Clark Fork-Big Hole River drainage divide area landform origins along continental divide, Granite and Deer Lodge Counties, Montana, USA

Authors


Abstract:

This essay uses topographic map evidence to interpret landform origins along the continental divide between Clark Fork and the Big Hole River in Granite and Deer Lodge Counties, Montana. The east-west continental divide crosses the study region in roughly a southwest to northeast direction along the Anaconda Range crest with drainage north of the continental divide flowing to north and northwest oriented Clark Fork and eventually reaching the Pacific Ocean. South of the continental divide drainage flows to the south oriented Big Hole River, which south and east of the study region makes a large U-turn to flow to the northeast oriented Jefferson River which flows to the north oriented Missouri River with water eventually reaching the Gulf of Mexico. Multiple mountain passes or former valleys cross the continental divide and link valleys of Clark Fork tributaries with valleys of south oriented Big Hole River tributaries. These former through valleys were eroded as south and southeast oriented flood flow channels at a time when the continental divide did not exist and the Anaconda Range was just beginning to emerge. Floodwaters were derived from a thick North American ice sheet and were flowing from western Canada across Montana. The thick ice sheet was located in a deep “hole” and the ice sheet weight was causing crustal warping that raised the Anaconda Range as melt water floods flowed across it. Floodwaters flowed across the emerging mountain range and carved deep valleys it. Headward erosion of deep southeast, northeast, and east oriented valleys, including headward erosion of the deep north oriented Missouri River valley from space in the deep “hole” being opened up by ice sheet melting, captured south oriented flood flow east and south of the present day continental divide. North of the present day continental divide headward erosion of deep valleys beheaded and reversed south oriented flood flow channels with floodwaters on north ends of beheaded flood flow channels reversing flow directions to erode the present day north oriented valleys. Subsequent to the immense melt water flood erosion events alpine glaciers formed in the newly emerged Anaconda Range and further deepened and streamlined some of the flood eroded valleys.

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 Clark Fork-Big Hole River drainage divide area landform origins along the continental divide in Granite and Deer Lodge Counties, 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 Clark Fork-Big Hole River drainage divide area landform evidence along the continental divide in Granite and Deer Lodge Counties, Montana will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm.

Clark Fork-Big Hole River drainage divide area location map

Figure 1: Clark Fork-Big Hole 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 Clark Fork-Big Hole River drainage divide area along the continental divide in Granite and Deer Lodge Counties, Montana and illustrates a region in southwest Montana with a small region in Idaho visible in the southwest corner of figure 1. The Missouri River is located in the east half of figure 1 and is formed at Three Forks at the confluence the north and northwest oriented Gallatin River, north oriented Madison River (not labeled in figure 1), and northeast, east, and northeast oriented Jefferson River. From Three Forks the Missouri River flows in a north and north-northwest direction to Canyon Ferry Lake (large reservoir flooding the Missouri River valley) and then to the Gates of the Rocky Mountains at the north edge of figure 1. North of figure 1 the Missouri River turns to flow in a northeast and east direction to North Dakota where it turns to flow in a southeast and south direction with water eventually reaching the Gulf of Mexico. The Jefferson River is formed at the confluence of Big Hole and Beaverhead Rivers near Twin Bridges. Note how the Big Hole River flows in a north and northeast direction west of the Pioneer Mountains and then turns to flow in an east-southeast direction to the town of Divide. From Divide the Big Hole River flows in a south and northeast direction to join the north-northeast oriented Beaverhead River. North of the south oriented Big Hole River segment are headwaters of north and northwest oriented Clark Fork, which flows from Warm Springs to Deer Lodge, Garrison, and Drummond and then to near the northwest corner of figure 1. North and west of figure 1 Clark Fork eventually joins the Columbia River with water finally reaching the Pacific Ocean. The Clark Fork-Big Hole River drainage divide area in Granite and Deer Lodge Counties is located in the Anaconda Range between north oriented Rock Creek (which flows to the northwest oriented Clark Fork segment) and the northeast oriented Big Hole River segment. The east-west continental divide extends along the crest of the Anaconda Range and drainage north and west of the Anaconda Range flows to the Pacific Ocean while drainage south and east of Anaconda Range flows to the Gulf of Mexico.

