Origin of North America east-west continental divide, Little Prickly Pear Creek-Blackfoot River drainage divide area, Montana, USA

Authors

Abstract:

Topographic map interpretation methods are used to interpret landform origins along the Montana Little Prickly Pear Creek-Blackfoot River drainage divide segment of North America’s east-west continental divide. Little Prickly Creek originates on the east side of the east-west continental divide and flows in an east, north, and north-northeast direction to join the Missouri River with water eventually reaching the Gulf of Mexico. Multiple southeast-oriented tributaries flow from the continental divide to reach the east-oriented Little Prickly Pear Creek segment. The Blackfoot River originates west of the continental divide and flows in a southwest and west direction with water eventually reaching the Pacific Ocean. Multiple northwest-oriented tributaries flow from the continental divide to join the west orientetd Blackfoot River. The east-west continental divide is crossed by northwest-southeast oriented through valleys (or mountain passes) linking the northwest-oriented Blackfoot River tributary valleys with the southeast-oriented Little Prickly Pear Creek tributary valleys. These northwest-southeast oriented through valleys are interpreted to have been eroded by diverging and converging southeast-oriented flood flow channels in what was once a large south and/or southeast-oriented anastomosing channel complex supplying flood waters to the actively eroding and deep east-oriented Little Prickly Pear Creek valley. At that time the deep Blackfoot River valley had not yet been eroded and flood waters were flowing on a high level erosion surface west of the continental divide which was at least as high if not higher than the continental divide (crustal warping since that time has probably significantly changed regional elevations). Headward erosion of the deep west-oriented Blackfoot River valley then captured the southeast-oriented flood flow channels in sequence from west to east. Flood waters on northwest ends of beheaded flood flow channels reversed flow direction to erode northwest-oriented Blackfoot River tributary valleys. Based on topographic map evidence from other Missouri River drainage basin drainage divide areas flood waters are interpreted to have been derived from a rapidly melting thick North American ice sheet, which was located in a deep “hole.” The deep “hole’s” western rim was located in western Alberta and eastern British Columbia and immense south and southeast-oriented ice-marginal melt water floods flowed from Canada into and across Montana along the and across the present day east-west continental divide.

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 available at this site may be found by selecting desired Missouri River tributaries and/or states from this essay’s sidebar category list.

Introduction

  • The purpose of this essay is to use topographic map interpretation methods to explore the east-west continental divide origin along the Montana Little Prickly Pear Creek-Blackfoot River drainage divide, USA. Map interpretation methods can be used to unravel many geomorphic events leading up to formation of present-day drainage routes and development of other landform features. While each detailed topographic map feature provides detailed evidence to be explained, the solution must be consistent with explanations for adjacent area map evidence as well as solutions to big picture map evidence puzzles. I invite readers to improve upon my solutions and/or to propose alternate solutions that better explain evidence and are also consistent with adjacent map area and big picture evidence. Readers may do so either by making comments here or by writing and publishing their own essays and then by leaving a link to those essays in 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 essays in the Missouri River drainage basin landform origins research project 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 the east-west continental divide origin along the Montana Little Prickly Pear Creek-Blackfoot River drainage divide will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm (see paradigm related essay link at top of this page). This essay is included in the Missouri River drainage basin landform origins research project essay collection.

