Abstract:

This essay uses topographic map evidence to interpret landform origins in the South St Vrain Creek-Boulder Creek drainage divide area in the Colorado Front Range. South St Vrain Creek originates near the east-west continental divide and flows in an east-northeast and northeast direction to join southeast and northeast oriented St Vrain Creek, which east of the Front Range flows to the north, northeast, southeast, and northeast oriented South Platte River. North Boulder Creek is located south of South St Vrain Creek, originates near the continental divide, and flows in a southeast and east-northeast direction to join Middle Boulder Creek and to form east oriented Boulder Creek, which east of the Front Range turns in a northeast direction to join northeast oriented St Vrain Creek. Fourmile Creek is an east, northeast, and southeast oriented Boulder Creek tributary located north of North Boulder Creek. Left Hand Creek originates near the continental divide and flows in an east-northeast, northeast, east, north, southeast, and northeast direction to St Vrain Creek and is located north of Fourmile Creek and south of South St Vrain Creek. James Creek is an east-northeast and east-southeast oriented Left Hand Creek tributary located north of Left Hand Creek. Headwaters of these east oriented streams are surrounded by mountain peaks rising to elevations of almost 4000 meters. East of the high mountains crests of Front Range drainage divides between the east-oriented valleys are less than 2800 meters and suggest the valleys were eroded into a former erosion surface. East of the Front Range on the Colorado Piedmont elevations are generally less than 1700 meters. Shallow north-to-south oriented through valleys cross drainage divides between the east oriented stream valleys and provide evidence of diverging and converging south oriented flood flow channels that were captured as the east oriented valleys eroded headward (in sequence from south to north) toward the present day continental divide. The shallow north-to-south oriented through valleys are present both in the Colorado Front Range and on the Colorado Piedmont east of the Front Range. Floodwaters were derived from the western margin of a thick North American ice sheet and flowed from western Canada to and across Colorado at a time when Colorado mountain ranges were beginning to emerge. The mountain ranges emerged as floodwaters flowed across them, as ice sheet related crustal warping raised the mountain masses and the entire region, and as deep valleys eroded headward into the region (from both the east and the west). The east-west continental divide marks the boundary between south oriented flood flow channels captured by headward erosion of the southwest oriented Colorado River valley and by headward erosion of the southeast and northeast oriented South Platte River valley. The north oriented South Platte River drainage route on the Colorado Piedmont was created by a reversal of south oriented flood flow, which also created northeast oriented Boulder Creek and St Vrain Creek and tributary drainage routes on the Colorado Piedmont as floodwaters reversed their flow direction to flow to deeper southeast and northeast oriented South Platte River valley.

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 South St Vrain Creek-Boulder Creek drainage divide area landform origins in the Colorado Front 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 South St Vrain Creek-Boulder Creek drainage divide area landform evidence in the Colorado Front Range will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm.

South St Vrain Creek-Boulder Creek drainage divide area location map

Figure 1: South St Vrain Creek-Boulder Creek 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 South St Vrain Creek-Boulder Creek drainage divide area in the Colorado Front Range and illustrates a region in north central Colorado. The Colorado Front Range is located just west of the cities of Fort Collins, Boulder, and Golden and the Colorado Piedmont is immediately east of the Front Range. The South Platte River flows in a north-northeast and north direction from the south edge of figure 1 (east half) to Denver and then to Milliken where it turns in a northeast and then southeast direction to flow to the east edge of figure 1 (north half). East of figure 1 the South Platte River turns to flow in a northeast direction to reach the Platte River in western Nebraska. Rocky Mountain National Park is located in the mountains near the center of figure 1. The Big Thompson River originates in Rocky Mountain National Park and flows in a southeast, northeast, and east-southeast direction to join the South Platte River near Milliken. The Colorado River originates in the northwest quadrant of Rocky Mountain National Park and then flows in a south direction to Lake Granby before turning in a southwest direction to eventually reach the Pacific Ocean. The east-west continental divide is located east of the south oriented Colorado River in Rocky Mountain National Park and then extends in roughly a south direction to Berthoud Pass and Loveland Pass and then to the south edge of figure 1. The unlabeled stream located south of the Big Thompson River and flowing in a southeast direction from Lyons to Longmont before turning in a northeast direction to join the South Platte River is St Vrain Creek. St Vrain Creek headwaters include North St Vrain Creek, Middle St Vrain Creek, South St Vrain Creek, and Boulder Creek with Boulder Creek being the east and northeast oriented St Vrain Creek tributary flowing near the city of Boulder. These major St Vrain Creek headwaters streams all originate near the east-west continental divide. South St Vrain Creek originates near the east west continental divide just south of Rocky Mountain National Park and flows in an east-northeast and northeast direction to join North St Vrain Creek near Lyons with Middle St Vrain Creek being a tributary to South St Vrain Creek. The tributary located between South St Vrain Creek and Boulder Creek and joining St Vrain Creek near Longmont is Left Hand Creek. The South St Vrain Creek-Boulder Creek drainage divide area investigated in this essay is located east of the continental divide, north and west of Boulder Creek, and south and east of South St Vrain Creek. The Colorado River-North St Vrain Creek drainage divide area essay illustrates and discusses regions along the continental divide.

