellowstone River valley from space in the deep “hole” being opened up by ice sheet melting beheaded and reversed the south oriented flood flow channels to erode north oriented valleys seen today. South and southeast oriented flood flow channels in which floodwaters were flowing across the figure 1 map area to 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 flood flow 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 and southeast oriented flood flow that eroded the north oriented Bighorn River valley, which then 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 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 so as to erode the north-northeast oriented Pryor Creek valley. Headward erosion of the actively eroding Pryor Creek valley then captured southeast oriented flood flow moving to the newly eroded north-northeast oriented Bighorn River valley in Montana, although south and southeast oriented floodwaters moving west of the newly reversed and now north oriented Bighorn River valley continued to flow to the actively eroding Shoshone River valley and tributary valleys and to other Bighorn River tributary valleys in Wyoming. The Pryor, Bighorn, and other mountain ranges emerged as floodwaters deeply eroded regions surrounding the rising 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 Pryor Creek-Bighorn River drainage divide area

Figure 2: Detailed location map Pryor Creek-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 Pryor Creek-Bighorn River drainage divide area in Yellowstone and Big Horn Counties, Montana. County boundaries are shown and Yellowstone and Big Horn Counties are labeled. The Yellowstone River flows in a northeast direction from the west center edge of figure 2 to Laurel, Billings, Huntley, and Pompeys Pillar before reaching the north edge of figure 2 (east half). Note several north and northwest oriented tributaries to the northeast oriented Yellowstone River downstream from Billings. The northwest oriented tributaries join the Yellowstone River as barbed tributaries. The brown shaded is the Crow Indian Reservation with the Bighorn Canyon National Recreation Area being the brighter brown shaded area adjacent to and within the Crow Indian Reservation. The Bighorn Canyon Recreation Area is located along the north-northeast and northeast oriented Bighorn River, which flows in a northeast direction across the south center region of figure 2 and then in a north-northeast direction to Harding and the north edge of figure 2 and which then joins the Yellowstone River north of figure 2. Two Bighorn River tributaries of importance in this essay are Big Woody Creek, which flows in a north-northeast, northeast and east-southeast direction across the Bad Lands in the Yellowstone County corner southeast of Billings, and northeast and east oriented Beauvais Creek, which originates west of the Bighorn Canyon National Recreation Area and which joins the Bighorn River near St. Xavier. Pryor Creek originates in the West Pryor Mountain area (southwest quadrant of figure 2) and flows in a northwest, north, and north-northeast direction to join the Yellowstone River near Huntley. Between Pryor Creek and the Bighorn River the longest north oriented Yellowstone River tributary is Fly Creek, which originates just north of the Bad Lands (southeast of Billings in Yellowstone County) and which flows in a northeast and north direction to join the Yellowstone River near Nibbe (just west of Pompeys Pillar). Pryor Creek tributaries of importance in this essay include northeast, north, northwest oriented East Fork Pryor Creek (in southeast corner of Yellowstone County, south and east of Billings), and the north and northwest oriented East Pryor Creek (which is located east of West Pryor Mountain and the Pryor Creek headwaters. Note how many Pryor Creek tributaries from the east are oriented in northwest directions. The northwest oriented tributary valleys were eroded by reversals of flood flow on northwest ends of southeast oriented flood flow channels beheaded by Pryor Creek valley headward erosion.