Little Prickly Pear Creek-Blackfoot River drainage divide area location map

Figure 1: Little Prickly Pear Creek-Blackfoot 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 Little Prickly Pear Creek-Blackfoot River drainage divide area and shows a region in central and western Montana with a small area of Idaho seen in the figure 1 southwest corner. Helena, the Montana state capitol, is located near the figure 1 center. The Missouri River is formed at the confluence of the Jefferson, Madison, and Gallatin Rivers (at Three Forks) and flows in a north and north-northwest direction between Helena and the Big Belt Mountains (east of Helena) to near Wolf Creek. Near Wolf Creek the Missouri River turns to flow in a northeast direction to Great Falls and the figure 1 north edge (east half) with water eventually reaching the Gulf of Mexico. Little Prickly Pear Creek is not shown on figure 1, but Little Prickly Pear Creek headwaters are located slightly north and west of Marysville (slightly west and north of Helena) and flow in an east direction before making an abrupt turn to flow in a north and north-northeast direction to join the Missouri near Wolf Creek (where the Missouri River turns from flowing in a north-northwest direction to flowing in a northeast direction). Little Prickly Pear Creek headwaters and its southeast and south-oriented tributaries originate along the continental divide, which is also not shown on figure 1. West of the continental divide are headwaters and tributaries of the west-oriented Blackfoot River, which is shown on figure 1, and which flows through the towns of Lincoln and Ovando before joining the northwest-oriented Clark Fork, which then flows to Missoula and the figure 1 west edge (north half) with water eventually reaching the Pacific Ocean. Clark Fork originates in the Butte region and flows in a west and north direction to Deer Lodge and then turns to flow in a northwest direction to Missoula and the figure 1 west edge. South of the Clark Fork headwaters is the Big Hole River, which flows around the Pioneer Mountains, and which then flows to the Jefferson River, which in turns flows to the Missouri River. North of the Little Prickly Pear Creek drainage basin is the east-oriented Dearborn River drainage basin, which also is bounded on the west by the east-west continental divide. This essay focuses on the Little Prickly Pear Creek-Blackfoot River drainage divide segment of the east-west continental divide. Links to essays  illustrating and describing other east-west continental divide segments can be found in the east-west continental divide category (see sidebar category list).
  • The east-west continental divide is one of North America’s best known geographic features, although to the best of my knowledge geologists have not previously used topographic map evidence to determine the continental divide origin. The continental divide is an erosional landform and topographic map evidence provides one of the best ways to study erosional landform evidence. As seen in the example topographic maps used in this essay the continental divide is crossed by numerous through valleys (or mountain passes). Some of these through valleys are deep while others appear as low saddles notched into high ridges. Yet each of these through valleys is evidence of a former water eroded channel that once crossed what is now the continental divide. Using such topographic map evidence at least some of these water eroded channels can be reconstructed to determine where the water came from and where the water was going. The number of water eroded through valleys that cross the present day continental divide is impressive and suggests the through valleys were not eroded by rivers and tributaries similar to those that are generally seen today, but instead were eroded by anastomosing flood flow channels, which diverged and then converged again. Reconstructing these anastomosing flood flow channels suggests they were eroded into a high level erosion surface, at least as high present day continental divide ridges, if not higher, although crustal warping has probably altered regional elevations since that time. Also reconstructing the anastomosing flood flow channels suggests they were oriented in south and southeast directions and can be traced headward into the Canadian Rocky Mountains in western Alberta and eastern British Columbia where flood waters are interpreted to have been derived from a rapidly melting thick North America ice sheet (located east of the Canadian Rocky Mountains). Reconstructions also show the south and southeast-oriented flood flow channels extended southward from Montana across Wyoming and Colorado along and across the present day continental divide. The east-west continental divide was carved as the anastomosing south and southeast-oriented flood flow channels were systematically captured by headward erosion of much deeper valleys (in sequence from south to north) from both the east and the west. North and northwest-oriented valleys were eroded by reversals of flood flow on north and northwest ends of beheaded flood flow channels.

