Abstract:

This essay uses topographic map evidence to interpret landform origins in the southeast Pioneer Mountains region between Grasshopper Creek and Beaverhead River in Beaverhead County, Montana. The Beaverhead River is today a north-northeast oriented drainage route east of the Pioneer Mountains and is joined by the north, northeast, southeast, south, and northeast oriented Big Hole River to form the Jefferson River. Grasshopper Creek is a south and southeast oriented tributary originating in the Pioneer Mountains and joins the Beaverhead River as a barbed tributary. Rattlesnake Creek is another barbed Beaverhead River tributary originating in the Pioneer Mountains. North of the Rattlesnake Creek headwaters are headwaters of east, southeast, and northeast oriented Birch Creek, which flows to the south oriented Big Hole River at the point where the Big Hole River begins its eastern U-turn. Well-defined through valleys cross drainage divides between the Birch Creek valley and the various Beaverhead River tributary valleys as well as drainage divides between the Beaverhead River tributary valleys. The through valleys and barbed tributaries provide evidence the entire region was once crossed by immense south and southeast oriented floods as the Pioneer Mountains were beginning to emerge. Floodwaters were derived from the west margin of a thick North American ice sheet located along the present day Canadian Rocky Mountain front. Crustal warping related to the thick ice sheet presence and deep flood erosion of present day mountain core regions combined with deposition of flood transported debris in adjacent valleys and basins was probably responsible for the emergence of the present mountain ranges including Pioneer Mountains. Emergence of the Pioneer Mountains and regional uplift combined with headward erosion of a deep southeast oriented flood flow channel along the Pioneer Mountains north flank resulted in the beheading and reversal of flood flow across what were then the emerging Pioneer Mountains. The reversal of flood flow in what had been a south oriented flood flow channel east of the emerging Pioneer Mountains to create the north oriented Beaverhead River drainage system occurred when headward erosion of what is today the deep north and northeast oriented Missouri River valley beheaded and reversed the south oriented flood flow channel. The deep north and northeast oriented Missouri River valley eroded headward from space in a deep “hole” the melting ice sheet had once occupied and the massive flood flow reversal in southwest Montana was probably aided by ice sheet related crustal warping that created topographic barriers blocking south oriented flood flow.

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 Grasshopper Creek-Beaverhead River drainage divide area landform origins in the southeast Pioneer Mountains region of Beaverhead County, Montana and 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 Grasshopper Creek-Beaverhead River drainage divide area landform evidence in the southeast Pioneer Mountains region of Beaverhead County, Montana will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm.

Grasshopper Creek-Beaverhead River drainage divide area location map

Figure 1: Grasshopper Creek-Beaverhead 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 Grasshopper Creek-Beaverhead River drainage divide in the southeast Pioneer Mountains region of Beaverhead County, Montana and illustrates a region in southwest Montana with Idaho located west and south of Montana. The Pioneer Mountains are labeled and Grasshopper Creek flows in a south and southeast direction from the Pioneer Mountains to join the north-northeast oriented Beaverhead River near Dillon, Montana. The Beaverhead River is formed at the confluence of north-northwest oriented Red Rock River and an unnamed east oriented tributary (Horse Prairie Creek) at an unnamed reservoir (Clark Canyon Reservoir) and flows in a north-northeast direction to join the Big Hole River near Twin Bridges, Montana and to form the northeast and east oriented Jefferson River. The Jefferson River joins the north oriented Madison and Gallatin River near the town of Three Forks, Montana (near northeast corner of figure 1) to form the north oriented Missouri River with water eventually reaching the Gulf of Mexico. The Big Hole River flows in a north direction west of the Pioneer Mountains and then turns to flow in a northeast, southeast, and south direction east of the Pioneer Mountains before making a U-turn to flow in a northeast direction to join the north-northeast oriented Beaverhead River. The unnamed north oriented drainage route in the Pioneer Mountains, which originates near the south oriented Grasshopper Creek headwaters, is the Wise River, which joins the Big Hole River near the town of Wise River, Montana. The Grasshopper Creek-Beaverhead River drainage divide area investigated in this essay is located east and north of Grasshopper Creek and north and west of the Beaverhead River and includes the southeast Pioneer Mountains region.

