Abstract:

This essay uses topographic map evidence to interpret landform origins in the region between Rotten Grass Creek and the Little Bighorn River in Big Horn County, Montana. Rotten Grass Creek is a northeast, north, and north-northwest oriented Bighorn River tributary originating in the high Bighorn Mountains near the Montana-Wyoming state line. The Little Bighorn River also originates in the high Bighorn Mountains in Wyoming south of the Rotten Grass Creek headwaters and flows in a northeast, north, and north-northwest direction to join the Bighorn River west and north of Rotten Grass Creek. Lodge Grass Creek is a north and northeast oriented Little Bighorn River tributary originating in the high Bighorn Mountains  between the Rotten Grass Creek and Little Bighorn River headwaters. The Bighorn River has eroded a deep northeast oriented across the north end of the Bighorn Mountains and then flows in a north-northeast direction to join the northeast oriented Yellowstone River. North of the Bighorn Mountains shallow through valleys cross drainage divides between the north oriented primary 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 tributary valleys. Today the valleys crossing the drainage divides provide evidence of what were once a maze of south oriented flood flow channels, but which were then systematically captured and diverted to flow in north directions. Floodwaters were derived from a rapidly melting thick North American ice sheet and were initially 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. The deep northeast oriented Yellowstone River valley and north-northeast oriented Bighorn 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.

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 Rotten Grass Creek-Little Bighorn River 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 Rotten Grass Creek-Little Bighorn River drainage divide area landform evidence in Big Horn County, Montana will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm.

Rotten Grass Creek-Little Bighorn River drainage divide area location map

Figure 1: Rotten Grass Creek-Little Bighorn River drainage divide area location map (select and click on maps to enlarge). National Geographic Society map digitally presented using National Geographic Society TOPO software.

Figure 1 provides a location map for the Rotten Grass Creek-Little Bighorn River drainage divide area in Big Horn County, Montana. The Montana-Wyoming state line extends in a west to east direction just south of the figure 1 center with Montana in the north. The Yellowstone River flows in an east direction from the west edge of figure 1 in Montana to near Laurel and then turns 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. The Little Bighorn River originates in the Bighorn Mountains just south of the state line and flows in a northeast direction to Wyola, Montana and then in a north and north-northwest direction to join the Bighorn River near Hardin, Montana. The northeast oriented tributary originating in the Bighorn Mountains just south of the state line and joining the Little Bighorn River near Lodge Grass is Lodge Grass Creek. Rotten Grass Creek is shown, but not labeled in figure 1 and originates in the Bighorn Mountains just north of the state line (and north of the Lodge Grass Creek headwaters) and first flows in a northeast direction, but then turns to flow in a north-northwest direction to join the Bighorn River near St. Xavier, Montana. The Rotten Grass Creek-Little Bighorn River drainage divide area illustrated and discussed in this essay is north of the state line, west and south of Rotten Grass Creek, and east of the Little Bighorn River and includes areas in the high Bighorn Mountains as well as regions north of the Bighorn Mountains.

Before looking a detailed map evidence a look at  the big picture erosion history may be useful. 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 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 surrounding regions and floodwaters flowed freely across the entire figure 1 map area. Ice sheet related crustal warping raised the Bighorn Mountains and other mountain ranges as immense melt water floods eroded regions surrounding the rising mountain masses. Headward erosion of the much deeper northeast oriented Yellowstone River valley and its north-northeast oriented Bighorn River tributary valley from space in the deep “hole” being opened up by ice sheet melting beheaded and reversed the major 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 flood flow channels further to the west and with captured flood flow were able to create significant north oriented drainage systems, such as the north oriented Bighorn River drainage system in Wyoming.

In the case of the figure 1 headward erosion of the deep northeast oriented Yellowstone River valley and its tributary Bighorn River valley beheaded and reversed south oriented flood flow channels that eroded north and northwest oriented tributary valley segments, including the north and northwest oriented Little Bighorn River and Rotten Grass Creek valley segments. As seen in the topographic maps illustrated here these north oriented valleys captured south and southeast oriented flood flow from further to the west, which provided water volumes to further erode the north oriented valleys. Some of the captured floodwaters were flowing across the high Bighorn Mountains, which strongly suggests the Bighorn Mountains emerged as floodwaters flowed across them and deeply eroded regions surrounding them. Deep flood water erosion of the rising mountain core and deposition of the eroded debris in adjacent basins may have contributed to the Bighorn Mountains uplift and adjacent basin formation, although the crustal warping was probably initiated by the thick ice sheet weight.