East Fork Pryor Creek-Fly Creek drainage divide area

Figure 3: East Fork Pryor Creek-Fly 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 East Fork Pryor Creek-Fly Creek drainage divide area. East Fork Pryor Creek flows in a northeast direction from near the southwest corner of figure 3 before turning to flow in a north and northwest direction to the northwest corner of figure 3. North and west of figure 3 East Fork Pryor Creek joins north oriented Pryor Creek. Note short northwest oriented tributaries to the northeast and north oriented East Fork Pryor Creek segments. Fly Creek originates in the south center area of figure 3 and flows in an east-northeast direction to the east edge of figure 3 (just south of center) and east of figure 3 turns to flow in a north direction to the Yellowstone River. Big Woody Creek is northeast and east-southeast oriented stream south of Fly Creek in the southeast quadrant of figure 3 and east of figure 3 Big Woody Creek flows in an east-southeast direction to join the north-northeast oriented Bighorn River. Note how there is a northwest to southeast oriented streamlining of the landscape and how there are shallow through valleys linking northwest oriented East Fork Pryor Creek tributary valleys with southeast oriented Fly Creek tributary valleys and northwest oriented Fly Creek tributary valleys with southeast oriented Big Woody Creek tributary valleys. The map contour interval for figure 3 is 20 meters and the through valleys are locally defined by two or three contour lines. However, note elevations on the upland along the west edge of the southwest quadrant of figure 3, which rise to more than 1240 meters. Also note elevations on the upland south of the north center edge of figure 3, which rise to more 1260 meters. Then note elevations of through valley floors crossing the East Fork Pryor Creek-Fly Creek drainage divide, one of which is less than 1140 meters with some others in the 1140 to 1160 meter range. In other words, between the two mentioned upland regions there is a broad northwest to southeast oriented through valley, which is more than 100 meters deep linking the northwest oriented East Fork Pryor Creek valley segment with the east oriented Fly Creek valley segment.  Similar through valleys cross the Fly Creek-Big Woody Creek drainage divide. The through valleys were eroded by southeast oriented flood flow. Headward erosion of the Big Woody Creek valley first captured the southeast oriented flood flow and diverted the flood flow to the Bighorn River valley. Next headward erosion of the east oriented Fly Creek valley captured the southeast oriented flood flow and diverted floodwaters to the newly formed north oriented Fly Creek valley (east of figure 3). Finally headward erosion of the deep Yellowstone River valley north of figure 3 beheaded south oriented flood flow on the Pryor Creek alignment, which caused a flood flow reversal that eroded the northwest oriented East Fork Pryor Creek valley segment seen in figure 3.

Detailed map of East Fork Pryor Creek-Fly Creek drainage divide area

Figure 4: Detailed map of East Fork Pryor Creek-Fly 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 East Fork Pryor Creek-Fly Creek drainage divide area seen in less detail in figure 3. Fly Creek flows in an east-northeast direction from the south center edge of figure 4 to the east edge of figure 4 (south half). Note southeast oriented Fly Creek tributaries, including tributaries joining Fly Creek east of figure 4. The East Fork Pryor Creek flows in a north direction along and across the west edge of figure 4 and can be seen near the southwest and northwest corners of figure 4. Note northwest oriented tributaries flowing to the north oriented East Fork Pryor Creek. The map contour interval for figure 4 is 20 feet.  Note elevations of floors of through valleys linking northwest oriented East Fork Pryor Creek tributary valleys with southeast oriented Fly Creek tributary valleys. The deeper through valley floor elevations at the drainage divide are between 3700 and 3720 feet. While not seen in figure 4 elevations just north of figure 4 (in section 17, the south half of which is seen in figure 4) rise to 3905 feet. North of figure 4 elevations greater than 4140 feet can be found. Elevations near the south edge of section 30 (in southwest quadrant of figure 4) rise to more than 3880 feet and elevations much greater than 4200 feet can be found in West Pryor Mountain, which is south and west of figure 4. These elevations suggest there is a broad northwest to southeast oriented through valley between the East Fork Pryor Creek valley and the Fly Creek valley, which by only using evidence in figure 4 is approximately 180 feet deep, although by use of evidence from a larger region is more than 400 feet deep. This large northwest to southeast oriented through valley was eroded by massive southeast oriented flood flow at a time when the north oriented Pryor Creek valley did not exist, nor did the deep Yellowstone River valley in the Billings area exist at that time. The southeast oriented flood flow was first captured by headward erosion of the deep north-northeast oriented Pryor Creek valley, which eroded headward from what was then the actively eroding northeast oriented Yellowstone River valley head. Continued headward erosion of the deep northeast oriented Yellowstone River valley next captured the southeast oriented flood flow.

