Abstract:

This essay uses topographic map evidence to interpret landform origins in the Cache la Poudre River-Big Thompson River drainage divide area in the Colorado Mummy Range. The Mummy Range is located in northeast Rocky Mountain National Park and the region immediately to the north. The Cache la Poudre River originates south and west of the Mummy Range and flows in a north and east direction around the north end of the Mummy Range before turning in a southeast direction to flow to the South Platte River. The South Fork Cache la Poudre River originates in the Mummy Range and flows in a northeast direction to join the east oriented Cache la Poudre River. South of the north oriented Cache la Poudre River headwaters is the south oriented Colorado River headwaters valley and southeast oriented headwaters of the Big Thompson River and its tributaries. The Big Thompson River then flows in a southeast, east, northeast, east, and southeast direction to join the South Platte River. Deep passes link the north oriented Cache la Poudre River headwaters valleys with the south oriented Colorado River headwaters valley and with the southeast oriented Big Thompson River headwaters valleys. Further mountain passes link northwest and west oriented Cache la Poudre River tributary valleys with the northeast oriented South Fork Cache la Poudre River valley or with southeast oriented South Fork tributary valleys. Valley orientations and through valleys (defined by the passes) are interpreted to have been eroded as anastomosing south oriented flood flow channels at a time when the Mummy Range and surrounding mountains were beginning to emerge. Floodwaters are interpreted to have been derived from the western margin of a thick North American ice sheet and were flowing from western Canada to and across emerging mountain ranges. The Mummy Range and surrounding mountain ranges emerged as floodwaters flowed across them and as deep valleys eroded headward into them to capture immense south oriented floods. After flood flow across the region had ended the mountain ranges had emerged flood eroded valleys in the newly uplifted mountains were filled with valley glaciers, which further modified the valleys, but which did not erode new valleys or significantly change valley orientations.

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 Cache la Poudre River-Big Thompson River drainage divide area landform origins in the Colorado Mummy Range. 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 Cache la Poudre River-Big Thompson River drainage divide area landform evidence in the Colorado Mummy Range will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm.

 

Cache la Poudre River-Big Thompson River drainage divide area location map

Figure 1: Cache la Poudre River-Big Thompson 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 Cache la Poudre River-Big Thompson River drainage divide area in the Colorado Mummy Range and illustrates a region in north central Colorado. Rocky Mountain National Park is shown and labeled. The South Platte River flows in a north-northeast direction from the south edge of figure 1 (east of center) through Denver to Brighton and then in a north and northeast direction to near Greeley where it turns to flow in a southeast direction to the east edge of figure 1. East of figure 1 the South Platte River turns to flow in a northeast direction into western Nebraska with water eventually reaching the Gulf of Mexico. The Cache la Poudre River originates in the northwest corner of Rocky Mountain National Park and flows in a north direction before turning to flow in an east, southeast, and east direction to join the South Platte River near Greeley (at the point where the South Platte River turns to flow in a southeast direction). The Big Thompson River originates a short distance south of the Cache la Poudre River headwaters and flows in a southeast and northeast direction to Estes Park and then in a northeast, east, and southeast direction to join the South Platte River near Milliken (after entering the South Platte River valley the Big Thompson River flows in a northeast direction before joining the northeast oriented South Platte River). The North Fork Big Thompson River (shown, but labeled in figure 1) originates in the northeast corner of Rocky Mountain National Park and flows in a southeast direction to join the Big Thompson River east of Rocky Mountain National Park. The Colorado River headwaters are not shown in figure 1, but originate near the Cache la Poudre and Big Thompson River headwaters and flow in a south direction near the west edge of Rocky Mountain National Park. At the southwest corner of Rocky Mountain National Park  the Colorado River turns to flow in a southwest, west, and southwest direction to Bond (near west edge in southwest quadrant of figure 1) where it makes a turn to flow in a northwest direction to the west edge of figure 1 with water eventually reaching the Pacific Ocean. The Mummy Range (not labeled in figure 1) straddles the Rocky Mountain National Park northern boundary and is primarily located between the east oriented Cache la Poudre River and the Big Thompson River headwaters. The Cache la Poudre River-Big Thompson River drainage divide area in the Mummy Range investigated in this essay is located in northern Rocky Mountain National Park and the region immediately to the north.