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

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

Figure 2 provides a more detailed location map for the Rotten Grass Creek-Little Bighorn River drainage divide area in Big Horn County, Montana. The Montana-Wyoming state line extends in a west to east direction near the south edge of figure 2. The brown shaded area in Montana is Crow Indian Reservation land with an area of the Northern Cheyenne Indian Reservation straddling the east edge of figure 2. The brighter brown area extending north and northeast from the south edge of figure 2 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-northeast direction from the south edge of figure 2 (west half) 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. The Little Bighorn River flows in a north and north-northwest direction from Parkman, Wyoming (near south edge, east of center) to join the Bighorn River near Hardin, Montana. 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 and floodwaters flowed across what was a 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 channels further to the west. While today the regional topography is such the flood flow routes would be impossible, at that time elevations of regions north and west of the Bighorn Mountain area were high enough 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 Mountains rock 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.

Bighorn River-Little Bighorn River drainage divide area

Figure 3: Bighorn River-Little Bighorn River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 3 provides a topographic map of north end of the Bighorn River-Little Bighorn River drainage divide area and is located north of the Bighorn Mountains and is also located north of where north-northwest oriented Rotten Grass Creek joins the Bighorn River. The Bighorn River flows in a north-northeast direction near the west edge of figure 3. The Little Bighorn River flows in a north-northwest direction from the east center edge of figure 3 to the north edge of figure 3. Crow Agency is the town near the northeast corner of figure 3. Note how Bighorn River tributaries from the east are generally oriented in northwest directions while Little Bighorn River tributaries from the west are generally oriented in northeast directions. These valley orientations suggest a south oriented flood flow channel may have been once located near the present day Bighorn River-Little Bighorn River drainage divide. A close look at the Bighorn River-Little Bighorn River drainage divide reveals shallow through valleys linking the northeast oriented Little Bighorn River tributary valleys with the northwest oriented Bighorn River tributary valleys. For example, note the through valley linking northwest oriented Humphrey Coulee with east oriented Squaw Creek. The map contour interval for figure 3 is 20 meters and the through valley floor elevation at the drainage divide is between 1040 and 1060 meters. North of the through valley there are elevations greater than 1120 meters and south of the through valley elevations also rise to more than 1120 meters. In other words the through valley is at least 60 meters deep. Similar through valleys link the northwest oriented Davis Coulee and Good Horse Creek valleys with the north and northeast oriented Onion Creek valley. Additional through valleys cross the drainage divide both to the north and to the south. These through valleys are water eroded features and were eroded by southeast oriented flood flow channels, which were captured by headward erosion of the deeper north-northwest oriented Little Bighorn River valley. At that time south and southeast oriented flood flow on the Little Bighorn River alignment had been beheaded and floodwaters on the north ends of the beheaded flood flow routes reversed flow direction to erode the north-northwest oriented Little Bighorn River valley. Also, at that time the Little Bighorn River valley was eroding headward faster than the Bighorn River valley and was able to capture southeast oriented flood flow south of the actively eroding Bighorn River valley head. Headward erosion of the north-northeast oriented Bighorn River valley beheaded the southeast and northeast oriented flood flow channels moving water to the Little Bighorn River valley. Floodwaters on northwest ends of the beheaded flood flow reversed flow direction to erode the northwest oriented Bighorn River tributary valleys and to create the Bighorn River-Little Bighorn River drainage divide.