Drainage routes in the Colorado Front Range and on the Colorado Piedmont developed during immense melt water floods from the western margin of a thick North American ice sheet. Floodwaters flowed from western Canada to and across Colorado at a time when the Front Range was beginning to emerge. The Front Range emerged as south oriented floodwaters flowed across it, as ice sheet related crustal warping raised the mountain mass and the entire region, and as deep valleys eroded headward into Colorado from the Gulf of Mexico in the east and from the Pacific Ocean in the west. Initially floodwaters moved in south directions, but flood flow directions were changed as crustal warping raised the mountains and as headward erosion of deep valleys captured and beheaded diverging and converging flood flow channels. The present day north oriented South Platte River drainage route and its north oriented tributaries on the Colorado Piedmont originated as south oriented flood flow channels, with floodwaters flowing to the southeast oriented Arkansas River valley south of figure 1. East oriented valleys eroded headward in sequence from south to north from these south oriented flood flow channels to capture south oriented flood flow in the emerging Colorado Front Range. At the same time the deep southwest oriented Colorado River valley eroded headward into the emerging mountains to also capture the south oriented flood flow. The east-west continental divide between Berthoud Pass and the Big Thompson River headwaters marks the boundary between south oriented flood flow channels captured by headward erosion of east oriented valleys from the present day Colorado Piedmont and the south oriented flood flow channels captured by headward erosion of the southwest oriented Colorado River valley. As the east oriented valleys were eroding headward into the emerging Front Range headward erosion of the deep southeast and northeast oriented South Platte River valley from western Nebraska beheaded the south oriented flood flow channels on the Colorado Piedmont. Floodwaters on north ends of the beheaded flood flow channels reversed flow direction to flow to the much deeper southeast and northeast oriented South Platte River valley. Because flood flow channels were beheaded in sequence from east to west and because the flood flow channels were anastomosing the newly reversed flood flow channels could capture yet to be beheaded flood flow in flood flow channels further to the west. Such captures of yet to be beheaded flood flow helped create significant north oriented South Platte River tributary drainage routes. The reversal of flood flow on the north-northeast and north oriented South Platte River drainage route captured significant south oriented flood flow still flowing in the emerging Front Range and that captured flood flow once on the Colorado Piedmont turned to flow in northeast and north directions to join the newly reversed flood flow route.

Detailed location map for South St Vrain Creek-Boulder Creek drainage divide area