Today the Mummy Range and adjacent mountains are high mountains, yet drainage routes in the present day South Platte River drainage basin, including in the Rocky Mountain National Park area, developed during immense south oriented melt water floods from the western margin of a thick North American ice sheet. The gigantic melt water floods flowed from western Canada to and across the present day South Platte River drainage basin at a time when Colorado mountain ranges were beginning to emerge. Colorado mountain ranges, including the Mummy Range, emerged as floodwaters flowed across them, as ice sheet related crustal warping raised the mountain masses and the entire region, and as floodwaters deeply eroded surrounding regions. The present day north oriented South Platte River drainage route south of Greeley and north oriented South Platte River tributary drainage routes east of Denver (e.g. Kiowa Creek) originated as south oriented flood flow channels east of the emerging Colorado Front Range. East oriented valleys eroded headward from these south oriented flood flow channels into the emerging Colorado Front Range to capture south oriented floodwaters flowing into, along, and across the emerging mountain masses further to the west. Headward erosion of these deep east oriented valleys was in sequence from south to north. For example, headward erosion of the Big Thompson River valley occurred in advance of headward erosion of the Cache la Poudre River valley, which then captured south and southeast oriented flood flow moving to the newly eroded Big Thompson River valley and tributary valleys. The north oriented Cache la Poudre River headwaters drainage route was created by a reversal of flood flow on the north end of flood flow channels to the deep south and southwest oriented Colorado River valley, which had also eroded headward into the region to capture the massive south oriented flood flow. Floodwaters on the north ends of the beheaded flood flow channels reversed flow direction to flow to the deeper east oriented Cache la Poudre River valley. Northeast oriented valley segments were formed by eroding headward across southeast oriented flood flow. Headward erosion of the southeast and northeast oriented South Platte River valley from western Nebraska in time captured flood flow moving on the Cache la Poudre River drainage route and beheaded the south oriented flood flow channels on the present day Colorado Piedmont. Floodwaters on north ends of the beheaded flood flow routes reversed flow direction to create north oriented South Platte River tributary drainage routes and the north oriented South Platte River drainage route. These flood flow reversals were probably greatly aided by ice sheet related crustal warping that was raising the entire region.

Detailed location map for Cache la Poudre River-Big Thompson River drainage divide area