Detailed location map for Little Prickly Pear Creek-Blackfoot River drainage divide area

Figure 2: Detailed location map for Little Prickly Pear Creek-Blackfoot River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

  • Focusing on the Little Prickly Pear Creek-Blackfoot River drainage divide area figure 2 provides a more detailed location map. Green shaded areas are National Forest lands, which generally are located in mountainous regions. The east-west continental divide is marked with a dashed line, is labeled near the figure 2 north center edge, and extends from the north center edge to the figure 2 south center edge. Helena is located along the figure 2 south edge (east half). The Missouri River flows in a north-northwest direction from the figure 2 east edge (north of southeast corner) to just beyond Holter Dam (near north edge in figure 2 northeast quadrant) and then turns to flow in a north-northeast direction to the figure 2 north edge. Little Prickly Pear Creek originates near the continental divide (south of Granite Butte) and flows in an east-northeast direction to a short distance beyond the town of Canyon Creek before turning to flow in a north and north-northeast direction to the town of Wolf Creek and then to join the Missouri River near the figure 2 north edge. The north-northeast oriented Little Prickly Pear Creek valley segment could be considered a south-southwest extension of the north-northeast oriented Missouri River valley segment north and east of figure 2 (see figure 1). The major Little Prickly Pear Creek tributary shown is south and southeast-oriented Canyon Creek, which originates near the continental divide (south of figure 2 north center edge) and which joins Little Prickly Pear Creek near the town of Canyon Creek. West of the continental divide in the figure 2 northwest quadrant highway 200 through the town of Lincoln roughly follows the route of the southwest and west oriented Blackfoot River, which flows to the figure 2 west edge. Poorman Creek is a west- and northwest-oriented tributary originating near Granite Butte and joining the Blackfoot River south of the town of Lincoln. Other unnamed northwest-oriented tributaries are shown originating near the continental divide and flowing to the southwest and west-oriented Blackfoot River. Nevada Creek originates near the continental divide and south of the Poorman Creek headwaters and flows in a southwest and west direction before turning to flow in a northwest direction to the figure 2 west center edge and then to join the west-oriented Blackfoot River west of the figure 2 map area. The Nevada Creek U-turn west of the continental divide, just like the Canyon Creek-Little Prickly Pear Creek U-turn east of the continental divide, provides evidence of significant reversals of flood flow on what had once been south- and southeast-oriented flood flow channels. Silver Creek, which flows from Marysville to near Silver City and then in a southeast, east, and north direction to join the Missouri River near Hauser Dam, is linked by a broad through valley with the north-oriented Little Prickly Pear Creek valley, but is today a completely independent Missouri River tributary (also making a U-turn).

Blackfoot River-Canyon Creek drainage divide area

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

 

  • Figure 3 illustrates the Blackfoot River-Canyon Creek drainage divide area. The east-west continental divide is marked with a dashed line, is labeled, and extends from the figure 3 north edge (east of center) to the figure 3 west edge (west half). Flesher Pass is located near the figure 3 center and is where highway 279 crosses the continental divide. Highway 200 extends from the figure 3 north center edge to the figure 3 west center edge and for most of the distance is following the southwest-oriented Blackfoot River. West of figure 3 the Blackfoot River turns to flow in a west direction. Highway 279 from Flesher Pass to its junction with highway 200 follows northwest oriented Willow Creek, which is a Blackfoot River tributary. South and east of Flesher Pass highway 279 enters the south-southwest oriented Canyon Creek valley, which south of the figure 3 map area drains to east, north, and north-northeast oriented Little Prickly Pear Creek. Note how the southeast-oriented Canyon Creek headwaters are aligned with northwest-oriented Beartrap Creek and Anaconda Creek, both of which flow to the west and southwest-oriented Blackfoot River. Follow the continental divide in a southwest direction from Flesher Pass and note how multiple northwest-southeast oriented through valleys (or mountain passes) link northwest-oriented Blackfoot River tributary valleys with southeast-oriented Canyon Creek tributary valleys. The figure 3 map contour interval is 50 meters and some of the through valleys are defined by three or more contour lines suggesting they are at least 100 meters deep. Each of these multiple northwest-southeast oriented through valleys (or mountain passes) is evidence of a water eroded valley that was eroded into an erosion surface as high as the present day continental divide, if not higher. The multiple northwest- southeast oriented through valleys suggest roughly parallel southeast-oriented diverging and converging flood flow channels, such as might be found in a large-scale southeast-oriented anastomosing channel complex. The southeast-oriented flood was probably flowing to what was then the actively eroding and very deep south-oriented Canyon Creek valley, which was eroding headward into the figure 3 map area. At that time there was no deep Blackfoot River valley north and west of the continental divide and flood waters were flowing on a high level erosion surface and could easily flow from north and west of figure 3 to the actively eroding south-oriented Canyon Creek valley. Headward erosion of the very deep southwest-oriented Blackfoot River valley then captured the southeast-oriented flood flow channels in sequence from west to east. Flood waters on northwest ends of the beheaded flood flow channels reversed flow direction to erode northwest-oriented Blackfoot River tributary valleys. Because flood flow channels were beheaded and reversed in sequence from west to east yet to be captured flood flow from east of the actively eroding Blackfoot River valley head could be captured by the newly beheaded and reversed flood flow channels to help erode deep and large northwest-oriented Blackfoot River tributary valleys.