Figure 2: Detailed location map of South St Vrain Creek-Boulder Creek 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 South St Vrain Creek-Boulder Creek drainage divide area in the Colorado Front Range. County lines are shown and Gilpin and Boulder Counties are labeled. The east-west continental divide follows the west boundary of Boulder and Gilpin Counties from the north edge of figure 2 (west half) and then continues in a south and southwest direction to Berthoud Pass near the southwest corner of figure 2. The red-brown region near the northwest corner of figure 2 is located in Rocky Mountain National Park. Green colored are National Forest lands located in the mountain regions. North St Vrain Creek originates just east of the continental divide in the southeast corner of Rocky Mountain National Park and flows in an east and northeast direction to the north center edge of figure 2 and north of figure 2 turns to flow in a southeast direction. Middle St Vrain Creek originates near the continental divide and just south of Rocky Mountain National Park and flows in a southeast and east direction to Peaceful Valley and then in a northeast direction to Raymond before turning in an east direction to join northeast oriented South St Vrain Creek. South St Vrain Creek also originates near the continental divide and flows in an east and northeast direction to join southeast oriented North St Vrain Creek at the north edge of figure 2 (east of center) and to form St Vrain Creek, which then flows in a southeast direction to Longmont before turning in an east and northeast direction to flow to northeast corner of figure 2. North and east of figure 2 St Vrain Creek flows to the north, northeast, southeast, and northeast oriented South Platte River. Left Hand Creek originates near the continental divide (north of Kiowa Creek and south of the South St Vrain Creek headwaters) and flows in an east-northeast direction to Jamestown before turning in a southeast, northeast, southeast, and northeast direction to join St Vrain Creek near Longmont. Boulder Creek has several tributaries converging near the city of Boulder and then flows in an east-northeast and northeast direction to join St Vrain Creek east of Longmont. The northernmost Boulder Creek tributary is east and southeast oriented Fourmile Creek. North Boulder Creek originates near the continental divide (south of Kiowa Peak) and flows in an east, southeast, and east-northeast direction to where it converges with other tributaries to form east oriented Boulder Creek. Middle Boulder Creek flows in an east and east-northeast direction through the towns of Eldora and Nederland before joining North Boulder Creek to form Boulder Creek. South Boulder Creek originates in the far west region of Gilpin County and flows in a northeast direction to East Portal and then in an east direction to Eldorado Springs before turning in a north direction to join Boulder Creek east of Boulder. The north oriented “River” west of the continental divide is the Fraser River, which is flowing on the alignment of a south oriented flood flow channel, which was beheaded and reversed by headward erosion of the much deep southwest oriented Colorado River valley.

South St Vrain Creek-North Boulder Creek drainage divide area

Figure 3: South St Vrain Creek-North Boulder Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 3 provides a topographic map of South St Vrain Creek-North Boulder Creek drainage divide area near the continental divide. The map contour interval for figure 3 is 50 meters. The east-west continental divide follows the crest of the high mountains near the west edge of the northwest quadrant of figure 3. South St Vrain Creek originates near the continental divide (south of Shoshoni Peak) and flows in an east-northeast and northeast direction to the north center edge of figure 3. North of figure 3 South St Vrain Creek flows in a northeast direction to join southeast oriented North St Vrain Creek and to form southeast and northeast oriented St Vrain Creek. Duck Lake is located south of the north center edge of figure 3. James Creek originates at Duck Lake and flows in an east-northeast direction to the north edge of figure 3 (east half). North of figure 3 James Creek turns in an east-southeast direction to join Left Hand Creek. Left Hand Creek originates at Left Hand Park Reservoir (in northwest quadrant of figure 3) and flows in an east-northeast, southeast, and east-northeast direction to the east edge of figure 3 (north half). East of figure 3 Left Creek flows in a northeast, north, southeast, and northeast direction to join St Vrain Creek. Fourmile Creek originates in the west center region of figure 3 and flows in a southeast, northeast, east, and northeast direction to the east edge of figure 3 (north of center). East of figure 3 Fourmile Creek turns in a southeast direction to join east and northeast oriented Boulder Creek. North Boulder Creek originates near the continental divide and flows in a southeast and east-northeast direction before turning in a southeast direction near Comforter Mountain to join northeast oriented Middle Boulder Creek and to form east oriented Boulder Creek, which flows to the east edge of figure 3. East of figure 3 Boulder Creek turns to flow in a northeast direction to join northeast oriented St Vrain Creek, which then joins the South Platte River. Middle St Vrain Creek flows in a northeast direction from the south center edge of figure 3 to join North Boulder Creek near Comforter Mountain. The North Fork Middle Boulder Creek flows in a southeast and south direction across the southwest corner of figure 3. A highway extends from the north center edge of figure 3 to the south center edge of figure 3. Elevations along the continental divide and the adjacent high ridges west of the highway exceed 3500 meters and in places exceed 4000 meters. East of the highway elevations along the drainage divides are much lower and are generally in the 2600-2800 meter range. The consistency of elevations along the drainage divides suggests the east oriented valleys eroded headward into a former erosion surface. A close look at that former erosion surface reveals evidence for shallow north-to-south oriented flood flow channels that were captured in sequence (from south to north) by headward erosion of the deeper east oriented valleys. An example of evidence for such a north-to-south oriented flood flow channel is found between Bald Mountain and Sugarloaf Mountain on the Fourmile Creek-North Boulder Creek drainage divide. Figure 4 provides a detailed topographic map of that region to better illustrate the evidence.