Detailed map of Davis Coulee-Onion Creek drainage divide area

Figure 4: Detailed map of Davis Coulee-Onion 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 Davis Coulee-Onion Creek drainage divide area seen in less detail in figure 3. Davis Coulee drains in a west-northwest direction in the northwest quadrant of figure 4. Good Horse Coulee drains in a north and then west direction in the southwest quadrant of figure 4. Onion Creek flows in a northeast and east direction from the south center edge of figure 4 to the east center edge of figure 4. Note the east oriented tributary that joins Onion Creek in the northwest quadrant of section 6 (near east center edge of figure 4) and how a through valley in the northeast quadrant of section 2 links that tributary valley with the northwest oriented Davis Coulee valley. The map contour interval for figure 4 is 20 feet and the through valley floor elevation at the drainage divide is between 3460 and 3480 feet. The hill in section 35 to the north rises to 3692 feet and the hill in the southwest quadrant of section rises to 3698 feet. In other words the through valley is more than 200 feet deep. Also note near the center of section 11 the through valley linking the west oriented Good Horse Coulee valley with the northeast oriented Onion Creek valley. The through valley floor elevation at the drainage divide is between 3500 and 3520 feet. As already noted the hill in section 2 to the north rises to 3698 feet and while not seen in figure 4 a hill just south of figure 4 rises to 3702 feet, which means this second through valley is also approximately 200 feet deep. The through valleys are water eroded features and were eroded by floodwaters moving in an east direction to what was then the actively eroding Little Bighorn River valley. The north oriented Onion Creek and Good Horse Coulee headwaters valleys were eroded by reversals of flood flow on the north ends of beheaded south oriented flood flow channels. Headward erosion of the deep north-northeast oriented Bighorn River valley west of figure 4 beheaded the southeast and east oriented flood flow to the newly eroded Little Bighorn River valley. Floodwaters on the northwest and west ends of the beheaded flood flow routes reversed flow direction to erode the northwest oriented Davis Coulee valley and the west oriented Good Horse Coulee valley segments. Through valleys crossing other drainage divides in figure 4 provide evidence of additional flood flow complications, although the general pattern of flood flow movement was as described by the through valleys described.

Rotten Grass Creek-Nest Creek drainage divide area

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

Figure 5 illustrates the Rotten Grass Creek-Nest Creek drainage divide area south and slightly west of figure 3 and includes an overlap area with figure 3. The Bighorn River can be seen flowing in a north-northeast direction near the west edge of figure 5. Rotten Grass Creek flows in a north-northwest direction from the south edge of figure 5 (just west of center) to join the Bighorn River near the northwest corner of figure 5. Headwaters of Good Horse Creek and of Onion Creek can be seen flowing parallel to each other in a north-northeast direction near the north center edge of figure 5 (and as seen in figures 3 and 4 north of figure 5 Good Horse Creek turns to flow in a west direction to the Bighorn River while Onion Creek turns to flow in a northeast and east direction to the Little Bighorn River).  The Bighorn River-Little Bighorn River drainage divide extends in a north to south direction from the north center edge of figure 5 to the south center edge of figure 5 and in the south half of figure 5 is the Rotten Grass Creek-Little Bighorn River drainage divide. Little Bighorn River tributaries are generally oriented in northeast directions. Bighorn River tributaries from the east are oriented northwest directions and most Rotten Grass Creek tributaries from the east are oriented in northwest directions. However, Ox Yoke Coulee drains in a southwest direction to join Rotten Grass Creek as a barbed tributary and East Dipping Vat Coulee after draining in a northwest direction turns to drain in a southwest direction to join north-northwest oriented Rotten Grass Creek as a barbed tributary. Shallow through valleys can be seen linking the west and northwest  oriented Bighorn River and Rotten Grass Creek tributary valleys with the northeast oriented Little Bighorn River tributary valleys. For example, the power line extending from the East Dipping Vat Coulee valley to the Nest Creek valley is located in a through valley linking those two valleys. The map contour interval for figure 5 is 20 meters and the through valley floor elevation at the drainage divide is between 1180 and 1200 meters. North of the through valley a hill on the drainage divide rises to more than 1260 meters and south of the through valley hills rising to more than 1260 can also be seen. In other words the through valley is at least 60 meters deep and provides evidence of a flood flow channel that once moved flood water to the actively eroding Little Bighorn River valley. At that time the deep Rotten Grass Creek valley and the deep Bighorn River valley to the west did not exist and floodwaters were able to flow freely across the region on an upland surface at least as high as the highest figure 5 elevations today.