East Pryor Creek-West Fork Big Woody Creek drainage divide area

Figure 5: East Pryor Creek-West Fork Big Woody Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 5 illustrates the East Pryor Creek-West Fork Big Woody Creek drainage divide area south and west of figure 3 and does not include an overlap area with figure 3.  Pryor Creek flows in a north direction along the west edge of the northwest quadrant of figure 5. East Pryor Creek flows in a northwest direction from the south edge of figure 5 (just west of center) to join north oriented Pryor Creek (slightly north of the west center edge area of figure 5). Birdhead Creek and Magpie Coulee are northwest oriented Pryor Creek tributaries near the north edge while Hay Creek is a northwest oriented tributary joining Pryor Creek near the west center edge of figure 5. Chilkoot Coulee is a west oriented East Pryor Creek tributary originating in the south center region of figure 5. Big Woody Creek originates just north of the south center edge of figure 5 and flows in a north-northeast direction along the west flank of Wild Horse Ridge and then turns to flow in a northeast direction to the northeast corner of figure 5. West Fork Big Woody Creek originates just east of the Chilkoot Coulee headwaters and flows in a north-northeast direction to join Big Woody Creek in the northeast quadrant of figure 5. The East Fork Pryor Creek (seen in figures 3 and 4) is the northeast oriented stream flowing through the small town of Soda Springs (in north center area of figure 5). Note how the north-northeast oriented West Fork Big Woody Creek valley is linked by a through valley with the west oriented Chilkoot Coulee valley.  The map contour interval in the southern 40% of figure 5 is 50 meters (20 meters in the northern 60%) and the through valley floor elevation at the drainage divide is between 1350 and 1400 meters. A spot elevation to the north reads 1422 meters while a spot elevation on Wild Horse Ridge just south of figure 5 reads 1473 meters (and much higher elevations are found on West Pryor Mountain a short distance further to the south. The through valley was eroded by south oriented floodwaters from the Pryor Creek alignment, which were captured by headward erosion of the Big Woody Creek valley from the deeper Bighorn River valley (which at first may have been a south oriented flood flow channel, but which was later beheaded and reversed so as to become a north oriented river valley). The captured floodwaters made a large U-turn in the Chilkoot Coulee area, first flowing in a southeast direction, then in an east direction, and next in a north-northeast and northeast direction before turning again to flow in an east-southeast direction to the Bighorn River valley.

Detailed map of East Pryor Creek-West Fork Big Woody Creek drainage divide area

Figure 6: Detailed map of East Pryor Creek-West Fork Big Woody 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 East Pryor Creek-West Fork Big Woody Creek drainage divide area seen in less detail in figure 5. East Pryor Creek flows in a north-northwest direction across the southwest corner of figure 6. Chilkoot Coulee originates in section 20 and drains in a west, northwest, and west direction to East Pryor Creek in the southwest corner of section 13. West Fork Big Woody Creek originates in section 16 and flows in a north-northeast direction to the north edge of figure 6 (at line between sections 9 and 10). The northeast oriented stream originating in section 8 (west of West Fork Big Woody Creek) is a Big Woody Creek tributary. Note how the drainage divide between the north-northeast oriented West Fork Big Woody Creek valley and the west oriented Chilkoot Coulee valley is a through valley (actually two adjacent through valleys separated by a low-rise or hill). The map contour interval for figure 6 is 20 feet and the through valley elevation at the drainage divide is between 4500 and 4520 feet. The spot elevation in the south half of section 17 to the west reads 4675 feet while the spot elevation in section 21 to the southeast reads 4695 meaning the through valley is at least 150 feet deep. A similar through valley near the south edge of section 8, with a similar elevation at the drainage divide, links the unnamed northeast oriented Big Woody Creek tributary valley with an unnamed west oriented East Pryor Creek tributary located north of Chilkoot Coulee. These two through valleys provide evidence of at least two roughly parallel flood flow channels, which first flowed in a southeast direction on the East Pryor Creek alignment before making U-turns to flow to the Big Woody Creek valley and the Bighorn River valley. Flood flow in these two flood flow channels was beheaded and reversed by headward erosion of the much deeper northeast oriented Yellowstone River valley, which caused headward erosion of the deep north-northeast oriented Pryor Creek valley. As seen in figures below not all southeast oriented flood flow was captured by headward erosion of the Big Woody Creek valley, and some southeast oriented flood flow continued to move in a southeast and south direction and flowed between Wild Horse Ridge and West Pryor Mountain to Beauvais Creek while other flood flow continued along the West Pryor Mountain east flank to the east oriented Dry Head Creek valley.