Detailed map of Fourmile Creek-North Boulder Creek drainage divide area

Figure 4: Detailed map of Fourmile Creek-North Boulder 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 Fourmile Creek-North Boulder Creek drainage divide area seen in less detail in figure 5. The map contour interval for figure 4 is 40 feet. Fourmile Creek flows in an east direction from the northwest corner of figure 4 to the northeast corner of figure 4. East of figure 4 Fourmile Creek turns in a southeast direction to join Boulder Creek. North Boulder Creek flows in a northeast and east direction across the southeast corner of figure 4. East of figure 4 North Boulder Creek turns in a southeast direction (with some significant twists and turns) to join northeast oriented Middle Boulder Creek and to form east oriented Boulder Creek. Gordon Gulch is the southeast, east, southeast, and south oriented North Boulder Creek tributary draining from section 29 along the west edge of figure 4. Bald Mountain is a high point in northeast corner of section 28 and reaches an elevation of 9147 feet. Sugarloaf Mountain is a high point in section 26 and reaches an elevation of 8917 feet. Farewell Gulch is a south oriented Gordon Gulch tributary originating in section 27 and is linked by a through valley or pass with the valley of a northeast oriented Fourmile Creek tributary. The through valley floor elevation is between 8440 and 8480 feet, which is at least 400 feet lower than the top of Sugarloaf Mountain. East of the through valley two more north-to-south oriented through valleys link the Fourmile Creek valley with valleys of south oriented North Boulder Creek tributaries. These three north-to-south oriented through valleys or passes were eroded by diverging and converging south oriented flood flow channels prior to headward erosion of the east oriented Fourmile Creek valley. Floodwaters were first captured by headward erosion of the North Boulder Creek valley and its tributary Gordon Gulch valley. Headward erosion of the east oriented Fouurmile Creek valley then beheaded the south oriented flood flow channels in sequence from east to west and ended flood flow to the North Boulder Creek valley and its tributary valleys. In the northwest quadrant of section 28 west of Bald Mountain a through valley links the north oriented Bear Gulch valley with a southeast oriented Gordon Gulch tributary valley. The through valley floor elevation is between 8600 and 8640 feet. Approximately four sections (miles) to the west elevations exceed the Bald Mountain elevation suggesting there is a broad north-to-south oriented through valley linking the Fourmile Creek valley with the North Boulder Creek valley. The broad through valley at its deepest points is at least 500 feet deep. These through valleys are evidence the east oriented valleys and their tributary valleys eroded headward across diverging and converging south oriented flood flow channels, which were eroding the present day high-level erosion surface seen in the Colorado Front Range.

South St Vrain Creek-Left Hand Creek drainage divide area

Figure 5: South St Vrain Creek-Left Hand Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 5 illustrates the South St Vrain Creek-Left Hand Creek drainage divide area north and east of figure 3 and there is an overlap area with figure 3. The map contour interval for figure 5 is 50 meters. The Colorado Front Range eastern flank can be seen along the east margin of figure 5. Niwot Mountain in the southwest corner of figure 5 has an elevation of 3496 meters and is an eastern extension of the high continental divide ridge west of figure 5 (and seen in figure 3). Elevations greater than 3000 meters are also found in the northwest corner region of figure 5. Elevations on the Colorado Piedmont east of figure 5 are generally less than 1700 meters. In between the continental divide high ridge elevations and the Front Range eastern flank elevations along Front Range drainage divides are generally in the 2500 to 2700 meters range again suggesting the east oriented valleys eroded headward into what is today an uplifted erosion surface. Left Hand Park Reservoir is located along the west edge of the southwest quadrant of figure 5. Left Hand Creek flows in an east-northeast and southeast direction from Left Hand Park Reservoir before turning in an east-northeast, northeast, and east direction to flow to the east margin of the Colorado Front Range where Left Hand Creek turns in a north direction before flowing an east direction to the east edge of figure 5 (north half). East of figure 5 Left Hand Creek flows in a southeast and northeast direction to join northeast oriented St Vrain Creek. South St Vrain Creek flows from the west edge of figure 5 (north of Left Hand Park Reservoir) in a northeast, east, north, and northeast direction to the north edge of figure 5 (east half). North and east of figure 5 South St Vrain Creek joins southeast oriented North St Vrain Creek to form southeast and northeast oriented St Vrain Creek. Duck Lake is located in the southwest quadrant of figure 5 between South St Vrain Creek and Left Hand Creek. James Creek originates at Duck Lake and flows in a east-northeast and east-southeast direction to join northeast and east oriented Left Hand Creek (in east center area of figure 5). Little James Creek is a southeast oriented tributary joining James Creek at its elbow of capture (where James Creek turns in an east-southeast direction). The highway in the west half of figure 5 extending from the north edge of figure 5 to the south edge of figure 5 is located in north-to-south oriented through valleys crossing drainage divides. Figure 6 provides a detailed topographic map of the region where the highway crosses South St Vrain Creek-James Creek drainage to better illustrate the through valley crossing that drainage divide.