Before looking at detailed maps of the Clark Fork-Big Hole River drainage divide in the Anaconda Range region a brief look at the big picture erosion history is appropriate. Large volumes of south and southeast oriented floodwaters once flowed across the 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 (at least initially) there were no high mountains or deep valleys or basins in western Montana or in the region south of figure 1 and floodwaters could freely flow across locations that today would be blocked by high drainage divides. Montana, Idaho, Wyoming, and other mountain ranges were formed by ice sheet related crustal warping that occurred as floodwaters flowed across them. In addition, deep flood water erosion of valleys and basins surrounding the rising mountain ranges contributed to the emergence of present day mountain ranges. In time the ice sheet related crustal warping combined with deep glacial erosion under the ice sheet created a deep “hole” in which the ice sheet was located. Eventually as the ice sheet melted there came a time when elevations on the ice sheet surface (at least in the south) were lower than elevations along the deep “hole” southwest rim in Montana where the immense south and southeast oriented ice marginal melt water floods were flowing. Deep northeast oriented valleys then eroded headward from space in the deep “hole” being opened up by the ice sheet melting to capture the south and southeast oriented melt water floods in present day eastern and central Montana. At the same time headward erosion of the south and west oriented Columbia River valley from the Pacific Ocean beheaded and reversed southeast oriented flood flow channels moving floodwaters to western Montana.

The northeast oriented Missouri River valley segment north of figure 1 and its east and northeast oriented tributary valleys eroded headward from the deep “hole” across the south and southeast oriented flood flow. Northwest oriented Missouri River tributary valleys and the north-northwest oriented Missouri River valley segment seen in figure 1 were eroded by reversals of flood flow on north and northwest ends of beheaded flood flow channels. The present day north oriented Madison River, north and northwest oriented Gallatin River, and north oriented Jefferson River tributaries alignments were established initially as south oriented flood flow channels, which were reversed and deepened during massive upper Missouri River drainage basin flood flow reversal. Uplift of the Yellowstone Plateau and mountain ranges south of figure 1 probably contributed significantly to the massive flood flow reversal. Reversal of flood flow in the present day north oriented Jefferson River valley captured south oriented flood flow on the present day north oriented Clark Fork alignment, which created the present day Big Hole River U-turn. The north oriented Big Hole River segment flows on the alignments of what began as south oriented flood flow channels.

A similar situation occurred north and west of the continental divide where headward erosion of the deep south and west oriented Columbia River valley and tributary valleys (north and west of figure 1) beheaded and reversed southeast and south oriented flood flow channels on the present day north and northwest oriented Clark Fork alignment to create the present day north and northwest oriented Clark Fork drainage system. The north, northwest, and north oriented Rock Creek valley was eroded by flood flow reversals on the north and northwest ends of beheaded flood flow channels. The flood flow reversals were probably caused by Anaconda Range uplift, which as seen in detailed topographic maps below occurred as floodwaters were flowing across the rising mountain range.

Detailed location map for Clark Fork-Big Hole River drainage divide area

Figure 2: Detailed location map Clark Fork-Big Hole 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 Clark Fork-Big Hole River drainage divide area along the continental divide in Granite and Deer Lodge Counties, Montana and shows drainage routes not seen in figure 1. County boundaries are shown and Granite, Deer Lodge, and Silver Bow Counties are labeled. Green shaded areas are National Forest lands, which generally are located in mountainous regions. The east-west continental divide is shown with a dashed line extending in a northeast and then southeast direction from the south edge of figure 2 (just east of the southwest corner) to near the southeast corner of figure 2 and is the Granite County-Deer Lodge County boundary in the northeast Anaconda Range region. Mill Creek is a stream in the northwest quadrant of figure 2 and is located just south of Anaconda and its headwaters flow just north of the continental divide in an east-southeast direction before turning to flow in a northeast direction to the town of Opportunity and then to join north oriented streams flowing to form Clark Fork, which flows to the north edge of figure 2 (near the town of Deer Lodge). The Big Hole River (unlabeled in figure 2) flows in a northeast and then southeast direction near the south center edge of figure 2 and is joined by several southeast oriented tributaries, including Fishtrap Creek (unlabeled in figure 2), which joins the Big Hole River near the town of Fishtrap. East and north of Fishtrap Creek is La Marche Creek, which has labeled West and East Forks. South of figure 2 the Big Hole River after turning to flow in a south direction makes a large U-turn to flow in a northeast direction to join the northeast oriented Beaverhead River and to form the northeast oriented Jefferson River with water eventually reaching the Gulf of Mexico. North of the La Marche and Fishtrap Creek headwaters and of the east-west continental divide are headwaters of north oriented Rock Creek (also unlabeled in figure 2), which flows through the Phillipsburg Valley to the north edge of figure 2 (just west of center) to join northwest oriented Clark Fork with water eventually reaching the Pacific Ocean. The region investigated in this essay extends west along the continental divide from the east-southeast oriented Mill Creek headwaters to the Fishtrap Creek headwaters and includes headwaters of the north oriented East and Middle Forks of Rock Creek.