Before looking at detailed maps of the Grasshopper Creek-Beaverhead River drainage divide area a brief look at the regional big picture erosion history will provide some background for the discussions related to the detailed maps shown below. Large volumes of south and southeast oriented floodwaters once flowed across the entire region shown by figure 1. Floodwaters were derived from the western margin of a rapidly melting thick North American ice sheet and were flowing in a south and southeast direction from southwest Alberta and southeast British Columbia to and across the figure 1 region. At that time the high mountain ranges and deep valleys and basins that exist today did not exist and floodwaters formed large anastomosing complexes of diverging and converging south oriented flood flow channels as they flowed freely across the region. The mountains ranges and valleys and valleys and basins emerged as crustal warping related to the presence of a huge continental ice sheet north and east of figure 1 and related to the massive erosion and deposition occurring as floodwaters flowed across the region changed the regional landscape. North oriented rivers in figure 1 are generally flowing in valleys that originated as south oriented flood flow channels. The south oriented Big Hole River segment east of the Pioneer Mountains and south and southeast oriented Grasshopper Creek seen in figure 1 are still flowing in their original south oriented direction. The north oriented Beaverhead River drainage system seen today was formed during massive flood flow reversals that occurred as mountain ranges and high plateaus were uplifted by ice sheet related crustal warping, which occurred as floodwaters flowed across the region. During these flood flow reversals south oriented flood flow along one route would be captured so as to flow in a north direction along an adjacent route. In addition, deep flood water erosion of valleys and basins surrounding the rising mountain ranges contributed to the emergence of present day mountain ranges.

Detailed location map for Grasshopper Creek-Beaverhead River drainage divide area

Figure 2: Detailed location map Grasshopper Creek-Beaverhead 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 Grasshopper Creek-Beaverhead River drainage divide area in the southeast Pioneer Mountains region of Beaverhead County, Montana and shows drainage routes not seen in figure 1. Madison County is the unnamed county east of Beaverhead County and is seen along the east margin of figure 2. Green shaded areas are National Forest lands, which generally are located in mountainous regions. The green shaded area in the northern area of figure 2 is located in the Pioneer Mountains. The east-west continental divide is the dashed line that extends in a south-southeast direction across the southwest corner of figure 2 and is located on the Beaverhead Mountains crest ridge. Grasshopper Creek originates near Torrey Mountain in the Pioneer Mountains (in north center area of figure 2) and flows in a northwest and then south and southeast direction to join the north-northeast oriented Beaverhead River as a barbed tributary just north of Henneberry Ridge (near south center edge of figure 2). The Beaverhead River flows in a north-northeast direction from the south center edge of figure 2 to Dillon and then to join the Big Hole River near Twin Bridges (near northeast corner of figure 2). The Big Hole River originates west of Selway Mountain near the continental divide in the southwest quadrant of figure 2 and flows in a north direction to the north edge of figure 2. North of figure 2 the Big Hole River turns to flow in a northeast and southeast direction around the north end of the Pioneer Mountains and then to flow in a south direction back into figure 2 along the east flank of the Pioneer Mountains before making a U-turn (seen in the northeast quadrant of figure 2) to join the north-northeast oriented Beaverhead River near the northeast corner of figure 2. The unlabeled north-northwest and north-northeast oriented stream originating near Torrey Mountain and flowing to the north edge of figure 2 is the Wise River, which joins the Big Hole River north of the Pioneer Mountains and north of figure 2. The Grasshopper Creek-Beaverhead River drainage divide area investigated in this essay is located east and north of Grasshopper Creek, west and north of the Beaverhead River, and south of Birch Creek, which is an east, southeast, and northeast oriented Big Hole River tributary originating near Torrey Mountain in the Pioneer Mountains. Rattlesnake Creek is a south-southeast and southeast oriented barbed Beaverhead River tributary also originating in the Torrey Mountain region and is located within the study region. Note also the unnamed south oriented Grasshopper Creek tributaries flowing from the Pioneer Mountains south flank.