Figure 2: Detailed location map Cache la Poudre River-Big Thompson River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 2 provides a detailed location map for the Cache la Poudre River-Big Thompson River drainage divide area in the Colorado Mummy Range. The red-brown region is Rocky Mountain National Park and green colored areas are National Forest lands in surrounding mountain regions. The cities of Fort Collins and Loveland near the east edge of figure 2 are located on the Colorado Piedmont, which is located along the eastern margin of the Colorado Front Range. The Mummy Range location is labeled and straddles the Rocky Mountain National Park northern border. The east-west continental divide is shown and extends in a north-northeast direction from the west edge of figure 2 along the crest of the Never Summer Range before turning in a south-southeast direction to the south edge of figure 2 (west half). The area enclosed by the continental divide in the southwest quadrant of figure 2 is drained by the Colorado River, which flows in a south direction in the Kawuneeche Valley to the south edge of figure 2. South of figure 2 the Colorado River turns in a southwest direction and eventually reaches the Pacific Ocean. The Cache la Poudre River headwaters are located on the north and northeast side of the continental divide and flow in a northwest, northwest, north, and northeast direction to Kinikinik near the north edge of the northwest quadrant of figure 2. From Kinikinik the Cache la Poudre River flows in an east direction to the east edge of the green colored area and then in a southeast direction to the east edge of figure 2 (east of Fort Collins). East of figure 2 the Cache la Poudre River turns to flow in an east direction to join the southeast and northeast oriented South Platte River. The South Fork Cache la Poudre River originates in the Mummy Range near the north margin of Rocky Mountain National Park and flows in a northeast direction to join the east oriented Cache la Poudre River near the north center edge of figure 2. The Big Thompson River originates near the Cache la Poudre headwaters at Milner Pass and flows in a southeast, east, and northeast direction to Estes Park. From Estes Park the Big Thompson River flows in a northeast direction to Drake and then in an east-southeast direction to the east edge of figure 2 (near Loveland). East of figure the Big Thompson River turns to flow in a southeast direction to enter the northeast oriented South Platte River valley near where the South Platte River turns from flowing in a north direction to flowing in a northeast direction. Fall River is a southeast oriented Big Thompson River tributary originating south of the north oriented Cache la Poudre River segment in Rocky Mountain National Park. The North Fork Big Thompson River originates in the northeast quadrant of Rocky Mountain National Park (south of the South Fork Cache la Poudre River headwaters) and flows in a northeast, east-southeast, and southeast direction to join the Big Thompson River near Drake. Headward erosion of the northeast oriented South Fork Cache la Poudre River valley captured south and southeast oriented flood flow moving to the North Fork Big Thompson River valley and its tributary valleys. Headward erosion of the east oriented Cache la Poudre River valley beheaded south oriented flood flow channels to the newly eroded Big Thompson River and Fall River valleys and to the actively eroding south oriented Colorado River valley. Headward erosion of these valleys and flood flow captures occurred as floodwaters were flowing across emerging mountains, which contributed the erosion of deep valleys.

Cache la Poudre River-South Fork Cache la Poudre River drainage divide area

Figure 3: Cache la Poudre River-South Fork Cache la Poudre River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 3 provides a topographic map of Cache la Poudre River-South Fork Cache la Poudre River drainage divide area. The map contour interval for figure 3 is 50 meters. The Cache la Poudre River flows from the south edge of figure 3 (near southwest corner) in a north direction and then turns flows in a northeast direction across the northwest corner of figure 3. After a short jog north of figure 3 the Cache la Poudre River flows in an east direction near the north edge of figure 3 to the northeast corner. East of figure 3 the Cache la Poudre River flows in an east, southeast, and east direction to join the southeast and northeast oriented South Platte River. The South Fork Cache la Poudre River flows in a northeast direction across the southeast corner of figure 3. East of figure 3 the South Fork flows in a northeast direction to join the east oriented Cache la Poudre River. The north end of the Mummy Range is located between the north and northeast oriented Cache la Poudre River near the west edge of figure 3 and the northeast oriented South Fork Cache la Poudre River seen near the east edge of figure 3. Tributaries from the Mummy Range to the north and northeast oriented Cache la Poudre River segments are oriented in northwest directions while tributaries from the Mummy Range to the northeast oriented South Fork Cache la Poudre River are oriented in southeast and east-southeast directions. For example Sheep Creek is a north-northwest oriented tributary with East and West Forks and flows to the northeast oriented Cache la Poudre River segment. Beaver Creek originates in the Mummy Range and flows in an east-southeast direction to join the South Fork Cache la Poudre River near the south edge of figure 3. The northwest oriented West Fork Sheep Creek valley is linked by a pass with the east-southeast oriented Beaver Creek valley. The pass elevation is between 3350 and 3400 meters. Elevations north and east of the pass rise to 3547 meters and elevations south and west of the pass rise to 3615 meters suggesting the pass is 150 meters deep or deeper. The pass is really a drainage divide on what was once a through valley, which had been eroded by southeast oriented flood flow moving to the east-southeast oriented Beaver Creek valley. At that time the Cache la Poudre River valley to the northwest did not exist and elevations west and north of the pass were at least as high as the pass elevation. The deep northeast oriented South Fork Cache la Poudre River valley had eroded headward from the actively eroding east oriented Cache la Poudre River valley to capture southeast oriented flood flow moving across the emerging Mummy Range. Headward erosion of the Cache la Poudre River valley then beheaded the southeast oriented flood flow routes to the actively eroding Beaver Creek valley and floodwaters on northwest ends of the beheaded flood flow routes reversed flow direction to flow to the deeper Cache la Poudre River valley and to create the northwest oriented Sheep Creek drainage route. Next headward erosion of the northeast and east oriented Cache la Poudre River valley beheaded a south oriented flood flow channel on the west side of the emerging Mummy Range. Floodwaters on the north end of the beheaded flood flow channel reversed flow direction to create the north oriented Cache la Poudre River drainage route seen along the west edge of figure 3.