Mill Creek-Sixmile Creek drainage divide area

Figure 3: Mill Creek-Sixmile 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 Mill Creek-Sixmile Creek drainage divide area. The east-west continental divide is shown with a labeled dashed line extending from the west edge of figure 3 (south half) to the east edge of figure 3 (just north of southeast corner). Drainage north of the continental divide flows to Clark Fork and eventually reaches the Pacific Ocean. Drainage south of the continental divide flows to the Big Hole River and eventually reaches the Gulf of Mexico. Anaconda is the city located near the northeast corner of figure 3 and is located in the valley of east-southeast oriented Warm Springs Creek (which east of figure 3 flows to north and northwest oriented Clark Fork). Mill Creek originates near Miller Lake (in west center area of figure 3) and flows in an east-southeast direction just north of the continental divide to the south center area of figure 3 and then turns to flow in a northeast direction to the east edge of figure 3 (north of center). West of Miller Lake are northwest oriented headwaters of north oriented Twin Lakes Creek, which is a Warm Spring Creek tributary. Note how a through valley (or mountain pass) links the northwest oriented Twin Lakes Creek headwaters valley with the east-southeast oriented Mill Creek valley. The map contour interval for figure 3 is 50 meters and the through valley floor elevation at the drainage divide is between 2800 and 2850 meters. Spot elevations on mountain peaks north and south of the through valley read more than 3100 meters suggesting the through valley is 250-300 meters deep. The through valley was eroded by southeast oriented flood flow moving to what at that time was the actively eroding east-southeast oriented Mill Creek valley. Just south of the Mill Creek elbow of capture (where it turns to flow in a northeast direction) and south of the continental divide are headwaters of south oriented Sixmile Creek, which south of figure 3 joins south oriented California Creek, which then joins south oriented Deep Creek, which joins the south oriented Big Hole River. A through valley across the continental divide links the Mill Creek elbow capture with the south oriented Sixmile Creek headwaters valley. The through valley floor elevation at the drainage divide is shown as 2306 meters. Immediately east of the Sixmile Creek headwaters is Grassy Mountain, which rises to 2435 meters and which suggests the Mill Creek-Sixmile Creek through valley is at least 129 meters deep. Numerous similar through valleys can be seen in figure 3. The through valleys are all water-eroded features and were eroded as south and southeast oriented flood flow channels at a time when the mountains in figure 3 had yet to emerge. Emergence of the mountains occurred as ice sheet related uplift raised the mountain range and as floodwaters eroded deep valleys across and into the rising mountain masses. Subsequently valley glaciers further modified and deepened some of the flood-eroded valleys, although the changes were relatively minor.

Detailed map of Mill Creek-Sixmile Creek drainage divide area

Figure 4: Detailed map of Mill Creek-Sixmile 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 Mill Creek-Sixmile Creek drainage divide area seen is less detail in figure 3. Mill Creek flows in a southeast direction from the northwest corner of figure 4 to section 36 and then turns to flow in a northeast direction to the northeast corner of figure 4 with water eventually reaching the Pacific Ocean. The east-west continental divide is shown with a dashed line and extends from the west edge of figure 4 in a southeast direction along the ridge south of the southeast oriented Mill Creek valley to Grassy Mountain (in section 7) and then continues in a southeast, east, and northeast direction to the east edge of figure 4 (in section 9). The south oriented stream originating south of the continental divide in section 1 is Sixmile Creek, which flows to the south edge of figure 4 (west of center) with water eventually reaching the Gulf of Mexico. Note the north-to-south oriented through valley crossing the continental divide in section 1 and which links the Mill Creek elbow of capture in section 36 with the south oriented Sixmile Creek valley.  The map contour interval for figure 4 is 40 feet and the through valley floor elevation at the continental divide is shown as being 7565 feet. Grassy Mountain to the east rises to 7990 feet and the continental divide near the west edge of figure 4 rises to 9500 feet. These elevations suggest the through valley is at least 1330 feet deep. This through valley was eroded by south oriented flood flow at a time when the deep northeast oriented Mill Creek valley did not exist. At that time the mountains were just beginning to emerge and floodwaters could freely flow across what are today insurmountable topographic barriers. Floodwaters were flowing in multiple south oriented flood flow channels and a much deeper north-to-south oriented through valley is found near the corner of sections 8,9, 16, and 17 (near southeast corner of figure 4) where the Anaconda-Ralston Road crosses the continental divide. The north oriented stream is a Mill Creek tributary while the south oriented stream flows to California Creek, which flows to Deep Creek, which in turn flows to the Big Hole River. The through valley floor elevation at the continental divide is 6772 feet or approximately 800 feet lower than the Mill Creek-Sixmile Creek through valley. This deeper through valley was also eroded as a south oriented flood flow channel, which converged with the south oriented Mill Creek-Sixmile Creek flood flow channel where Sixmile Creek joins California Creek south of figure 4.