Grasshopper Creek-Rattlesnake Creek drainage divide area

Figure 3: Grasshopper Creek-Rattlesnake 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 Grasshopper Creek-Rattlesnake Creek drainage divide area in the Pioneer Mountains. Grasshopper Creek flows in a south direction near the west edge of figure 3 and south of figure 3 turns to flow in a southeast direction to join the north-northeast oriented Beaverhead River as a barbed tributary. Note how most Grasshopper Creek tributaries from the west are oriented in southeast directions and many tributaries from the east are oriented in southwest directions suggesting the Grasshopper Creek drainage basin has not experienced a major flow reversal. The high Pioneer Mountains crest ridge extends in a north-to-south direction just west of the center of figure 3 and high points include Sawtooth Mountain, Alturas No 2 Mountain, Alturas No 1 Mountain, and Baldy Mountain. The crest ridge forms the drainage divide between the south-oriented Grasshopper Creek drainage basin to the west and the southeast and east oriented Birch Creek and south-southeast oriented Rattlesnake Creek drainage basins to the east. The Birch Creek drainage basin in located in the northeast quadrant of figure 3 with Birch Creek flowing in a southeast, east-southeast, northeast and southeast direction from the north edge of figure 3 (near center) to the east edge of figure 3 (north half). East of figure 3 Birch Creek flows in a southeast and northeast direction to join the Big Hole River near where the south oriented Big Hole River makes its U-turn to join the north-northeast oriented Beaverhead River. Mule Creek is northeast oriented Birch Creek tributary of interest in this essay. South of the northeast oriented Mule Creek headwaters is Minneopa Lake and the south-southeast oriented stream flowing from Minneopa Lake to the south edge of figure 3 is Rattlesnake Creek, which south of figure 3 turns to flow in a southeast direction to join the north-northeast oriented Beaverhead River as a barbed tributary. Note how the northeast oriented Mule Creek valley is linked by a well-defined through valley with the south-southeast oriented Rattlesnake Creek valley. The map contour interval for figure 3 is 50 meters and the through valley floor elevation at the drainage divide is between 2550 and 2600 meters. Trent Mountain to the west rises to 3107 meters while an unnamed mountain to the east rises to 2932 meters suggesting the through valley is more than 300 meters deep. Study of the drainage divide between the “east” oriented Birch Creek drainage basin and the south oriented Rattlesnake Creek drainage basin reveals other, although shallower, north-to-south oriented through valleys. The through valley provide evidence of multiple south oriented flood flow channels that were eroded into a surface as high, if not higher, than some of the highest elevations seen in figure 3 today. At that time the Pioneer Mountains did not stand high above the surrounding region and floodwaters could freely flow across the region.

Detailed map of Mule Creek-Rattlesnake Creek drainage divide area

Figure 4: Detailed map of Mule Creek-Rattlesnake 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 Mule Creek-Rattlesnake Creek drainage divide area seen in less detail in figure 3. The high Pioneer Mountains crest ridge extends in a north-to-south direction near the west edge of figure 4. Landforms in the crest ridge area appear to have been shaped by alpine glaciers, although the glaciers were located in preexisting valleys. In this essay the concern is with the valley origins and not with the alpine glaciation, which occurred after water had eroded the valleys and created the drainage divides. Birch Creek originates at Chan Lake (near northwest corner of figure 4) and flows to Pear Lake and then in a northeast direction to Boot Lake and next to near the north center edge of figure 4 before flowing in a southeast and northeast direction to near the northeast corner of figure 4. Mule Creek originates in the southeast corner of section 9 and flows in a northeast direction to join Birch Creek in section 2. Rattlesnake Creek originates in section 8 south of Trent Mountain and flows in a southeast direction to Trent Lake and then to Minneopa Lake. Rattlesnake Creek flows in a south direction from Minneopa Lake to the south center edge of figure 4. A through valley in the northeast corner of section 16 links the south oriented Rattlesnake Creek valley with the northeast oriented Mule Creek valley. The map contour interval for figure 4 is 40 feet and the through valley floor elevation at the drainage divide is between 8440 and 8480 feet. The high point near the north edge of section 14 to the east is 9618 feet while the Trent Mountain elevation to the west reaches 10,193 feet suggesting the through valley is at least 1100 feet deep. The through valley is a water-eroded feature and was eroded by south oriented flood water flowing to what was then the actively eroding south oriented Rattlesnake Creek valley. To reach the south oriented through valley the floodwaters had to flow in a southwest direction from east of Torrey Mountain, which is located just north of figure 4. However, before the through valley was eroded the floodwaters probably flowed in a south direction across what are now high ridges in the Torrey Mountain area. If so the floodwaters must have been flowing on a surface at least as high as the present day Pioneer Mountains crest ridge elevations, which are seen along the west edge of figure 4. At that time the Pioneer Mountains could not have been the high mountain range they are today and the high Pioneer Mountains as we see them today have emerged since that time.