Detailed map of Sheep Creek-Beaver Creek drainage divide area

Figure 4: Detailed map of Sheep Creek-Beaver 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 Sheep Creek-Beaver Creek drainage divide area seen in less detail in figure 5. The map contour interval for figure 4 is 40 feet. Beaver Creek originates in section 8 and flows in a northeast direction before turning to flow in an east-southeast direction to Comanche Lake and the east edge of figure 4 (near southeast corner). East of figure 4 Beaver Creek flows to the northeast oriented South Fork Cache la Poudre River. A Beaver Creek tributary originates in the northeast corner of section 4 and flows in a southeast direction to Browns Lake and then in a southeast, east, and south direction to join Beaver Creek. The East Fork Sheep Creek originates north of Browns Lake and south of Crown Point and flows in a southwest direction before turning in a northwest direction to flow to the north edge of figure 4 (west half). West Fork Sheep Creek flows in a north-northeast and north-northwest direction in section 5 and then flows in a north-northwest direction to the north edge of figure 4 (near northwest corner). North of figure 4 the East and West Forks join to form north-northwest oriented Sheep Creek, which then flows to the northeast and east oriented Cache la Poudre River. A pass in the southeast quadrant of section 5 links the north-northwest oriented West Fork Sheep Creek valley with the east-southeast oriented Beaver Creek valley. The pass floor elevation is between 11,000 and 11,040 feet. A second pass just north of the northeast corner of section 4 links the northwest oriented East Fork Sheep Creek valley with the southeast oriented Beaver Creek tributary valley. The second pass floor elevation is between 11,160 and 11,200 feet. Elevations just north of section 2 rise to 11,631 feet while elevations in section 17 near the south edge of figure 4 rise to 11,859 feet. These elevations suggest the section 5 pass is approximately 600 feet deep and the second pass is more than 400 feet deep. These parallel and adjacent passes were eroded by diverging and converging southeast oriented flood flow channels at a time when elevations west and north of the passes were at least as high as the pass elevations (which means the deep Cache la Poudre River valley had not eroded headward into the region north and west of figure 4). Headward erosion of the deep Cache la Poudre River valley beheaded the southeast oriented flood flow channels and floodwaters on the northwest ends of the beheaded flood flow channels reversed flow direction to flow to the much deeper Cache la Poudre River valley and to create the northwest oriented Sheep Creek drainage route.