Detailed map of South St Vrain Creek-James Creek drainage divide area

Figure 6: Detailed map of South St Vrain Creek-James 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 South St Vrain Creek-James Creek drainage divide area seen in less detail in figure 5. The map contour interval for figure 6 is 40 feet. South St Vrain Creek flows in an east-northeast and north-northeast direction from the southwest corner of figure 6 to the north edge of figure 6 and north of figure 6 turns to flow in an east-southeast direction across the northeast corner of figure 6. East and north of figure 6 South St Vrain Creek turns again to flow in a north and northeast direction to join southeast and northeast oriented St Vrain Creek. James Creek flows in an east-northeast and east direction from the south edge of figure 6 (in section 31) to the east edge of figure 6 (in section 33). East of figure 6 James Creek flows in an east-northeast and east-southeast direction to join Left Hand Creek, which then flows in a north, southeast, and northeast direction to join northeast oriented St Vrain Creek. A through valley in the north center area of section 32 links the north-northeast oriented South St Vrain Creek valley with the east oriented James Creek valley. The through valley floor elevation is between 8880 and 8920 feet (probably closer to 8880 feet). Elevations in the northwest quadrant of section 28 rise to 9096 feet with much higher elevations located west of the through valley. These elevations suggest the through valley is approximately 200 feet deep. The through valley was eroded by south oriented flood flow, which was flowing on the present day north-northeast oriented South St Vrain Creek alignment. The south oriented floodwaters were first captured by headward erosion of the east oriented James Creek valley. At that time the east-southeast oriented South St Vrain Creek valley in the northeast corner of figure 6 did not exist. Headward erosion of that east-southeast oriented valley (from a reversed flood flow channel on a north oriented South St Vrain Creek valley segment which had been beheaded and reversed by headward erosion of the northeast oriented South St Vrain Creek valley north and east of figure 6-see figure 5) next beheaded and reversed the south oriented flood flow in figure 6 to create the north-northeast oriented South St Vain Creek drainage route.

Left Hand Creek-Boulder Creek drainage divide area

Figure 7: Left Hand Creek-Boulder Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 7 illustrates the Left Hand Creek-Boulder Creek drainage divide area south of figure 5 and includes an overlap area with figure 5. The map contour interval for figure 7 is 50 meters. The city of Boulder is located along the eastern flank of the Colorado Front Range with the Colorado Piedmont to the east of figure 7. Middle Boulder Creek flows in a northeast direction from the south edge of figure 7 (west half) to near Comforter Mountain where it is joined by east-northeast and southeast oriented North Boulder Creek to form east oriented Boulder Creek, which then flows to the city of Boulder and the east edge of figure 7. East of figure 7 Boulder Creek turns to flow in a northeast direction to join northeast oriented St Vrain Creek, which then flows to the South Platte River. Fourmile Creek flows in an east direction from the west edge of figure 7 (slightly south of center) to Wallstreet and then in a northeast direction to Salina. From Salina Fourmile Creek turns to flow in a southeast direction to join Boulder Creek near Orodell. Left Hand Creek flows in a southeast direction from the west edge of figure 7 (north half) and then turns to flow in an east-northeast, northeast, east, and north direction to the north edge of figure 7 (east half). North of figure 7 Left Hand Creek turns to flow in a southeast and northeast direction to join St Vrain Creek. Elevations on the Colorado Piedmont east of the Front Range are generally less than 1700 meters while elevations on drainage divides seen in figure 7 range from 2400 to 2800 meters. The lack of really high mountains suggests the Front Range area seen in figure 7 is an uplifted erosion surface into which the east oriented valleys eroded. Study of drainage divides between the east oriented streams reveals additional evidence for shallow north-to-south oriented through valleys suggesting the east oriented valleys eroded headward in sequence (from south to north) across diverging and converging south oriented flood flow channels. The south oriented flood flow was probably responsible for creating the erosion surface, which was probably being uplifted as floodwaters flowed across it. Figure 8 provides a detailed topographic map of the Left Hand Creek-Fourmile Creek drainage divide area near Gold Hill to better illustrate evidence for former flood flow channels in that region.