Taylor Creek-Ermont Gulch drainage divide area

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

Figure 5 illustrates the Taylor Creek-Ermont Gulch drainage divide area south of figure 3 and there is an overlap area with figure 3. Figure 5 shows a region along the south flank of the Pioneer Mountains. Grasshopper Creek flows in a south and southeast direction from near the northwest corner of figure 5 to the south edge of figure 5 (west half) and south of figure 5 joins the north-northeast oriented Beaverhead River as a barbed tributary. Note south oriented Grasshopper Creek tributaries in the west half of figure 5. Taylor Creek is a south oriented Grasshopper Creek tributary flowing to the south edge of figure 5 (just west of center) and joins Grasshopper Creek south of figure 5. The south oriented Grasshopper Creek tributaries are flowing in valleys that were initiated as south oriented flood flow channels, which were captured by headward erosion of the deeper southeast oriented Grasshopper Creek valley. Rattlesnake Creek flows in a south direction from the north edge of figure 5 (slightly east of center) to Kelley Reservoir and then turns to flow in a southeast direction to the east center edge of figure 5. Ermont Gulch is a southeast oriented drainage route located south of the southeast oriented Rattlesnake Creek segment and drains to the east edge of figure 5 and joins Rattlesnake Creek east of figure 5. A north-to-south oriented ridge near the center of figure 5 serves as the Taylor Creek-Rattlesnake Creek drainage divide. Badger Pass, which is used by the highway is a through valley eroded across the Grasshopper Creek-Rattlesnake Creek drainage divide. Another through valley across the drainage divide can be seen west of the Ermont No 2 Mine to the north. The map contour interval for figure 5 is 50 meters and the Badger Pass elevation at the drainage divide is between 2050 and 2100 meters. Elevations south of Badger Pass rise to more than 2250 meters and to the north elevations in the Pioneer Mountains rise much higher suggesting Badger Pass is at least 150 meters deep. The through valleys were eroded by southeast and east oriented flood flow channels moving floodwaters from the south oriented flood flow channels on the Taylor Creek alignment to south oriented flood flow channels on what is today the north oriented Beaverhead River alignment. Headward erosion of the deep southeast oriented Grasshopper Creek valley enabled a much deeper south oriented Taylor Creek valley to erode headward (or northward) so as to behead the southeast and east oriented flood flow channels crossing the present day Taylor Creek-Ermont Gulch drainage divide. Headward erosion of deeper south oriented valleys (or flood flow channels) west of Taylor Creek next captured southeast and east oriented flood moving to the newly deepened south oriented Taylor Creek valley.