Cache la Poudre River-North Fork Big Thompson River drainage divide area

Figure 5: Cache la Poudre River-North Fork Big Thompson River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 5 illustrates the Cache la Poudre River-North Fork Big Thompson River drainage divide area south and slightly east of figure 3 and there is an overlap area with figure 3. The map contour interval for figure 5 is 50 meters. The Cache la Poudre River flows in a north, northwest, and north direction from the south edge of figure 5 (west half) to the north edge of figure 5 (near northwest corner). North of figure 5 the Cache la Poudre River turns to flow in a northeast and then east direction and eventually turns to flow in a southeast direction before joining the South Platte River. Comanche Peak is located near the center of figure 5 and Mummy Pass is located south of Comanche Peak. The South Fork Cache la Poudre River originates south of Mummy Pass and flows in a north direction to near Mummy Pass and then in a northeast direction to the northeast corner of figure 5. North and east of figure 5 the South Fork flows to the east and southeast oriented Cache la Poudre River. Rowe Peak is located south of Mummy Pass and Hagues Peak is south of Rowe Peak. The North Fork Big Thompson River originates between Rowe Peak and Hagues Peak and flows in a northeast, east, and east-southeast direction to the east edge of figure 5 (south half). East of figure 5 the North Fork Big Thompson River turns to flow in a southeast direction to join the Big Thompson River. Icefield Pass is located east and slightly south from Mummy Pass. While many other passes are visible in figure 5 this essay will focus on the history of Mummy Pass and Icefield Pass. Today Mummy Pass links the northeast oriented South Fork Cache la Poudre valley with the west oriented Mummy Pass Creek valley, which drains to the west oriented Hague Creek valley, which in turn drains to the northwest and north oriented Cache la Poudre River valley. Mummy Pass has an elevation of between 3400 and 3450 meters. Comanche Peak to the north rises to 3872 meters and Hagues Peak to the south rises to 4133 meters. These elevations suggest Mummy Pass is at least 420 meters deep. While the region has been glaciated and the glaciers probably have deepened and otherwise altered some of the valleys the glaciers did not erode new valleys, which means the Mummy Pass valley is a water eroded valley. The Mummy Pass valley was eroded by east oriented flood flow diverging from a south oriented flood flow channel on the present day north oriented Cache la Poudre River alignment and moving floodwaters to what at that time was the actively eroding northeast oriented South Fork Cache la Poudre River valley. Icefield Pass provides evidence of an earlier stage when the east oriented flood flow moved to the actively eroding east and southeast North Fork Big Thompson River valley. Headward erosion of the much deeper northeast oriented South Fork Cache la Poudre River valley captured the east oriented flood flow that had been moving to the North Fork Big Thompson River valley.

Detailed map of Mummy Pass Creek-North Fork Big Thompson River drainage divide area

Figure 6: Detailed map of Mummy Pass Creek-North Fork Big Thompson River 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 Mummy Pass Creek-North Fork Big Thompson River drainage divide area seen in less detail in figure 5. The map contour interval for figure 6 is 40 feet. Hagues Peak is located near the south center edge of figure 6 and Rowe Peak is located north of Hagues Peak. Rowe Glacier is located between Rowe Peak and Hagues Peak and the North Fork Big Thompson River originates at Rowe Glacier and then flows in a northeast and east-southeast direction to the east edge of figure 6 (south of center). East of figure 6 the North Fork Big Thompson River turns to flow in a southeast direction to join the Big Thompson River. Icefield Pass is located west of the elbow of capture where the northeast oriented North Fork Big Thompson River turns to flow in an east-southeast direction. Mummy Pass is located west of Icefield Pass and is slightly north and west of the center of figure 6. The north oriented drainage route flowing just east of Mummy Pass and then flowing in a northeast and north direction to the north edge of figure 6 (near northeast corner) is the South Fork Cache la Poudre River. North and east of figure 6 the South Fork Cache la Poudre River flows in a northeast direction to join the east and southeast oriented Cache la Poudre River. Mummy Pass Creek flows in a west direction from Mummy Pass and then turns in a south-southwest direction to enter the much deeper north-northwest and west oriented Hague Creek valley. West of figure 6 Hague Creek flows to the north, northwest, north, northeast, and east oriented Cache la Poudre River. Mummy Pass has an elevation of between 11,240 and 11,280 feet. Fall Mountain to the north rises to 12,258 feet and Comanche Peak (north of figure 6) rises to 12,702 feet. Hague Peak to the south rises to 13,500 feet. These elevations suggest Mummy Pass is approximately 1400 feet deep. While the region in figure 6 has been glaciated with valley glaciers deepening and otherwise altering many of the valleys the glaciers filled existing valleys and did not erode new valleys. In other words, while Mummy Pass has probably been altered by glacial activity the Mummy Pass through valley was present before the Mummy Range was glaciated. The same can be said for Icefield Pass and Flint Pass (south of Mummy Pass) and probably for most other passes seen in figure 6. Perhaps the most intriguing pass is the pass between Rowe Peak and Hagues Peak at the top of Rowe Glacier. Compared to Mummy Pass the Rowe Glacier Pass is shallow (perhaps 200 feet deep) and what looks like a deep cirque is located on the west side, yet that pass links the northeast oriented North Fork Big Thompson River headwaters valley with the northwest and oriented Hague Creek valley. An argument could be made the Rowe Glacier pass was formed by glacial erosion, but an argument could also be made the Rowe Glacier pass originated as a water-eroded valley at a time when floodwaters were flowing on an erosion surface that has since been almost completely removed as floodwaters deeply eroded the surrounding region and as crustal warping raised the Mummy Range. In either case Mummy Range glaciation did not occur until after flood flow across the region had ended and until after the Mummy Range had been uplifted to become the high mountain range it is today.