Detailed map of Willow Creek-Canyon Creek drainage divide area

Figure 4: Detailed map of Willow Creek-Canyon 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 Willow Creek-Canyon Creek drainage divide area near Flesher Pass, which was seen in less detail in figure 3 above. The east-west continental divide is marked with a dashed lined, is labeled, and extends in southwest direction from the figure 4 north edge (east half) to the figure 4 south edge (west half). Flesher Pass is located near the figure 4 center. The south-southwest oriented stream flowing from the figure 4 northeast corner to the figure 4 south edge (east half) is Canyon Creek, which south of the figure 4 map area joins east, north, and north-northeast oriented Little Prickly Pear Creek, which flows to the Missouri River with water eventually reaching the Gulf of Mexico. The southwest and west-northwest oriented stream in the figure 4 northwest quadrant is Willow Creek, which flows to the southwest and west-oriented Blackfoot River with water eventually reaching the Pacific Ocean. Note how Flesher Pass is a through valley (or mountain pass) between a north-northwest oriented Willow Creek tributary valley and a south-southeast oriented Canyon Creek tributary valley. The figure 4 contour interval is 40 feet and the Flesher Pass elevation is given as 6131 feet. The continental divide in the section 10 southwest corner area rises to more than 6800 feet and near the figure 4 north edge rises to more than 7400 feet. In the other direction the continental divide rises to more than 7040 feet in the figure 4 southwest corner area. These elevations and the adjacent valley orientations suggest Flesher Pass may have been a south-southeast oriented flood flow channel as much as 900 feet deep which was eroded into a high level erosion surface that once existed in the figure 4 region. Other less deep northwest-southeast and north-south oriented through valleys (or mountain passes) can be seen in sections 10 and 11 to the northeast of Flesher Pass and in sections 16 and 20 to the southwest of Flesher Pass. Each of these through valleys is evidence of a southeast and/or south-oriented flood flow channel, although it is possible these flood flow channels were eroded onto the floor of a much broader southeast-oriented flood flow channel with valley wall remnants preserved by higher continental divide elevations near the figure 4 north edge and the figure 4 southwest corner. Headward erosion of the much deeper southwest-oriented Blackfoot River valley (north and west of figure 4) beheaded and reversed the southeast-oriented flood flow channels, with the deep Willow Creek valley being eroded by the reversed flood flow and by southeast-oriented flood flow captured from east of the actively eroding Blackfoot River valley head (note southwest-oriented Willow Creek headwaters valley).

Blackfoot River-Virginia Creek drainage divide area

Figure 5: Blackfoot River-Virginia Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