Storm Lake Creek-Seymour Creek drainage divide area

Figure 5: Storm Lake Creek-Seymour Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 5 illustrates the Storm Lake Creek-Seymour Creek drainage divide area west of figure 3 and includes a significant overlap area with figure 3. The continental divide is shown with a labeled dashed line and extends in an east and northeast direction from the west edge of figure 5 (near southwest corner) to Mount Tiny and then extends in more of an east direction across Storm Lake Pass to the east edge of figure 5 (just south of center). Just north of Storm Lake Pass is Storm Lake and the headwaters of north and north-northeast oriented Storm Lake Creek, which flows to the north center edge of figure 5 and then north of figure 5 joins east-southeast oriented Warm Springs Creek, which flows to north and northwest oriented Clark Fork. Twin Lakes Creek is the north oriented stream flowing to the north edge of figure 5 (just east of Storm Lake Creek). Mill Creek flows in an east-southeast direction to the east center edge of figure 5 and east of figure 5 flows to north and northwest oriented Clark Fork. The north oriented stream near the west edge of figure 5 flowing to East Fork Reservoir (near northwest corner) is the East Fork Rock Creek, which north of figure 5 flows to north oriented Rock Creek, which in turn flows to northwest oriented Clark Fork. Page Creek is the northwest and west oriented East Fork Rock Creek tributary originating west of Mount Tiny. Seymour Creek is the southeast oriented stream originating just south of Mount Tiny and flowing to the south edge of figure 5 (just east of center) and then south of figure 5 to the Big Hole River. Note how the southeast oriented Seymour Creek valley is linked by a through valley at Storm Lake Pass with the north oriented Storm Lake Creek valley and also by a through valley (or mountain pass) just south of Mount Tiny with the northwest oriented Page Creek valley. The map contour interval for figure 5 is 50 meters and the Storm Lake Pass elevation at the continental divide is between 2750 and 2800 meters. The Page Creek-Seymour Creek through valley has a similar elevation. Mount Tiny rises to 3000 meters and higher elevations are found along the continental divide on either side of the two through valleys suggesting the two through valleys are at least 200 meters deep. Two flood flow channels converging in the southeast oriented Seymour Creek valley south and east of Mount Tiny eroded these two through valleys. Numerous other similar through valleys cross the present day continental divide and provide evidence of an anastomosing south oriented channel complex that once crossed the region. Ice sheet related crustal warping that raised the Anaconda Range as south and southeast oriented floodwaters flowed across it eventually created a topographic barrier that resulted in flood flow reversals responsible for eroding the north oriented valleys. The flood flow reversals probably occurred in sequence, which meant newly reversed flood flow channels could capture large volumes of flood flow from yet to be reversed flood flow channels. These flood flow captures provided water volumes required to erode the deep north oriented valleys. Alpine glaciation probably further deepened and streamlined some of the valleys, although the valley orientations were not significantly changed.