Detailed map of Taylor Creek-Ermont Gulch drainage divide area

Figure 6: Detailed map of Taylor Creek-Ermont Gulch 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 Taylor Creek-Ermont Gulch drainage divide area seen in less detail in figure 5. Cottonwood Creek flows in a south-southwest direction from the north edge of figure 6 (near northwest corner) to the west edge of figure 6 (near center). Taylor Creek is the south and south-southwest oriented stream flowing from the north edge of figure 6 (west half) to the south edge of figure 6 (near southwest corner). Ermont Gulch drains in a southeast direction from section 34 (near north center of figure 6) to the east edge of figure 6 (near center). Badger Pass is located in the south center area of figure 6 and is where the Big Hole Road crosses Badger Ridge. The map contour interval for figure 6 is 20 feet and the Badger Pass elevation where the road crosses the drainage divide is between 6740 and 6760 feet. In the southeast corner of section 33 to the north another road crosses the Taylor Creek-Ermont Gulch drainage divide at an elevation of 6879 feet. Note the southwest oriented Taylor Creek tributary originating near that northern pass. The southeast oriented Ermont Gulch valley originates on the northeast side of that northern pass. In section 3 to the southeast of that northern pass is the head of a south oriented valley that is drained by a south, east, and east-northeast oriented Ermont Gulch tributary. These three south oriented valleys diverge from a south oriented through valley north of figure 6. That south oriented through valley links these three diverging south oriented valleys with the south oriented Rattlesnake Creek valley near the point where it turns to flow in a southeast direction. Prior to headward erosion of the deep southeast oriented Rattlesnake Creek valley floodwaters flowed in a south direction to the section 34 area in figure 6 and then diverged along the three different south oriented flood flow channels. The south, east, and east-northeast oriented Ermont Gulch tributary valley located just east of Badger Pass began to capture the south oriented flood flow first. Next headward erosion of the deep south-oriented Taylor Creek valley and its southwest oriented tributary valley from the actively southeast oriented Grasshopper Creek valley (south of figure 6) captured the south oriented flood flow. Finally headward erosion of the southeast oriented Ermont Gulch valley captured the south oriented flood flow, although probably floodwaters were simultaneously flowing in three diverging flood flow channels for a period of time. Headward erosion of the deep southeast oriented Rattlesnake Creek valley north of figure 6 next captured the south oriented flood flow and ended the south oriented flood flow. Study of figure 6 and the surrounding region reveals other through valleys crossing drainage divides, which means there were many additional flood flow movements in addition to those few movements I have described. Hopefully the described movements will provide guidance on how to interpret flood flow movements in the other through valleys seen.

Taylor Creek-Cold Spring Creek drainage divide area

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

Figure 7 illustrates the Taylor Creek-Cold Spring Creek drainage divide area south and east of figure 5 and includes an overlap area with figure 5. The Beaverhead River flows in a north-northeast direction across the southeast corner of figure 7. Southeast oriented Ermont Gulch (labeled Gulch in figure 7) and south-southeast oriented Rattlesnake Creek (unlabeled) can be seen crossing the northeast corner of figure 7 and they meet east of figure 7 before Rattlesnake Creek joins the north-northeast oriented Beaverhead River. Grasshopper Creek flows in a south, southeast, and east direction from the northwest corner of figure 7 to join the Beaverhead River near the southeast corner of figure 7. Taylor Creek flows in a south and southwest direction to join south oriented Grasshopper Creek near the northwest corner of figure 7. A north-to-south oriented forested ridge is located in the west half of figure 7 and just east of that ridge is south oriented Cold Spring Creek, which flows to southeast oriented Grasshopper Creek. Hangmans Gulch is a south oriented valley just west of the ridge draining to Grasshopper Creek (south and east of Taylor Creek). Note how Grasshopper Creek has eroded a deep water gap across the forested ridge. The map contour interval for figure 7 is 50 meters and Grasshopper Creek crosses the 1750-meter contour line at the southeast end of the water gap. Elevations on the ridge north of the water rise to 2250 meters and elevations south of the water gap rise to 2221 meters suggesting the water gap is almost 500 meters deep. The water gap was eroded as the deep southeast oriented Grasshopper Creek valley eroded headward across the region to capture south oriented flood flow, which means at that time the high forested ridge did not exist. The ridge emerged as south oriented floodwaters eroded deep south oriented valleys adjacent to the ridge. The north oriented Madigan Gulch valley south of Cold Spring Creek was eroded by a reversal of flood flow on what had been initiated as a south oriented flood flow channel. Floodwaters probably reached the reversed Madigan Gulch flood flow channel from west of the forested ridge prior to being beheaded by headward erosion of the deep Grasshopper Creek valley and its tributary valleys. It is also possible the ridge was being uplifted as floodwaters flowed across the region, although in this region deep erosion on either side of the ridge appears to be the more probable explanation.