East Pryor Creek-Beauvais Creek drainage divide area

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

Figure 7 illustrates the East Pryor Creek-Beauvais Creek drainage divide area south and east of figure 5 and includes an overlap area with figure 5. The northeast corner of West Pryor Mountain can be seen in the southwest corner of figure 7. Deep Creek flows in a north and northwest direction from the north flank of West Pryor Mountain to the northwest corner of figure 7 and joins north oriented Pryor Creek a short distance north and west of figure 7. Hay Creek is located east of Deep Creek and originates on West Pryor Mountain near the southwest corner of figure 7 and flows in a northeast, north, and northwest direction to the north edge of figure 7 (west half) and joins Pryor Creek north of figure 7.  East Pryor Creek is located east of Hay Creek and flows in a north, north-northeast, and north-northwest direction from the south edge of figure 7 (west of center) to the north edge of figure 7 (west of center). Wild Horse Ridge is located in the northeast quadrant of figure 7 and Big Woody Creek can be seen flowing in a north-northeast direction along the west flank of Wild Horse Ridge. South and east of Wild Horse Ridge is northeast oriented Beauvais Creek, which flows to the east edge of figure 7 (north half) and east of figure 7 turns to flow in an east-southeast direction to the north oriented Bighorn River. Note how the Beauvais Creek headwaters are linked by a through valley with the north oriented East Pryor Creek valley. The through valley is located between Wild Horse Ridge and West Pryor Mountain and the deepest through valley links a north-northwest oriented East Pryor Creek tributary valley with the east oriented Beauvais Creek headwaters valley. The map contour interval for figure 7 is 50 meters and the through valley floor elevation at the drainage divide is between 1300 and 1350 meters. The spot elevation on Wild Horse Ridge just north of the through valley reads 1473 meters and elevations on West Pryor Mountain to the south are much higher suggesting the through valley is at least 125 meters deep. The through valley was eroded by south-southeast oriented flood flow moving on the East Pryor Creek alignment, which was captured by headward erosion of the Beauvais Creek valley and which made a U-turn to flow in a northeast direction to the east-southeast oriented Beauvais Creek valley.

Detailed map of East Pryor Creek–Beauvais Creek drainage divide area

Figure 8: Detailed map of East Pryor Creek-Beauvais 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 East Pryor Creek-Beauvais Creek drainage divide seen in less detail in figure 7 above. West Pryor Mountain is south and west of figure 8 and north dipping hogbacks along the north flank of West Pryor Mountain can be seen near the south edge of figure 8. Hay Creek flows in a north direction near the west edge of figure 8. East Pryor Creek flows in a north direction in a deep water gap across section 26 (along south margin of the southwest quadrant of figure 8) and then turns in section 23 to flow in a north-northeast direction to the north edge of figure 8 (just east of center).  Beauvais Creek flows in an east direction in sections 18, 17, and 16 to the east edge of figure 8 (just north of center) and east of figure 8 turns to flow in a northeast and then east-southeast direction to join the Bighorn River. Note in sections 7 and 8 multiple northwest to southeast oriented through valleys linking northwest oriented East Pryor Creek tributary valleys with southeast oriented Beauvais Creek tributary valleys. The map contour interval for figure 8 is 20 feet and elevations of the deepest through valleys at the drainage divide are between 4500 and 4520 feet. Wild Horse Ridge in the northeast corner of figure 8 rises to more than 4840 feet while much higher elevations can be seen on the hogbacks along the south margin of figure 8. These elevations suggest the deepest through valleys are more than 300 feet deep and that there is a broad northwest to southeast oriented through valley between Wild Horse Ridge and West Pryor Mountain linking the north-northwest oriented East Pryor Creek valley segment with the east oriented Beauvais Creek segment. The through valley was eroded by southeast oriented flood flow moving across the region prior to headward erosion of the deep north oriented Pryor Creek valley (west of figure 8) and of the deep northeast oriented Yellowstone River north and west of figure 8. Headward erosion of the deep Yellowstone River and its tributary Pryor Creek valley beheaded and reversed the southeast oriented flood flow to erode the north-northwest oriented East Pryor Creek valley segment and northwest oriented East Pryor Creek tributary valleys and to create the East Pryor Creek-Beauvais Creek drainage divide.