Detailed map of Left Hand Creek-Fourmile Creek drainage divide area

Figure 8: Detailed map of Left Hand Creek-Fourmile 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 Left Hand Creek-Fourmile Creek drainage divide area seen in less detail in figure 7. The map contour interval for figure 8 is 40 feet. Fourmile Creek flows in an east, northeast, and southeast direction from near the southwest corner of figure 8 to near the southeast corner of figure 8. East of figure 8 Fourmile Creek flows in a southeast direction to join east and northeast oriented Boulder Creek, which joins northeast oriented St Vrain Creek. Left Hand Creek flows in an east-northeast direction from the west edge of figure 8 (north half) to the north center edge of figure 8. North and east of figure 8 Left Hand Creek flows in an east-northeast, northeast, north, southeast, and northeast direction to join northeast oriented St Vraiin Creek. The town of Gold Hill is located on the Left Hand Creek-Fourmile Creek drainage divide (in sections 11 and 12 in the west half of figure 8). Gold Run is a south, northeast, east-southeast, and southeast oriented Fourmile Creek tributary originating near Gold Hill. The south oriented Gold Run headwaters valley is linked by a through valley with the north-northwest oriented Lick Skillet Gulch valley, which drains to Left Hand Creek. The through valley floor has an elevation of between 8200 and 8240 feet. Elevations on Bighorn Mountain to the east rise to more than 8520 feet and elevations in section 11 to west rise to more than 8760 feet suggesting the Gold Hill through valley is almost 300 feet deep. Another interesting north-to-south oriented through valley is Sunshine Saddle located in the southeast corner of section 5 (near northeast corner of figure 8). Fourmile Canyon Creek flows in a southeast direction through the town of Sunshine in section 8 and then turns to flow in an east direction to the east edge of figure 8. East of figure 8 Fourmile Canyon Creek flows in an east and southeast direction to join Boulder Creek. Sunshine Saddle has a floor elevation of between 7480 and 7520 feet. Lee Hill to the east rises to more than 7880 feet and Butzel Hill to the west rises to more than 8200 feet suggesting Sunshine Saddle is at least 360 feet deep. Other much shallower north-to-south oriented through valleys or notches can be seen crossing the Left Hand Creek-Fourmile Creek and in the Gold Run-Fourmile Creek drainage divides in figure 8. These north-to-south oriented through valleys or notches are evidence of diverging and converging south oriented flood flow channels that existed prior to headward erosion of the deep east-northeast oriented Left Hand Creek valley. Headward erosion of the deep east-northeast oriented Left Hand Creek valley beheaded the south oriented flood flow channels in sequence from east to west and ended flood flow across the present day Left Hand Creek-Fourmile Creek drainage divide. The entire Front Range region seen in figure 8 was probably being uplifted relative to the Colorado Piedmont as the deep east oriented valleys eroded headward into the region.

