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.

• The purpose of this essay is to use topographic map interpretation methods to explore the Little Prickly Pear Creek-Missouri River drainage divide area landform origins in Lewis and Clark County, Montana, 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 by leaving a comment here with a link to those essays.

• 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 Little Prickly Pear Creek-Missouri River drainage divide area landform evidence in Lewis and Clark County, Montana will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm (see link to paradigm related essay in menu at top of page). This essay is included in the Missouri River drainage basin landform origins research project essay collection.

Figure 1: Little Prickly Pear Creek-Missouri 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 Montana Little Prickly Pear Creek-Missouri River drainage divide area in Lewis and Clark County. Helena, the Montana state capitol, is located near the figure 1 center. The Missouri River is located between Helena and the Big Belt Mountains (east of Helena) and flows in a north and north-northwest direction from Three Forks 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. Three Forks is where the Jefferson, Madison, and Gallatin Rivers join to form the Missouri River. The east-west continental divide extends in a south-southeast direction from the figure 1 north edge (west half) along or near the Lewis and Clark Range crest and then between Helena and Elliston before continuing in a south direction west of Basin and east of Butte and then in a west direction north of the Big Hole River. Of importance in this essay is the region just north and west of Helena where Little Prickly Pear Creek is located. Little Prickly Pear Creek is not shown on figure 1, but originates in the region north and slightly west of Marysville (just north and west of Helena) and flows in an east and north-northeast direction to the town of Wolf Creek and then to join the northeast-oriented Missouri River. The Little Prickly Pear Creek-Missouri River drainage divide area investigated in this essay is located west of Holter Lake and of the Gates of the Rocky Mountains located on the Missouri River. The Origin of the North America east-west continental divide, Little Prickly Pear Creek-Blackfoot River drainage divide, Montana essay illustrates and discusses the continental divide region north and west of the east and north-northeast oriented Little Prickly Pear Creek drainage basin (find essay under east-west continental divide category on sidebar category list) . North of the Little Prickly Pear Creek drainage basin on the east side of the continental divide is the Dearborn River drainage basin. Essays illustrating and discussing Dearborn River drainage basin drainage divide areas can be found under the Dearborn River category

• Before focusing on the Little Prickly Pear Creek-Missouri River drainage divide area a brief overview of the figure 1 drainage history is needed to provide a big picture view. All figure 1 drainage routes on both sides of the continental divide can be best be explained in the context of immense south and southeast-oriented melt water floods from a rapidly melting thick North American ice sheet. The thick ice sheet was located in a deep “hole”, which was created by deep glacial erosion (under the ice sheet) and by crustal warping of regions elsewhere, including along the ice sheet’s western and southwestern margin. The Canadian Rocky Mountains in Alberta and British Columbia emerged along the deep “hole’s” western rim while the Missouri River drainage basin in Montana and northern Wyoming is the deeply eroded deep “hole” southwest wall. Massive south and southeast-oriented ice-marginal meltwater floods flowed from the deep “hole’s” western rim in Canada across the figure 1 map area. Initially flood waters flowed on erosion surfaces now preserved (if they are preserved at all) by the highest level erosion surfaces and mountain ridges in the figure 1 map area and eroded south and southeast-oriented anastomosing channel complexes into those high level erosion surfaces. Much deeper valleys then eroded headward into the figure 1 map area from both the east and the west to capture the huge south and southeast-oriented melt water floods and to dismember the south and southeast-oriented anastomosing flood flow channels. Flood waters on north and northwest ends of south and/or southeast-oriented flood flow channels beheaded by much deeper valleys often reversed flow direction to erode what are today north and northwest-oriented valleys. Some of the flood flow reversals affected large regions as can be seen by the large number and size of the northwest, north-northwest, and north oriented drainage routes visible on figure 1. The north-northwest Missouri River valley east of Helena is an example of a valley eroded by a reversal of south-oriented flood flow on the north end of a beheaded south-oriented flood flow channel.

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

 

Turning now to the Little Prickly Pear Creek-Missouri River drainage divide area figure 2 provides a much more detailed location map. The green shaded areas are National Forest lands and are generally located in mountain regions. The east-west continent divide is shown with a dashed line and is labeled and extends along high mountain ridges from the figure 2 north center edge to the figure 2 south center edge. Helena is the city located along the figure 2 southeast quadrant south edge. The Missouri River flows in a north-northwest direction from the figure 2 east edge (near southeast corner) to Hauser Dam, Upper Hauser Lake, Holter Lake and Holter Dam and then turns to flow in a north-northeast direction to the figure 2 north edge (east half). Little Prickly Pear Creek originates near the continental divide (south of Granite Butte) and flows in an east direction to east of Canyon Creek where it then turns to flow in a north and north-northeast direction to the town of Wolf Creek and then to join the Missouri River. Little Prickly Pear Creek tributaries shown are from the north and west and include south and south-southeast oriented Canyon Creek, southeast-oriented Lyons Creek, and southeast-oriented Wolf Creek. Several mountains are marked in the Little Prickly Pear Creek-Missouri River drainage divide area and include Beartooth Mountain, Stony Point, and Baldy Mountain. South of the east-oriented Little Prickly Pear Creek headwaters is east, southeast, east, and north oriented Silver Creek, which flows to the flooded Missouri River valley a short distance upstream from Hauser Dam. The Little Prickly Pear Creek-Silver Creek drainage divide area is included in this essay’s study region. North of the east-oriented Little Prickly Pear Creek headwaters, and north and west of the continental divide, are northwest oriented headwaters and tributaries of the west oriented Blackfoot River. These northwest and west oriented streams flow in valleys that originated as southeast-oriented flood flow channels to what was at that time the actively eroding east-oriented Little Prickly Pear Creek valley. Headward erosion of the much deeper west- and southwest-oriented Blackfoot River valley beheaded the southeast-oriented flood flow channels. Flood waters on northwest ends of the beheaded flood flow channels reversed flow direction to erode northwest-oriented Blackfoot River headwaters and tributary valleys. The south and south-southeast oriented Canyon Creek valley eroded headward along a south-oriented flood flow channel formed by two converging flood flow channels, the eastern one of which was subsequently beheaded by headward erosion of the much deeper Dearborn River valley (east of the continental divide) while the western channel was beheaded by headward erosion of the much deeper Blackfoot River valley (west of the continental divide).