  • Figure 5 illustrates the Blackfoot River-Virginia Creek drainage divide area south and west of the figure 3 map area and includes overlap areas with figure 3. The east-west continental divide extends in a southwest and south direction from the figure 5 northeast corner to the figure 5 south center edge. Flesher Pass is located near the figure 5 northeast corner. The Blackfoot River flows in a southwest and west direction in the figure 5 northwest quadrant. Note the northwest-oriented tributaries flowing from the continental divide to the Blackfoot River. Named northwest-oriented tributaries from the southwest to the northeast include South Fork, Humbug Creek, Seven-Up-Pete Creek, Hogum Creek and its Black Diamond Creek tributary, Horsefly Creek, and Willow Creek. Stemple Pass crosses the continental divide in the figure 5 south center region. Poorman Creek is the west, southwest, and west-oriented stream originating on the west side of Stemple Pass and west of the figure 5 map area turns to flow in a northwest direction to join the Blackfoot River. The east and east-southeast oriented stream originating on the east side of Stemple Pass is Virginia Creek, which joins south-oriented Canyon Creek near the figure 5 east edge, with Canyon Creek then flowing in a southeast direction to the figure 5 east edge (just north of southeast corner). Note how both Virginia Creek and Canyon Creek have southeast-oriented tributaries originating near the east-west continental divide. A close look at the continental divide reveals northwest-southeast oriented through valleys or mountain passes linking the northwest-oriented Blackfoot River tributary valleys with the southeast-oriented Canyon Creek and Virginia Creek tributary valleys. The figure 5 contour interval is 50 meters and some of the through valleys are defined by two or more contour lines suggesting they may be 100 meters or more deep. Orientation of the tributary valleys on both sides of the continental divide and the northwest-southeast oriented through valleys provide evidence of what were once multiple southeast-oriented flood flow channels moving flood waters to what were then the actively eroding Canyon Creek and Virginia Creek valleys. At that time the region north and west of the continental divide had not yet been eroded by headward erosion of the deep Blackfoot River valley and flood waters could freely flow on a high level erosion surface from north and west of figure 5 to the actively eroding and deep Canyon Creek and Virginia Creek valleys. Headward erosion of the deep Blackfoot River valley then beheaded the southeast-oriented flood flow channels in sequence from the west to the east. Flood waters on northwest ends of the beheaded flood flow channels reversed flow direction to erode the northwest-oriented Blackfoot River tributary valleys. Remember flood flow channels were beheaded and reversed one channel at a time, which meant reversed flow channels could capture flood water from yet to be beheaded and reversed flood flow channels. Such captures provided water volumes required to erode deep valleys.

Detailed map of Black Diamond Creek-Trout Creek drainage divide area

Figure 6: Detailed map of Black Diamond Creek-Trout 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 Black Diamond Creek-Trout Creek drainage divide area seen in less detail in figure 5 above. The east-west continental divide is marked and labeled and extends in a southwest direction (with some significant jogs) from the figure 6 northeast corner to near the figure 6 southwest corner. Black Diamond Creek flows from the north half of section 23 to the figure 6 northwest corner and north and west of figure 6 joins northwest-oriented Hogum Creek which flows to the Blackfoot River. The north-oriented stream originating in section 19 and flowing to the figure 6 north edge (east of center) is a tributary to northwest-oriented Willow Creek, which also flows to the Blackfoot River. South and east of the continental divide Trout Creek originates in the section 24 southwest quadrant and flows in a south direction into section 25 and then in a southeast direction to the figure 6 south edge (east half) and south of the figure 6 map area joins east-southeast oriented Virginia Creek. Note how in the south half of section 19 a north-south oriented through valley (or mountain pass) crosses the continental divide and links the north-oriented Willow Creek tributary valley with a south-southeast oriented Trout Creek tributary valley. The figure 6 map contour interval is 40 feet and the through valley floor elevation at the continental divide is between 6560 and 6600 feet. To the west near the section 19 west edge the continental divide elevation rises to 7083 feet while to the east in section 20 the continental divide elevation rise to more than 7040 feet. In other words the section 19 through valley is at least 440 feet deep. The through valley is a water eroded feature and was eroded by south-oriented flood flow moving to what was then the actively eroding Trout Creek valley. At that time the deep northwest-oriented Willow Creek valley to the north of figure 6 did not exist and flood waters were flowing on a high level erosion surface at least as high as the through valley floor elevation today (although the region has probably experienced crustal warping since). Headward erosion of the deep Willow Creek valley beheaded and reversed the south-oriented flood flow to erode the north-oriented Willow Creek tributary valley. Several other through valleys can be seen in figure 6 crossing the east-west continental divide and provide evidence of other south- and southeast-oriented flood flow channels. For example in the section 23 northeast quadrant a northwest-southeast oriented through valley links the northwest-oriented Black Diamond Creek valley with the south-oriented Trout Creek headwaters valley. The through valley floor elevation is shown as 6725 feet while the continental divide elevation rises to 7083 feet near the section 19 west edge to the east and to 7090 feet in section 23. These elevations suggest the Black Diamond Creek-Trout Creek through valley in the section 23 northeast quadrant is at least 350 feet deep. Headward erosion of the deep Blackfoot River valley to the north and west of figure 6 beheaded and reversed the southeast-oriented flood flow channel to erode the northwest-oriented Black Diamond Creek valley.