East Pryor Creek-Dry Head Creek drainage divide area

Figure 9: East Pryor Creek-Dry Head Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 9 illustrates the East Pryor Creek-Dry Head Creek drainage divide area and is located south and east of figure 7 and includes a small overlap area with figure 7. West Pryor Mountain is the upland seen along the west edge of figure 9. Dry Head Creek flows in a north and northeast direction in the southwest corner of figure 9 and then flows in an east and southeast direction to join the northeast oriented Bighorn River in the southeast corner of figure 9. Little Mountain is the labeled west to east oriented ridge north of the east oriented Dry Head Creek segment. Hoodoo Creek is the southeast and south oriented stream flowing from the east end of Little Mountain to join Dry Head Creek, just before Dry Head Creek joins the north oriented Bighorn River. Hoodoo Creek and Dry Head Creek join the Bighorn River as barbed tributaries and provide evidence the Bighorn River valley was initially a south oriented flood flow channel prior to the flood flow reversal that caused the north oriented Bighorn River valley to be eroded. East Pryor Creek originates on West Pryor Mountain just north of Dry Head Creek near the southwest corner of figure 9 and then flows in a north-northeast and north direction to the north edge of figure 9 (west half). Note how along the east flank of West Pryor Mountain there is a deep north to south oriented through valley linking the north oriented East Pryor Creek valley with a south-southeast oriented Dry Head Creek tributary valley.  The map contour interval for figure 9 is 50 meters and the through valley floor elevation at the drainage divide is between 1700 and 1750 meters. Elevations on Little Mountain to the east rise to 1917 meters while much higher elevations are found on West Pryor Mountain to the west. These elevations suggest the through valley is approximately 200 meters deep and also that there is a broad northwest to southeast oriented through valley between highest Little Mountain elevations and West Pryor Mountain linking the north oriented East Pryor Creek valley with the east oriented Dry Head Creek valley. The through valleys, both the deepest channel along the West Pryor Mountain east flank and the broader northwest to southeast oriented through valley, were eroded by south and southeast oriented flood flow moving from the present day Pryor Creek drainage basin to the Dry Head Creek drainage basin and at least at first was probably moving to a south oriented flood flow channel on what is today the north oriented Bighorn River valley alignment.

Detailed map of East Pryor Creek-Dry Head Creek drainage divide area

Figure 10: Detailed map of East Pryor Creek-Dry Head Creek drainage divide area at Pryor Gap. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 10 provides a detailed topographic map of the East Pryor Creek-Dry Head Creek drainage divide area. West Pryor Mountain is the highland located in the west half of figure 10. East Pryor Creek flows in a north-northeast direction from the west edge of figure 10 (near southwest corner) to the north edge of figure 10 (near northwest corner of section 35). Dry Head Creek flows in an east-northeast and east direction from the south edge of figure 10 (west half) to the east edge of figure 10 (in section 8 near southeast corner of figure 10). Note in the northwest quadrant of figure 10 near the west edge of section 35 a through valley links the north oriented East Pryor Creek valley with a south-southeast oriented Dry Head Creek tributary valley. The map contour interval for figure 10 is 40 feet and the through valley floor elevation at the drainage divide is between 5480 and 5520 feet. Elevations in the northeast quadrant of section 35 rise to more than 6020 feet making the through valley at least 500 feet deep.  While not seen in figure 10 the highest point on Little Mountain to the northeast of figure 10 is 6288 feet, which means the through valley is at least 768 feet deep because elevations on West Pryor Mountain seen along the west margin of figure 10 exceed 6800 feet. This deep through valley is a water eroded feature and was eroded by south oriented flood water moving to the Dry Head Creek valley. Probably at that time West Pryor Mountain was still being uplifted as floodwaters were eroding regions around the rising mountain mass. Headward erosion of the deep northeast oriented Yellowstone River valley north of figure 10 from space in a deep “hole” being opened up by the melting of the thick ice sheet that had once filled the deep “hole” captured the south and southeast oriented flood flow and diverted the floodwaters in a northeast direction to space in the deep “hole”. This massive reversal of flood flow  was probably aided by crustal warping that was raising regional mountain masses, such as West Pryor Mountain as floodwaters were flowing across the region.

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.