Detailed map of Storm Lake Creek-Seymour Creek drainage divide area

Figure 6: Detailed map of Storm Lake Creek-Seymour 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 Storm Lake Creek-Seymour Creek drainage divide area seen in less detail in figure 5. The continental divide is shown with a labeled dashed line extending in a north direction from the south edge of figure 6 (west of center) to Mount Tiny and then in an east direction to the east edge of figure 6 (south of center). The southeast oriented stream originating in section 31 south of Mount Tiny is a tributary to southeast oriented Seymour Creek with water eventually reaching the Gulf of Mexico. The north and northwest oriented stream originating north of Goat Flat in section 36 and flowing to the west edge of figure 6 (north half) is Page Creek, which north and west of figure 6 joins north oriented East Fork Rock Creek with water eventually reaching the Pacific Ocean. The north oriented stream originating in the Storm Lake area is Storm Lake Creek, which north of figure 6 flows to Warm Springs Creek, with water eventually reaching the Pacific Ocean. The north oriented stream in section 25 is Dry Creek, which is another Warm Springs Creek tributary. Note the Page Creek-Seymour Creek through valley (or mountain pass) located in section 36 between Goat Flat and Mount Tiny. The map contour interval for figure 6 is 40 feet and the through valley elevation at the drainage divide is between 9120 and 9160 feet. Mount Tiny reaches 9848 feet in elevation and just south of figure 6 the continental divide rises to more than 10,100 feet suggesting the through valley is approximately 700 feet deep if not deeper. It is difficult to read the elevation at Storm Lake Pass in section 31, but it appears to be similar to the elevation of the section 36 through valley. The high point on the continental divide near the east edge of section 31 is 9989 feet, which is approximately 140 feet higher than the top of Mount Tiny. While today the mountain passes eroded across the continental divide in the Anaconda Range appear to be about the last place to look for flood eroded channels, these through valleys were eroded as south and southeast oriented flood flow channels in the region south of Storm Lake Pass. At that time the Anaconda Range did not stand high like it does today and floodwaters could freely flow across the region. Headward erosion of the east-southeast oriented Warm Springs Creek valley north of figure 6 beheaded and reversed the south oriented flood flow on the alignments of the present day north oriented Storm Lake Creek and Dry Creek valleys. Headward erosion of deep southeast oriented valley on the alignment of the present day northwest oriented Clark Fork (north of figure 6 and also north of east-southeast oriented Warm Springs Creek) beheaded and reversed flood flow on the alignment of the present day northwest oriented Page Creek valley. The flood flow reversals were probably greatly aided by Anaconda Range uplift, which was occurring as floodwaters flowed across what developing into a major topographic barrier.

East Fork Rock Creek-La Marche Creek drainage divide area

Figure 7: East Fork Rock Creek-La Marche Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 7 illustrates the East Fork Rock Creek-La Marche Creek drainage divide area west and somewhat south of figure 5 and includes a significant overlap area with figure 5. Storm Lake and Mount Tiny are located near the northeast corner of figure 7. The east-west continental divide is shown with a labeled dashed line and serves as the Granite County-Deer Lodge County border in the region between the southwest corner of figure 7 and Mount Tiny. Except in the northeast corner of figure 7 streams north and west of the continental divide flow to north oriented Rock Creek, which flows to northwest oriented Clark Fork. As previously described streams north of the continental divide in the Storm Lake area flow to east-southeast oriented Warm Springs Creek, which then flows to north and northwest oriented Clark Fork. Streams south of the continental divide in figure 7 flow to the Big Hole River. Cutaway Pass is located near the center of figure 7. The north oriented stream originating near Cutaway Pass and flowing to the north center edge of figure 7 is the East Fork Rock Creek.  South of Cutaway Pass is northeast and southeast oriented West Fork La Marche Creek. The Middle Fork La Marche Creek is located north and east of the West Fork La Marche and after originating south of Queener Mountain flows in an east-southeast and south direction to join the West Fork La Marche Creek near the southeast corner of figure 7. Note how the West Fork La Marche Creek originates near Warren Lake. The south oriented stream south of Warren Lake is the West Fork of Fishtrap Creek. The southeast oriented East Fork Fishtrap Creek originates in the Saddle Mountains (just north of south center edge of figure 7) and flows parallel to the southeast oriented West Fork La Marche Creek segment to the south edge of figure 7 (east half). Note how south of Warren Lake the northeast oriented West Fork La Marche Creek headwaters are linked by a deep through valley with south oriented West Fork Fishtrap Creek. The map contour interval for figure 7 is 50 meters and the through valley floor elevation at the drainage divide is between 2600 and 2650 meters. West Goat Peak to the east rises to 3290 meters and Warren Peak to the northwest rises to 3189 meters suggesting the through valley may be as much as 500 meters deep. This deep through valley provides evidence of a former south oriented flood flow channel that was beheaded by headward erosion of a deeper northeast oriented West Fork La Marche Creek valley. Cutaway Pass across the continental divide also provides evidence of a south oriented flood flow channel that existed prior to the flood flow reversal that created the north oriented East Fork Rock Creek drainage system. The floor of Cutaway Pass at the continental divide appears to have an elevation of between 2700 and 2750 meters. Fish Peak to the east rises to 3119 meters while Warren Peak to the west rises to 3189 meters suggesting Cutaway Pass may be 450 meters deep. Numerous other through valleys (or mountain passes) can be seen crossing the continental divide and other figure 7 drainage divides. These through valleys provide evidence of anastomosing flood flow channels that were eroded into the rising Anaconda Range as the mountains were emerging.