Poorman Creek-Little Prickly Pear Creek drainage divide area

Figure 7: Poorman Creek-Little Prickly Pear Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

  • Figure 7 illustrates the Poorman Creek-Little Prickly Pear Creek drainage divide area south of the figure 5 map area and includes overlap areas with figure 5. The east-west continental divide is marked with a dashed line and is labeled and extends in roughly a south direction from the figure 7 north edge (east of center) to the figure 7 south center edge. Stemple Pass is located on the continental divide near the figure 7 north edge and marks the drainage divide between east and east-southeast oriented Virginia Creek (which joins Canyon Creek near the figure 7 east edge, with Canyon Creek then flowing to east, north, and north-northeast oriented Little Prickly Pear Creek) and southwest, west, and northwest oriented Poorman Creek (which flows to the figure 7 west edge, north half, and then to the west-oriented Blackfoot River). Little Prickly Pear Creek flows in an east-northeast direction across the figure 7 southeast corner and east of figure 7 is joined by southeast-oriented Canyon Creek before it turns to flow in a north and north-northeast direction to join the Missouri River. South of Stemple Pass along the continental divide is Granite Butte and south and west of Granite Butte is the drainage divide between northwest-oriented South Fork Poorman Creek and southeast-oriented Marsh Creek, which flows to Little Prickly Pear Creek near the figure 7 southeast corner. Continuing south and west along the continental divide is the drainage divide between southeast-oriented North Fork Little Prickly Pear Creek and west- and southwest oriented Nevada Creek headwaters, which flow to the figure 7 south edge (west half). South and west of the figure 7 map area Nevada Creek makes a U-turn to flow in a northwest direction to join the west-oriented Blackfoot River. Following the continental divide further in either direction reveals additional west-east and northwest-southeast oriented through valleys eroded across it. Each of these through valleys is a water eroded feature and was eroded by east or southeast oriented flood flow channels delivering flood waters to what was at that time the actively eroding and deep Little Prickly Pear Creek valley. Initially flood waters west of the continental divide were flowing on a high level erosion surface, which had yet to be eroded by the deep valleys seen today. Headward erosion of the deep Blackfoot River valley beheaded and reversed the east- and southeast-oriented flood flow channels, with the southern west-oriented valleys being eroded headward to the present day continental divide location first and the more northern valleys beheading south-oriented flood flow channels to what were then the newly eroded valleys further to the south (see Poorman Creek-Nevada Creek drainage divide in figure 7 west half).