Cache la Poudre River-Fall River drainage divide area

Figure 7: Cache la Poudre River-Fall River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 7 illustrates the Cache la Poudre River-Fall River drainage divide area south and west of figure 5 and includes an overlap area with figure 5. The map contour interval for figure 7 is 50 meters. The Larimer-Grand County line near extends in an east and south direction from near the west edge of figure 7 (north half) to the south edge of figure 7 (west half) and is located along the east-west continental divide. The south oriented drainage route in Grand County, which originates on the south side of La Poudre Pass (in northwest quadrant of figure 7) and which flows to the southwest corner of figure 7, is the Colorado River. South and west of figure 7 the Colorado River turns in a southwest direction and eventually reaches the Pacific Ocean. The Cache la Poudre River originates near Milner Pass (in the southwest quadrant of figure 7 where highway crosses the continental divide) and flows in a northeast and north direction to the north edge of figure 7 (slightly west of center). The northeast oriented stream on the north side of La Poudre Pass is La Poudre Pass Creek, which joins the north oriented Cache la Poudre River just north of the north edge of figure 7 (west of center). La Poudre Pass and Milner Pass are deep through valleys eroded across the continental divide and link the north oriented Cache la Poudre River headwaters with the south oriented Colorado River headwaters. The passes were eroded by diverging flood flow channels on the present day north oriented Cache la Poudre River headwaters alignment, which converged again in the south oriented Colorado River valley south and west of Milner Pass. Headward erosion of the east oriented Cache la Poudre River valley north of figure 7 beheaded the south oriented flood flow channels and floodwaters on north ends of the diverging flood flow channels reversed flow direction to flow to the deeper east oriented Cache la Poudre River valley. Chapin Creek is a north oriented Cache la Poudre River tributary near the center of figure 7 and originates just north of Chapin Pass. The east-southeast oriented drainage route south of Chapin Pass is the Fall River, which originates near Fall River Pass (west of Chapin Pass) and which flows to the east edge of figure 7. East of figure 7 the Fall River flows to the Big Thompson River. Chapin Pass has an elevation of between 3350 and 3400 meters. Mount Chapin to the east rises to 3796 meters and an unnamed mountain west of Chapin Pass rises to more than 3750 meters. These elevations suggest Chapin Pass is at least 350 meters deep. Chapin Pass was eroded by south oriented flood flow moving from the present day north oriented Cache la Poudre River alignment to the east-southeast oriented Fall River valley. Forest Canyon is a deep southeast oriented valley draining to the south center edge of figure 7 and is drained by the southeast oriented Big Thompson River headwaters. The Big Thompson River originates near Forest Canyon Pass, which is located a short distance north and east of Milner Pass. Forest Canyon Pass is another deep pass linking the north oriented Cache la Poudre River headwaters valley with a southeast oriented drainage route, although Forest Canyon Pass history is more complex than the drainage histories of Chapin, Milner, and La Poudre Passes.