Detailed map of East Fork Rock Creek-La Marche Creek drainage divide area

Figure 8: Detailed map of East Fork Rock Creek-La Marche 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 Fork Rock Creek-La Marche Creek drainage divide area seen in less detail in figure 7. The continental divide serves as the Granite County-Deer Lodge County boundary and extends along the high Anaconda Range crest ridge from the southwest corner of figure 8 to the east edge of figure 8. Regions north of the continental divide drain to north oriented Rock Creek, which flows to northwest oriented Clark Fork with water eventually reaching the Pacific Ocean. Regions south of the continental divide drain to La Marche Creek, which flows to the Big Hole River with water eventually reaching the Gulf of Mexico. Cutaway Pass is located in the northwest quadrant of section 17 and is a deep notch eroded into the high Anaconda Range crest ridge and links the north oriented East Fork Rock Creek valley with a south oriented tributary valley draining to the point where northeast oriented West Fork La Marche Creek turns to flow in a southeast direction. The map contour interval for figure 8 is 40 feet and the Cutaway Pass elevation at the continental divide is given as 9036 feet. Fish Peak to the east rises to 10,233 feet while Beaverhead Mountain to the west rises to 9858 feet. Following the continental divide south and west of figure 8 leads to an unnamed mountain peak with an elevation greater than 10,400 feet. Depending on which elevations are used as reference points Cutaway Pass is 800 or more feet deep. Following the continental in either direction reveals other somewhat shallower notches cut into the high continental divide ridge linking north oriented valleys with the south oriented valleys. These high-level notches or mountain passes eroded across the present day continental divide are also through valleys and are evidence of water-eroded valleys that once crossed the region. The large number of through valleys (or notches) suggests the region was once crossed by a large number of anastomosing flood flow channels. At that time floodwaters were flowing on a high-level surface equivalent in elevation to some of the highest elevations seen in figure 8. However, at that time the Anaconda Range did not stand high above surrounding areas as it does today and floodwaters from the north could freely flow across the region. The high ridge, which forms the continental divide today, was formed as floodwaters eroded deep valleys headward from the south at the same time as ice sheet related crustal warping was raising the Anaconda Range. Uplift of the Anaconda Range combined with headward erosion of a deep southeast oriented valley on the alignment of the present day northwest oriented Clark Fork valley systematically resulted in the beheading and reversal of flood flow channels north of the present day drainage divide. The flood flow reversals occurred one flood flow channel or valley at a time, with newly reversed flood flow channels capturing large volumes of yet to be reversed flood flow from yet be beheaded and reversed flood flow channels, which provided water volumes required to erode significant north oriented valleys. Subsequent to the flood flow erosion alpine glaciers further deepened and streamlined valleys, although valley orientations and most other characteristics were established by the previous flood flow erosion.