Detailed map of South Fork Poorman Creek-Marsh Creek drainage divide area

Figure 8: Detailed map of South Fork Poorman Creek-Marsh 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 South Fork Poorman Creek-Marsh Creek drainage divide area seen in less detail in figure 7 above. The east-west continental divide is marked with a dashed line and is labeled and extends in a northeast direction from the figure 8 southwest corner to Granite Butte (south of figure 8 north center edge) and then in a west-northwest direction to the figure 8 north edge (west half). The South Fork Poorman Creek originates in the west half of section 27 as a south-oriented stream and turns in section 28 to flow in a northwest direction to the figure 8 west edge (south of northwest corner). North and west of figure 8 South Fork Poorman Creek flows to west and northwest oriented Poorman Creek, which flows to the west-oriented Blackfoot River. Marsh Creek originates in section 34 and flows in an east, southeast, and east-northeast direction to the figure 8 east edge (south half). East of figure 8 Marsh Creek turns to flow in a southeast direction to join Little Prickly Pear Creek. Note near the section 34 northwest corner a northwest-southeast oriented through valley linking the northwest-oriented South Fork Poorman Creek valley with the southeast-oriented Marsh Creek headwaters valley. The figure 8 map contour interval is 40 feet and the through valley elevation where the road crosses the drainage divide is shown as 6742 feet. Granite Butte to the north and east rises to 7600 feet while the continental divide just south of the figure 8 southwest corner rises to more than 7700 feet. In other words the through valley may be as much as 950 feet deep and represents a former southeast-oriented flood flow channel, which was eroded headward from the actively eroding and very deep Little Prickly Pear Creek valley into a high level erosion surface as high or higher than the highest figure 8 elevations today (crustal warping in the region while flood waters were flowing across it and following the deep flood erosion have probably significantly altered the figure 8 elevations). At the time the through valley flood flow channel was eroded the deep Blackfoot River valley and its deep west- and northwest-oriented Poorman Creek tributary valley did not exist. Headward erosion of the deep Blackfoot River valley beheaded the southeast-oriented flood flow channel. Flood waters on the northwest end of the beheaded flood flow channel reversed flow direction to eroded the northwest- and west-oriented Poorman Creek valley and the northwest-oriented South Fork Poorman Creek valley. The south-oriented South Fork Poorman Creek headwaters valley was eroded by yet to be beheaded south-oriented flood flow (yet to be beheaded by Blackfoot River valley headward erosion), which was captured by the much deeper reversed flood flow route and which helped erode the deep northwest-oriented South Fork Poorman Creek valley.

Nevada Creek-Little Prickly Pear Creek drainage divide area

Figure 9: Nevada Creek-Little Prickly Pear Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

  • Figure 9 illustrates the Nevada Creek-Little Prickly Pear Creek drainage divide area south and slightly west of the figure 7 map area and includes overlap areas with figure 7. The east-west continental divide is again marked with a dashed line and is labeled and extends from the figure 9 north edge (east of center) in a southwest and then south direction to Black Mountain (north of figure 9 south center edge) and then in an east direction to the figure 9 east edge (just north of southeast corner). Nevada Mountain is located near the continental divide just south of the figure 9 center. East of Nevada Mountain (and the continental divide) are headwaters of east-northeast oriented South Fork Little Prickly Pear Creek, which join southeast-oriented North Fork Little Prickly Pear Creek to form east-northeast oriented Little Prickly Pear Creek which flows to the figure 9 east edge (north of center). Southeast-oriented Marsh Creek flows from near the continental divide (near figure 9 north edge) to join Little Prickly Pear Creek near the figure 9 east edge. Deadwood Creek is the northeast, east-northeast, and north oriented Little Prickly Pear Creek tributary originating east of Black Mountain. Note how north-oriented Deadwood Creek tributaries are linked by through valleys to south-oriented streams flowing to the figure 9 south edge. South of figure 9 the south-oriented streams flow to the west-oriented Little Blackfoot River, which in turn flows to northwest-oriented Clark Fork. Nevada Creek originates in the figure 9 north center area (just east of the Powell County northeast corner) and flows in a west, southwest, and west direction to the figure 9 west edge (south of center) and then turns to flow in a northwest direction to join the Blackfoot River. Note southwest and northwest-oriented Shingle Mill Creek and Mitchell Creek, which are Nevada Creek tributaries, south of Nevada Creek in the figure 9 southwest quadrant.
  • South and east of Mitchell Creek is south-southwest oriented McKay Creek which south of the figure 9 map area splits with half making a U-turn to flow in a north-northwest direction across the figure 9 southwest corner as Halfway Creek, which is a Nevada Creek tributary, and the other half flowing in a south-southwest and south direction to join the west-oriented Little Blackfoot River. The large north-northwest to south-southeast oriented valley seen in the figure 9 southwest corner is the Avon Valley and is a through valley linking the west-oriented Blackfoot River valley with the west-oriented Little Blackfoot River valley. While probably related to underlying geologic structures the Avon Valley is also an erosional landform. The large and deep Avon Valley did not exist when east and southeast-oriented through valleys across the continental divide in figure 9 were eroded as east and southeast-oriented flood flow channels. Headward erosion of the deep west-oriented Little Blackfoot River valley first captured south-oriented flood flow in Avon Valley and then headward erosion of the deeper Nevada Creek valley (from the actively eroding west-oriented Blackfoot River valley) beheaded south-southeast oriented flood flow in the actively eroding Avon Valley. Flood waters on the north-northwest end of the beheaded Avon Valley flood flow channel reversed flow direction and captured the south flood flow eroding the southwest-oriented Shingle Mill Creek, Mitchell Creek, and McKay Creek valleys, creating the present day U-turns seen in the figure 9 southwest quadrant. Continued headward erosion of the Nevada Creek valley next beheaded south- and southeast-oriented flood flow routes to what was then the actively eroding Little Prickly Pear Creek tributary valleys and created the continental divide.