Detailed map of Rotten Grass Creek-North Sand Creek drainage divide area

Figure 6: Detailed map of Rotten Grass Creek-North Sand 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 Rotten Grass Creek-North Sand Creek drainage divide area seen in less detail in figure 5. Rotten Grass Creek flows in a north and north-northwest direction near the west edge of the southwest quadrant of figure 6. Note how several of the Rotten Grass tributaries are oriented in northwest directions. The northwest oriented tributary valleys were eroded by reversals of flood flow on what had been southeast oriented flood flow channels. North Sand Creek originates near the corner of sections 25, 30, 31, and 36 and flows in a north and northeast direction to the east edge of figure 6 (in section 20 near northeast corner of figure 6). Through valleys link the northwest oriented Rotten Grass Creek tributary valleys with the north and northeast oriented North Sand Creek valley. The map contour interval for figure 6 is 20 feet. The deepest through valley is located in the southeast quadrant of 25 and has a floor elevation at the drainage divide of between 3960 and 3980 feet. To the southeast a hill on the drainage divide in section 31 rises to more than 4220 feet while to the north a hill on the drainage divide rises to 4180 feet, which means the through valley is approximately 200 feet deep. Further, between the two high points mentioned the drainage divide elevations rise and fall indicating the presence of a much broader through valley with several channels eroded into its floor. Probably the best explanation for the tributary valley orientations on either side of the drainage divide is at one time the drainage divide was the approximate location of a south oriented flood flow channel (which flowed on a surface now totally removed). Southeast oriented flood flow coming from the west and southwest oriented flood flow from the east met to form the south oriented flood flow channel. The reversal of flood flow that caused headward erosion of the much deeper Little Bighorn River valley beheaded and reversed the southwest oriented flood flow channels and the reversal of flood flow that caused headward erosion of the much deeper Rotten Grass Creek valley beheaded and reversed the southeast oriented flood flow, with the drainage divide being formed where the floodwaters parted. However, the parting of the floodwaters was not simple and floodwaters spilled across the developing drainage divide from one side to the other, as the deeper tributary valleys were eroded headward toward the drainage divide location.

Lodge Grass Creek-Little Bighorn River drainage divide area

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

Figure 7 illustrates the Lodge Grass Creek-Little Bighorn River drainage divide area south and east of figure 5 and does not include an overlap area with figure 5. Lodge Grass is the town located near the northeast corner of figure 7. The Little Bighorn River flows in a north direction near the east edge of figure 7. Lodge Grass Creek flows in a northeast direction from the south edge of figure 7 (west half) to join the Little Bighorn River near the town of Lodge Grass. North of Lodge Grass Creek is east oriented Good Luck Creek, which is a Lodge Grass Creek tributary. South and east of Lodge Grass Creek are northeast oriented Alligator Creek and White Man Runs Him Creek, both of which flow to the north oriented Little Bighorn River. Note the shallow northwest to southeast oriented through valleys crossing Lodge Grass Creek-Alligator Creek and Alligator Creek-White Man Runs Him Creek drainage divides and the shallow through valleys crossing the northeast oriented South Fork Good Luck Creek-Lodge Grass Creek drainage divide. These through valleys provide evidence of south and southeast oriented flood flow routes that crossed the region prior to headward erosion of the deep northeast oriented valleys. Probably the deep valleys were eroded in sequence, with the southern valleys being eroded prior to the northern valleys, which means additional factors influenced the flood flow movements. Rotten Grass Creek flows in a northeast and then north direction from the southwest corner of figure 7 to the north edge of figure 7 (near northwest corner).  Note west to east oriented through valleys linking the north oriented Rotten Grass Creek valley with the northeast oriented Lodge Grass Creek valley. Perhaps the easiest through valley to see is near the north edge of figure 7 and links a northwest oriented Rotten Grass Creek tributary valley with the east oriented North Fork Good Luck Creek valley. The map contour interval for figure 7 is 20 meters and the through valley floor elevation at the drainage divide is between 1220 and 1240 meters. To the north elevations rise to more than 1300 meters and to the south elevations rise to more than 1320 meters, which suggests the through valley is at least 60 meters deep. The through valleys in figure 7 record a complex history of flood flow movements as headward erosion of the deep north oriented valleys systematically beheaded and captured south oriented flood flow moving across the region.