Middle Fork Rock Creek-Fishtrap Creek drainage divide area

Figure 9: Middle Fork Rock Creek-Fishtrap Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 9 illustrates the Middle Fork Rock Creek-Fishtrap Creek drainage divide area south of figure 7 and includes an overlap area with figure 7. The continental divide is also the Granite County southeast boundary and extends in a northeast direction from near the southwest corner of figure 9 to the north edge of figure 9 (west of center). Streams west and north of the continental divide flow to north oriented Rock Creek, which flows to northwest oriented Clark Fork with water eventually reaching the Pacific Ocean. Streams south and east of the continental divide flow to the Big Hole River with water eventually reaching the Gulf of Mexico. The northeast oriented Big Hole River can just barely be seen in the southeast corner of figure 9. East and south of figure 9 the Big Hole River turns to flow in a southeast and south direction before making another major U-turn to flow in a northeast direction. Saddle Mountain is located just north of the center of figure 9. The West Fork La Marche Creek flows in a northeast and southeast direction north of Saddle Mountain and after joining the Middle Fork La Marche Creek flows to the east edge of figure 9 (south half). Fishtrap Creek is the east-southeast oriented stream south of Saddle Mountain, which joins the Big Hole River in the southeast corner of figure 9. The West Fork Fishtrap Creek originates south of Warren Peak and flows in a south, southeast, and south direction to join southeast and northeast oriented Palisade Creek south of Saddle Mountain to form Fishtrap Creek. Note how just to the southeast of the Palisade Creek elbow of capture (where Palisade Creek changes from flowing in a southeast direction to flowing in a northeast direction) the southeast oriented East Fork Mudd Creek originates at Mudd Lake. Mudd Lake is located in a northwest to southeast oriented through valley, which was originally eroded by southeast oriented flood flow on the Palisade Creek alignment. The West Fork Fishtrap Creek, Palisade Creek, and East Fork Mudd Creek valleys in this region provide evidence of former converging and diverging flood flow channels such as might be found in an a southeast oriented anastomosing channel complex. West of the Palisade Creek headwaters and of the continental divide are headwaters of north-northwest oriented Falls Fork Rock Creek, which flows from Johnson Lake to near the northwest corner of figure 9. Note how the north-northwest oriented Falls Fork Rock Creek valley is linked by several deep notches (or valley remnants) with the southeast oriented Fishtrap Creek headwaters valleys. While today these deep notches are simply passes across a high mountain ridge the notches are evidence of southeast oriented flood flow channels that once crossed the region. At that time the Anaconda Range did not stand high above surrounding regions as it does today and floodwaters could freely across what is today an insurmountable topographic barrier.

Detailed map of Falls Fork Rock Creek-West Fork Fishtrap Creek drainage divide area

Figure 10: Detailed map of Falls Fork Rock Creek-West Fork Fishtrap 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 Falls Fork Rock Creek-West Fork Fishtrap Creek drainage divide area seen in less detail in figure 9. The labeled county line follows the continental divide along the high ridge from near the southwest corner of figure 10 to the north edge of figure 10 (east half). Falls Fork Rock Creek is the north and north-northwest oriented stream in the west half of figure 10 flowing from the Johnson Lake area to near the northwest corner of figure 10. North of figure 10 the north-northwest oriented Falls Fork joins north oriented Middle Fork Rock Creek. Warren Peak is the high mountain located south of the north center edge of figure 10. The West Fork Fishtrap Creek originates south of Warren Peak and flows in a southeast and south direction to the south edge of figure 10 (east half). A south oriented tributary originates in section 36 south of Warren Lake. The northeast oriented stream flowing from Warren Lake to the east edge of figure 10 is the West Fork La Marche Creek, which was seen in figures 7 and 8 and which east of figure 10 turns to flow in a southeast direction. Note how the Warren Lake basin is linked by a through valley with the south oriented West Fork Fishtrap Creek tributary valley. The map contour interval for figure 10 is 40 feet and the through valley floor elevation where the Continental Divide Trail crosses the drainage divide is between 8560 and 8600 feet. East of section 36 along the east edge of figure 10 is West Goat Peak (the western peak on Saddle Mountain) and the West Goat Peak elevation is 10,793 feet.  The Warren Peak elevation to the northwest is 10,458 feet. Based on elevations of these nearby mountain peaks the through valley could be as much as 2000 feet deep. The through valley was eroded by south oriented flood flow from north of the present day continental divide. Today the northeast oriented stream east of Warren Peak flows to northwest and west oriented Carpp Creek, which flows to the north oriented Middle Fork Rock Creek. Between Warren Peak and an unnamed peak near the east edge of section 27 is a deep notch carved in the high continental divide ridge. The elevation at the deepest point on the notch is shown as 9348 feet while the unnamed peak to the east has an elevation of 10,258 feet, suggesting the notch is 900 feet deep. Probably much of the south oriented flood flow that eroded the south oriented West Fork Fishtrap Creek tributary valley in section 36 moved through that notch. The south oriented flood flow was subsequently captured by headward erosion of the deeper northeast oriented West Fork La Marche Creek valley, which ended flood flow to the Fishtrap Creek drainage basin. Next the flood flow was beheaded and reversed north of the present day continental divide by headward erosion of the deep northeast oriented Carpp Creek headwaters valley. Finally, after the Anaconda Range had fully emerged as a high mountain range, alpine glaciation further modified some of the 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.

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