Detailed map of Nevada Creek-North Fork Little Prickly Pear Creek drainage divide area

Figure 10: Detailed map of Nevada Creek-North Fork Little Prickly Pear 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 Nevada Creek-North Fork Little Prickly Pear Creek drainage divide area seen in less detail in figure 9 above. The east-west continental divide is marked and labeled and extends from the figure 10 northeast corner in a southwest and south direction to the figure 10 south edge (east of center). The North Fork Little Prickly Pear Creek originates in section 5 and flows in an east-southeast direction to the figure 10 south edge (just north of southeast corner). Nevada Creek originates just west of the continental divide in section 6 and flows in a west and southwest direction to the figure 10 southwest corner. South and west-northwest oriented South Fork Poorman Creek can be seen along the north edge of the figure 10 northeast quadrant. North-oriented streams flowing to the figure 10 north edge are tributaries to northwest-oriented South Fork Poorman Creek or west oriented Poorman Creek north of the figure 10 map area. Note in section 33 the north-south oriented through valley crossing the continental divide and linking a north-oriented South Fork Poorman Creek tributary valley with a south-oriented North Fork Little Prickly Pear Creek tributary valley. This through valley was eroded as a south-oriented flood flow channel to the actively eroding North Fork Little Prickly Pear Creek valley. Headward erosion of the deep South Fork Poorman Creek valley beheaded and reversed the south-oriented flood flow. Note in the northwest corner of section 5 a north-south oriented through valley linking a west and south oriented Nevada Creek tributary valley with the southeast-oriented North Fork Little Prickly Pear Creek valley. Also note in the east half of section 32 (just to the north) a north-south oriented through valley linking a north-oriented South Fork Poorman Creek tributary valley with the west- and south-oriented Nevada Creek valley. The two through valleys are remnants of the same south-oriented flood flow channel to the actively eroding North Fork Little Prickly Pear Creek headwaters valley. Headward erosion of the deep west- and south-oriented Nevada Creek tributary valley captured the south-oriented flood flow before headward erosion of the deeper South Fork Poorman Creek valley beheaded and reversed the flood flow to erode the north-oriented South Fork Poorman Creek tributary valley. Study of figure 10 drainage divides reveals many other flood flow channel remnants, which suggest the entire figure 10 map area was crossed by diverging and converging flood flow channels which were systematically dismembered by headward erosion of much deeper valleys from both the east and west to create the present day east-west continental divide.

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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