Detailed map of Chapin Creek-Fall River drainage divide area

Figure 8: Detailed map of Chapin Creek-Fall River 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 Chapin Creek-Fall River drainage divide area seen in less detail in figure 7. The map contour interval for figure 8 is 40 feet. The east-west continental divide is the dashed line extending from the north edge of figure 8 (west half) to the south edge of the figure 8 west half). The Cache la Poudre River originates at Poudre Lake (north and east of continental divide in the southwest quadrant of figure 8) and then flows in a northeast direction to the north edge of figure 8 (east of center). North of figure 8 the Cache la Poudre River turns to flow in a north, northwest, north, northeast, east, southeast, and east direction to join the southeast and northeast oriented South Platte River with water eventually reaching the Gulf of Mexico. Forest Canyon Pass is a short distance north and east of the Cache la Poudre River headwaters. The Big Thompson River originates near Forest Canyon Pass and flows in a southeast direction to the south edge of figure 8 (east of center). South and east of figure 8 the Big Thompson River flows in east, northeast, east, and southeast directions to reach the northeast, southeast, and northeast oriented South Platte River. Fall River Pass is located north and east of the center of figure 8 and Fall River originates on the southeast side of Fall River Pass. Fall River flows in an east-southeast, southeast, and east direction to the east edge of figure 8 (south of center) and east of figure 8 joins the northeast, east, and southeast oriented Big Thompson River. Chapin Pass is east of Fall River Pass and north and northwest oriented Chapin Creek originates north of Chapin Pass and flows to the north edge of figure 8 (east half). North of figure 8 Chapin Creek joins the north oriented Cache la Poudre River. Chapin Pass has an elevation of between 11,120 and 11,160 feet, Fall River Pass has an elevation of 11,796 feet, and Forest Canyon Pass has an elevation of between 11,280 and 11,320 feet. Milner Pass (at the southwest end of Poudre Lake and west of figure 8) has an elevation of 10,758 feet and links the northeast oriented Cache la Poudre River headwaters with south-southwest oriented Beaver Creek, which south and west of figure 8 flows to the south and southwest oriented Colorado River. Using elevations located south of figure 8 Milner Pass is approximately 2000 feet deep and is approximately 400 feet deeper than Chapin Pass and more than 500 feet deeper than Forest Canyon Pass, which is almost 500 feet deeper than Fall River Pass. Forest Canyon Pass and Fall River Pass link the northeast oriented Cache la Poudre River valley with southeast oriented valleys and were probably eroded by southeast oriented flood flow channels prior to headward erosion of a deeper southwest oriented flood flow channel on the present day northeast oriented Cache la Poudre River alignment, which beheaded the southeast oriented flood flow channels and diverted floodwaters in a southwest direction to the actively eroding Colorado River valley. The region was being uplifted as floodwaters flowed across it and floodwaters eroded deeper and deeper valleys into the rising mountain masses until headward erosion of deeper valleys beheaded the flood flow channels and in the case of the Cache la Poudre River and Chapin Creek caused flood flow reversals to create north oriented drainage routes.

Fall River-Big Thompson River drainage divide area

Figure 9: Fall River-Big Thompson River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 9 illustrates the Fall River-Big Thompson River drainage divide area south and east of figure 7 and there is an overlap area with figure 7. The map contour interval for figure 9 is 50 meters. Chapin Creek originates near Chapin Pass and flows in a north direction near the northwest corner of figure 9. North of figure 9 Chapin Creek flows to the north, east, and southeast oriented Cache la Poudre River, which flows to the southeast and northeast oriented South Platte River. The Big Thompson River flows in a southeast direction from the west center edge of figure 9 through Forest Canyon before turning to flow in an east and northeast direction to Estes Park and the east center edge of figure 9. East of figure 9 the Big Thompson River flows in a northeast, east-southeast, east, and southeast direction to join the northeast, southeast, and northeast oriented South Platte River. The Fall River flows from the west edge of figure 9 (west of Chapin Pass) in an east-southeast direction to join the Big Thompson River near Estes Park. Fish Creek is the north-northeast oriented stream flowing from the south edge of figure 9 (east half) to join the Big Thompson River at Estes Lake. In the southwest quadrant of figure 9 there are many erosional residuals surrounded by present day drainage routes and/or through valleys which link the present day drainage routes. Several of these erosional residuals are named and include Deer Mountain, Eagle Cliff Mountain, Prospect Mountain, Castle Mountain, and many others. Some of these erosional residuals rise more than 300 meters above the surrounding valley floors. The maze of valleys between these erosional residuals was eroded by a former anastomosing complex of flood flow channels that eroded headward into the region as the Big Thompson River valley and its tributary valleys beheaded south and southeast oriented flood flow channels flowing across the region. The landscape is complicated because it was probably eroded by an east oriented anastomosing channel complex that was capturing a south oriented anastomosing channel complex and was subsequently further altered by valley glaciers in the mountain valleys. The South Lateral Moraine and Moraine Park on the south side of the Big Thompson River (north of the south center edge of figure 9) provide evidence the valley glaciers extended eastward in the Big Thompson River valley to at least that location. The valley glaciers did fill and modify pre-existing valleys, but the valleys had been eroded by flood flow prior to the glaciation. Glaciation occurred after all flood flow across the region had ended and after the mountains had been uplifted to create the high mountains seen today.