Left Hand Creek-South Boulder Creek drainage divide area

Figure 9: Left Hand Creek-South Boulder Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 9 illustrates the Left Hand Creek-South Boulder Creek drainage divide area east and slightly north of figure 7 and there is an overlap area with figure 7. The map contour interval for figure 9 is 50 meters. The east flank of the Colorado Front Range can be seen along the west edge of figure 9. East of the Front Range is the Colorado Piedmont. The city of Boulder is located in southwest corner of figure 9. The grid of roads east of Dominion along the north edge of figure 9 is the south edge of the city of Longmont. Boulder Creek flows in an east, east-northeast, and northeast direction from near the southwest corner of figure 9 to the east edge of figure 9 (north half). East and north of figure 9 Boulder Creek joins northeast oriented St Vrain Creek, which then flows to the north, northeast, southeast, and northeast oriented South Platte River. Fourmile Canyon Creek flows in an east, southeast, and east direction from the west edge of figure 9 along the north edge of the city of Boulder to join Boulder Creek near Valemont. South Boulder Creek flows in a north direction from the south edge of figure 9 (west half) to join Boulder Creek near Valemont (east of Boulder). Left Hand Creek flows in a north, southeast, and northeast direction from the west edge of figure 9 (north half) to the north edge of figure 9 (east half-just east of Longmont) and north of figure 9 joins east and northeast oriented St Vrain Creek. Boulder Reservoir is located in the west center region of figure 9 between the southeast-northeast Left Hand Creek elbow of capture and the north oriented South Boulder Creek valley (south of Valemont). A close look at figure 9 shows Boulder Reservoir is located in a north-to-south oriented through valley between Gun Barrel Hill to the east and the Colorado Front Range to the west. The 50-meter contour interval hides much of the through valley detail. Figure 10 provides a detailed topographic map to better illustrate the through valley features. The through valley was eroded by south oriented flood flow flowing to the present day north oriented South Boulder Creek alignment. The flood flow was moving in a south direction along the east flank of what at that time was the emerging Front Range and was probably derived from east oriented valleys eroding headward into the emerging Front Range to capture south oriented flood flow. East of figure 9 south oriented flood flow was also moving on the present day north oriented South Platte River alignment. Headward erosion of the southeast and northeast oriented South Platte River valley from Nebraska (north of figure 9) beheaded and reversed flood flow on the South Platte River alignment and northeast oriented valleys then eroded headward from that reversed flood flow to capture south oriented flood flow channels west of the South Platte River alignment. Headward erosion of the northeast oriented Boulder Creek valley captured the south oriented flood flow moving in the Boulder Reservoir through valley. Floodwaters on the north end of the beheaded flood flow channel reversed flow direction to create the north oriented South Boulder Creek drainage route. Next headward erosion of the northeast oriented Left Hand Creek valley captured the south oriented flood flow and ended flood flow in the Boulder Reservoir through valley.

Detailed map of Left Hand Creek-Boulder Creek drainage divide area

Figure 10: Detailed map of Left Hand Creek-Boulder 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 Left Hand Creek-Boulder Creek drainage divide area seen in less detail in figure 9. The map contour interval for figure 10 is 10 feet except near the west edge where the contour interval is 40 feet. Boulder Creek flows in an east direction near the southeast corner of figure 10. East of figure 10 Boulder Creek turns to flow in a northeast direction to join northeast oriented St Vrain Creek, which then joins the north, northeast, southeast, and northeast oriented South Platte River. Left Hand Crek can be seen flowing in a southeast and then northeast direction near the north center edge of figure 10. North of figure 10 Left Hand Creek joins east and northeast oriented St Vrain Creek. Dry Creek is a northeast oriented stream flowing from Boulder Reservoir to the northeast corner of figure 10. North and east of figure 10 Dry Creek joins St Vrain Creek. Gun Barrel Hill can be seen along the east edge of figure 10 and rises to 5420 feet. Elevations greater than 5420 feet are seen along the west edge of figure 10 and west of figure 10 elevations rise much higher. The Dry Creek-Boulder Creek drainage divide east of the Boulder Reservoir dams has an elevation of less than 5150 feet suggesting the Dry Creek-Boulder Creek through valley is at least 370 feet deep. South of the Dry Creek-Boulder Creek through valley and south of figure 10 is the north oriented South Boulder Creek valley. The Dry Creek-Boulder Creek through valley was eroded by south oriented flood flow moving to the present day north oriented South Boulder Creek alignment. Left Hand Creek appears to have been flowing on the surface of a southeast oriented pediment or alluvial fan and was probably initially captured by headward erosion of a southwest oriented valley on the present day northeast oriented Dry Creek alignment. Flood flow on the Dry Creek alignment was then reversed by the reversal of flood flow on the present day north oriented South Platte River alignment (east of figure 10). The reversed flood flow changed the alluvial fan or pediment orientation near the northeast corner of figure 10 to a northeast oriented slope and in the process created the southeast-northeast oriented Left Hand Creek elbow of capture and ended south oriented flood flow in the Dry Creek-Boulder Creek through valley.

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.