Rotten Grass Creek-Lodge Grass Creek drainage divide area

Figure 8: Rotten Grass Creek-Lodge 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 Rotten Grass Creek-Lodge Grass Creek drainage divide south and west of figure 7 and does not include an overlap area with figure 7. Steep slopes along the Bighorn Mountains east-northeast facing flank can be seen near the west edge of figure 8. Rotten Grass Creek flows in a north-northeast, east, and northeast direction from the southwest corner of figure 8 to the north edge of figure 8 (east of center). Soap Creek is the northeast oriented Bighorn River tributary flowing across the northwest corner of figure 8. Note the deep through valley linking the Soap Creek and Rotten Grass Creek valleys. The through valley is drained in the north by a north oriented Soap Creek tributary and in the south by an east oriented Rotten Grass Creek tributary. The map contour interval for figure 8 is 20 meters and the through valley floor elevation at the drainage divide is between 1440 and 1460 meters. To the east elevations rise to at least 1600 meters and to the west elevations rise much higher, meaning the through valley is approximately 140 meters deep. Lodge Grass Creek flows in a north-northeast direction from the south center edge of figure 8 to the north edge of figure 8 (east half). Note also the through valley along the Bighorn Mountains base linking the Rotten Grass Creek valley with the Lodge Grass Creek valley. Beaver Creek drains that through valley’s north end and Percheron Creek drains the through valley’s south end. The through valley floor elevation at the drainage divide is between 1560 and 1580 meters. Elevations to the east of the through valley rise to more than 1760 meters and to the west elevations rise even higher, meaning the through valley is approximately 180 meters deep. The Little Bighorn River can be seen flowing in an east-northeast direction across the southeast corner of figure 8. While not seen in figure 8 a deep through valley along the Bighorn Mountains base also links the Lodge Grass Creek valley with the Little Bighorn River valley. Each through valley turns in an east direction near its south end, which provides evidence the through valleys were eroded by south oriented flood flow, which was captured by headward erosion of the deep northeast or north-northeast oriented valley to the south. In other words the deep northeast or north-northeast oriented valleys were eroded in sequence, with the Little Bighorn River valley being eroded first, the Lodge Grass Creek valley second, the Rotten Grass Creek valley third, and the Soap Creek valley fourth. The deep Bighorn River valley, which is located north of west of figure 8, was eroded even later, and once eroded ended all flood flow across the region in figure 8.

Trout Creek-Lodge Grass Creek drainage divide area

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

Figure 9 illustrates the Trout Creek-Lodge Grass Creek drainage divide area south and west of figure 8 and includes an overlap area with figure 8. The Montana-Wyoming state line is the west to east oriented dashed line in the south half of figure 9.  The northern Bighorn Mountains crest ridge is labeled in Montana and extends south into Wyoming along the Sheridan-Big Horn County line. Lodge Grass Creek flows in a north direction from the south center edge of figure 9 (just east of the county line) and then turns to flow in a northeast direction to the east edge of figure 9 (north of center). The North Fork Lodge Grass Creek originates just north of the state line and east of the Bighorn Mountains crest ridge and flows in a northeast and east direction north of Tepee Pole Ridge to join Lodge Grass Creek near the east edge of figure 9. The deep north-northwest oriented canyon draining to the north edge of figure 9 (just west of center) is Black Canyon and north of figure 9 Black Canyon Creek drains in a north-northwest direction to the northeast oriented Bighorn River. Big Bull Elk Canyon is labeled and is located just west of the Bighorn Mountains crest ridge and drains in a northwest direction to the west edge of figure 9  (north of Big Bull Elk Ridge) and then to the northeast oriented Bighorn River. Trout Creek flows in a northwest direction in Big Horn County, Wyoming to the west edge of figure 9 and then flows in a west direction to join northwest and west oriented Porcupine Creek, which flows to the Bighorn River. Rotten Grass Creek is located east of Black Canyon and flows in a north-northeast direction to the north edge of figure 9 (east of center). While Bighorn Mountains elevations are more than 1500 meters higher than areas seen along the Rotten Grass Creek-Little Bighorn River drainage divide further to the north there is evidence in figure 9 suggesting multiple flood flow channels once crossed the high drainage divides. One interesting through valley crossing the high Bighorn Mountains and which looks like a high mountain pass is near the south edge of figure 9 in section 32 on the Sheridan-Big Horn County line and links the deep north and northeast oriented Lodge Grass Creek valley in the east with the deep northwest oriented Trout Creek valley in the west. The map contour interval for figure 9 is 20 meters and the through valley floor elevation at the drainage divide is between 2700 and 2720 meters. Just south of figure 9 the drainage divide rises to more than 2980 meters and near the state line the drainage divide rises to at least 2800 meters. Continuing north along the drainage divide, but still south of the state line, is another through valley with a valley floor elevation of between 2680 and 2700 meters. These through valleys are more than 100 meters deep and cross the present day Trout Creek-Lodge Grass Creek drainage divide. The through valleys are water eroded features and were eroded by flood flow moving across what is today the crest of the high Bighorn Mountains. Study of figure 9 reveals many similar through valleys crossing the high drainage divides. The through valleys and the much deeper present day stream valleys provide evidence of a maze of anastomosing flood flow channels which were carved into what was an emerging mountain mass.