Detailed map of Fall River-Big Thompson River drainage divide area

Figure 10: Detailed map of Fall River-Big Thompson River 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 Fall River-Big Thompson River drainage divide area seen in less detail in figure 9. The map contour interval for figure 10 is 40 feet. The Big Thompson River flows in an east direction from the west edge of figure 10 (near southwest corner) through Moraine Park and south of Eagle Cliff Mountain turns to flow in a northeast direction to the east edge of figure 10 (slightly south of center). East of figure 10 the Big Thompson River flows in a northeast, east-southeast, east, and southeast direction to join the northeast, southeast, and northeast oriented South Platte River. The Fall River flows in a southeast and east direction from the north edge of figure 10 (west half) through Horseshoe Park to the north center edge of figure 10 (north of Deer Mountain) and then turns to flow in a southeast direction to the east edge of figure 10 (slightly north of center) and joins the Big Thompson River east of figure 10. Large erosional residuals seen in figure 10 include Deer Mountain, Eagle Cliff Mountain, Castle Mountain, and Oldman Mountain and are bounded by present day drainage routes and/or through valleys linking present day drainage routes. For example a north to south oriented through valley west of Deer Mountain links the Fall River valley with east-southeast oriented Beaver Brook, which flows to the northeast oriented Big Thompson River. A through valley west of Eagle Cliff Mountain links the Beaver Brook valley with the east oriented Big Thompson River valley. The through valley west of Deer Mountain crosses Deer Ridge and has an elevation of between 8960 and 9000 feet at the drainage divide. Deer Mountain to the east rises to 10,000 feet and the mountains to the west rise even higher suggesting the Fall River-Beaver Creek through valley is at least 1000 feet deep. Horseshoe Park to the north of Deer Ridge has an elevation of between 8480 and 8520 feet, which suggests the Fall River valley floor was lowered by approximately 500 feet since the time south oriented flood flow moved across Deer Ridge or that Deer Ridge is a depositional feature (e.g. lateral moraine) formed after the through valley had been eroded. Lateral moraines are definitely present on both sides of the Big Thompson River valley in Moraine Park to the south. South Lateral Moraine is located along the south edge of figure 10 and the north lateral moraine is located between Beaver Brook and Moraine Park and extends across the north to south oriented through valley west of Eagle Cliff Mountain. Elevations in Beaver Meadows to the north of the north lateral moraine are between 8200 and 8400 feet while elevations in Moraine Park are slightly greater than 8000 feet. Eagle Cliff Mountain rises to 8906 feet suggesting the north to south oriented through valley is between 500 and 700 feet deep depending on where and how it is measured. The north to south oriented through valley west of Eagle Cliff Mountain and of Deer Mountain was eroded by south oriented flood flow prior to headward erosion of the Big Thompson River valley, which was prior to headward erosion of the Beaver Brook valley, which was prior to headward erosion of the Fall River valley. The region seen in figure 10 was deeply eroded as deep valleys eroded headward to capture immense south oriented floods moving across the emerging mountain mass. Valley glaciers did not fill the valleys until after all flood flow across the region had ended and all valleys had been eroded and until after the high mountains had fully emerged.

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.