Detailed map of Taylor Creek-Cold Spring Creek drainage divide area

Figure 8: Detailed map of Taylor Creek-Cold Spring 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 Taylor Creek-Cold Spring Creek drainage divide area seen in less detail in figure 7. Grasshopper Creek flows in a southeast direction across the southwest corner of figure 8 and the deep Grasshopper Creek water gap can be seen near the south edge of figure 8 (west half). The map contour interval for figure 8 is 20 feet (40 feet near the west edge) and Grasshopper Creek crosses the 5780-foot contour line in Bannack State Park on the west side of the water gap. Elevations on the ridge in section 4 to the north of the water gap rise to 7400 feet while south of figure 8 ridge elevations rise to more than 7340 feet. These elevations suggest the water gap is as much as 1500 feet deep. Taylor Creek flows in a south-southwest direction across the northwest quadrant of figure 8 and joins south oriented Grasshopper Creek west of figure 8. Hangmans Gulch originates in section 29 and drains in a southwest and south direction to join Grasshopper Creek in Bannack State Park. East of the high Badger Ridge is south oriented Cold Spring Creek, which is formed at the confluence of a west oriented tributary and a southeast oriented tributary near the corner of sections 2, 3, 34, and 35. Cold Spring Creek joins southeast oriented Grasshopper Creek south of figure 8. Note in the east half of section 28 (in north center of figure 8) how the southeast oriented Cold Spring Creek tributary valley is linked by a through valley with a northwest and west oriented Taylor Creek tributary valley. The Bench Mark at the through valley drainage divide reads 6759 feet. To the south Badger Ridge rises to 7400 feet and to the northeast there is a high point labeled “Badger” with an elevation of 7508 feet suggest the through valley may be as much as 640 feet deep. The through valley is a water-eroded feature and was eroded by southeast oriented flood flow diverging from a south oriented flood flow channel on the Taylor Creek alignment to converge with a south oriented flood flow channel east of the emerging Badger Ridge and to erode what is today the south oriented Cold Spring Creek valley. At that time the Grasshopper Creek valley was still eroding the deep southeast oriented water gap headward across what is today Badger Ridge. Once the deep southeast oriented Grasshopper Creek valley had eroded headward to the Badger Ridge west side the deep south oriented Taylor Creek valley eroded headward along the west side of Badger Ridge and beheaded the southeast oriented flood flow channel to the actively eroding Cold Spring Creek valley.