Detailed map of Trout Creek-Lodge Grass Creek drainage divide area

Figure 10: Detailed map of Trout Creek-Lodge Grass Creek 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 Trout Creek-Lodge Grass Creek drainage divide area seen in less detail in figure 9 and is located just south of the state line in Wyoming. The Sheridan County-Big Horn County boundary is the located along the high Bighorn Mountains crest ridge and is shown by a dashed line extending from the north edge of figure 10 (east of center) to the south edge of figure 10 (also east of center). Lodge Grass Creek flows in a north and north-northeast direction from the south edge (near southeast corner) to the east edge (north half) and north and east of figure 10 joins the Little Bighorn River. Trout Creek headwaters originate as two streams in sections PB 44 and 36 and flow in west and northwest directions to join and then flows in a northwest direction to the west edge of figure 10 (north half) and west and north of figure 10 joins west oriented Porcupine Creek, which flows to the northeast oriented Bighorn River. The map contour interval for figure 10 is 40 feet. Two through valleys linking the west oriented Trout Creek valley with the north and northeast oriented Lodge Grass Creek valley can be seen in sections 32 and 29.  The section 32 through valley has a floor elevation at the drainage divide of between 8840 and 8880 feet. The section 29 through valley is slightly higher and has a floor elevation at the drainage divide of between 8880 and 8920 feet. In the southeast corner of section 24 there is a slightly deeper through valley linking the Trout Creek valley with the east oriented Line Creek valley, with Line Creek being a tributary to Lodge Grass Creek. This section 24 through valley floor elevation at the drainage divide is between 8800 and 8840 feet. Sheep Mountain near the figure 10 south edge has an elevation of 9813 feet. A spot elevation of 9222 feet can be seen at the north center edge of figure 10 and slightly higher elevations are found north of figure 10. These elevations suggest the through valleys are between 300 and 400 feet deep. Today the through valleys are little more than high passes crossing a high mountain ridge, but the through valleys are water eroded features and provide evidence of multiple flood flow channels that once crossed the Bighorn Mountains. At the time floodwaters crossed the Bighorn Mountains the Bighorn Mountains did not stand high above surrounding regions as they do today and floodwaters from the north and northwest could freely flow onto and across what is today a high mountain range. Probably the Bighorn Mountains rock mass was rising as floodwaters flowed across it and deep erosion of the mountain core may have contributed to the mountain uplift. The Bighorn Mountains emerged as crustal warping raised the mountain mass and as floodwaters deeply eroded easily eroded bedrock materials surrounding the erosion resistant mountain core. Floodwaters were derived from the western margin of a rapidly melting thick North American ice sheet and were flowing from western Alberta across Montana and into Wyoming. Headward erosion of the deep northeast oriented Yellowstone River valley and its north-northeast oriented Bighorn River tributary valley captured the south and southeast oriented flood flow and diverted the floodwaters to space being opened up in a deep “hole” a melting ice sheet had formerly occupied. Floodwaters on north and northwest ends of beheaded flood flow channels reversed flow direction to erode north and northwest oriented tributary valleys.

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.