Birch Creek-Beaverhead River drainage divide area

Figure 9: Birch Creek-Beaverhead River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 9 illustrates the Birch Creek-Beaverhead River drainage divide area east and north of figure 5 and includes an overlap area with figure 5. The Beaverhead River flows in a north-northeast direction from the south edge of figure 9 (east half) to the east edge of figure 9 (north half). Rattlesnake Creek flows in a southeast direction across the southwest corner of figure 9. Birch Creek flows in a southeast direction from just south of Middle Mountain (near northwest corner of figure 9) and then turns to flow in a northeast direction to the north center edge of figure 9. North of figure 9 Birch Creek joins the south, southeast, and northeast oriented Big Hole River, which can be seen at the south end of its U-turn along the north edge of the northeast quadrant of figure 9 (just north of the forested ridge labeled Hogback). Apex is a highway intersection and railroad siding south of Birch Creek and west of the north center region of figure 9 and is located in a large north-to-south oriented through valley linking the northeast oriented Birch Creek valley with the north-northeast oriented Beaverhead River valley. North of Apex and of figure 9 is the south oriented Big Hole River valley on the east side of the Pioneer Mountains. The large through valley is evidence of a major south oriented flood flow channel along the east side of the Pioneer Mountains prior to the flood flow reversal that resulted in development of the north-northeast oriented Beaverhead River valley and the capture of the south oriented Big Hole River flood flow channel (and southeast oriented Birch Creek flood flow channel seen in the northeast quadrant of figure 9). The massive flood flow reversal east of the Pioneer Mountains was probably caused by a combination of factors. First crustal warping related to the presence of a thick continental ice sheet north and east of this essay’s study region raised mountain ranges in southwest Montana and adjacent states, which gradually created topographic barriers that blocked the south oriented flood flow. Second, the crustal warping combined with deep glacial erosion created a deep “hole” in which the thick ice sheet was located. Over time ice sheet melting opened up space in the deep “hole” that was lower in elevation than elevations along the ice sheet’s southwest margin in Montana. Deep northeast oriented valleys then eroded headward from space in the deep “hole” being opened up by the ice sheet melting and these deep northeast oriented valleys beheaded south and southeast oriented flood flow channels in Montana. Floodwaters on north ends of the beheaded flood flow channels reversed flow direction to erode north oriented valleys. Often reversed flow flood flow channels captured south oriented floodwaters from west of the actively eroding northeast oriented valley heads. For example, south oriented flood flow on the south oriented Big Hole River alignment north of figure 9 was captured by reversed flood flow in the north oriented Missouri River valley (seen in the northeast corner of figure 1). As seen in earlier figures south oriented flood flow moving across the emerging Pioneer Mountains first moved to a south oriented flood flow channel on the Beaverhead River alignment, but was captured when flood flow on the Beaverhead River alignment was beheaded and reversed. The Humbolt Mountain region, which is north of the west center edge area of figure 9, is seen in more detail in figure 10 below.

Detailed map of Sheep Creek-Trout Creek drainage divide area

Figure 10: Detailed map of Sheep Creek- Trout 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 Sheep Creek-Trout Creek drainage divide area seen in less detail near the west center edge of figure 9. The map contour interval for figure 10 is 40 feet in the north half and 20 feet in the south half. Birch Creek flows in a southeast direction across the northeast quadrant of figure 10. Humbolt Mountain is located slightly south and west of the center of figure 10 and reaches an elevation of 9713 feet. North of Humbolt Mountain is Armstrong Mountain, which reaches an elevation of 8455 feet, and the east and northeast oriented Birch Creek tributary flowing between Armstrong Mountain and Humbolt Mountain is Sheep Creek. The Sheep Creek valley floor between the two mountains is approximately 1000 feet lower than the Armstrong Mountain top. Note how Sheep Creek has a north oriented tributary originating in the northeast corner of section 30 west of Humbolt Mountain. South of the north oriented Sheep Creek tributary in section 30 are headwaters of south oriented Trout Creek with its water flowing south and west of figure 10 to southeast oriented Rattlesnake Creek. Note how in the west half of section 29 there is a north-to-south oriented through valley linking the north oriented Sheep Creek tributary valley with the south oriented Trout Creek valley. The through valley floor elevation at the drainage divide is between 8580 and 8600 feet and Tower Mountain to the northwest rises to 9288 feet suggesting the through valley may be as much as 700 feet deep. East of Humbolt Mountain another somewhat shallower north-to-south oriented through valley links the northeast oriented Bridge Gulch valley with the Long John Gulch valley. Still further east still another north-to-south oriented through valley links the northeast oriented Canyon Gulch valley with the southeast and east oriented Van Dyke Gulch valley. Numerous other similar through valleys can be seen crossing drainage divides in figure 10. These through valleys were eroded by south and southeast oriented flood flow at the time the Pioneer Mountains were beginning to emerge. Initially deep valleys eroded headward along the south oriented flood flow channels, but floodwaters flowing to these actively eroding valleys were beheaded and sometimes reversed by headward erosion of the deeper southeast oriented Birch Creek valley and its northeast oriented tributary valleys. Headward erosion of the deep south oriented Big Hole River valley along the Pioneer Mountains east flank and then of the southeast oriented Big Hole River valley north of the emerging Pioneer Mountains beheaded all of the south oriented flood flow routes to the region seen in figure 10.

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.