Figure 3: North end of Little Prickly Pear Creek-Missouri River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

Figure 3 illustrates the north end of the Little Prickly Pear Creek-Missouri River drainage divide area. The Missouri River valley is flooded by Holter Lake, which is upstream from Holter Dam, although otherwise the Missouri River flows in a north-northwest direction from the figure 3 south edge (east half) almost to the north edge and then turns to flow in a north-northeast direction to the figure 3 north edge (east half). Little Prickly Pear Creek flows in a north and north-northeast direction from the figure 3 south edge (west half) to the town of Wolf Creek and then joins the Missouri River near the figure 3 north edge. The north-northeast oriented Little Prickly Pear Creek valley can be considered a south-southwest extension of the north-northeast oriented Missouri River valley segment north of the figure 3 map area. Little Prickly Pear Creek has at least two north-northwest oriented tributaries from the south and east, the largest of which is Sheep Creek. Note how the north-northwest oriented Sheep Creek valley is linked by a high level through valley with the south-oriented Towhead Gulch valley, which drains to the figure 3 south center edge. That south-oriented through valley is a water eroded feature and was eroded as a south-oriented flood flow channel. Headward erosion of the much deeper north-northeast oriented Little Prickly Pear Creek valley beheaded and reversed the flood flow. Flood water to erode the deep north-northwest oriented Sheep Creek valley was yet to be beheaded flood flow from west of the actively eroding Little Prickly Pear Creek valley head and was captured by the reversed flow on the newly beheaded Sheep Creek flood flow channel. At that time regions west and south of the figure 3 map area had not yet been deeply eroded and flood waters were flowing on a high-level erosion surface at least as high as the Sheep Creek-Towhead Gulch drainage divide today (probably regional uplift and other crustal warping significantly altered elevations since). The figure 3 map contour interval is 50 meters and the through valley floor elevation at the drainage divide is between 1700 and 1750 meters or more than 700 meters above Holter Lake, which gives a minimal measure of the depth of flood flow erosion..

• The north-northwest oriented Missouri River valley was eroded by reversed flood flow on the north end of a beheaded south-oriented flood flow channel, although the S-shaped incised meander at Oxbow Bend provides evidence the situation was somewhat more complicated and that there were multiple parallel flood flow channels. The westernmost north-oriented segment of the S-shaped incised meander was eroded by a reversal of flood flow when the much deeper Missouri River valley beheaded the main south-oriented flood flow channel. The reversed flood flow then eroded a much deeper valley headward to the south end of the western most channel, which then eroded headward along a parallel south-oriented flood flow channel to the north end of the south-oriented channel segment. Headward erosion of the deep valley on the south-oriented channel segment then beheaded south-oriented flood flow on the eastern most north-oriented channel segment and the deep valley then eroded headward (upstream on the present day north-oriented Missouri River valley). North-oriented Missouri River tributary valleys (Falls Gulch and Log Gulch) were eroded by reversals of flood flow on north ends of beheaded flood flow routes. Note how the north-oriented Falls Gulch valley is linked by a high-level through valley with south-oriented Towhead Gulch providing evidence of a former south-oriented flood flow channel. Additional water eroded through valleys crossing figure 3 drainage divides can be seen and provide evidence of other high level flood flow channels. Other evidence of the initial south-oriented flood flow are southwest-oriented and barbed Missouri River tributaries from the east and southeast-oriented and barbed Little Prickly Pear Creek tributaries from the west and north.

Figure 4: Detailed map of Sheep Creek-Towhead Gulch 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-Towhead Gulch drainage divide area seen in less detail in figure 3 above. The south and western north oriented segments of the Missouri River valley S-shaped incised meander can be seen along the figure 4 east edge. Falls Gulch originates in the section 32 northwest quadrant and drains in a north-northeast direction across section 29 to the figure 4 north edge (east half) and then to the Missouri River. Towhead Gulch originates in the south half of section 32 and drains in a south-southwest direction across section 5 and the section 8 northwest corner to the figure 4 south center edge. South of figure 4 Towhead Gulch turns to flow in a southeast direction to join the Missouri River. Sheep Creek originates in sections 1 and 6 and flows in a north and north-northwest direction across section 25 to the figure 4 north edge (west half) and as seen in figure 3 flows to north-northeast oriented Little Prickly Pear Creek. Rose Gulch is the north- and northeast-oriented Sheep Creek tributary in section 36. The figure 4 map contour interval is 40 feet and the Holter Lake surface elevation is between 3560 and 3600 feet. Note near the south edge of section 6 a through valley (looks like a saddle on a narrow ridge) linking the north-oriented Sheep Creek valley with the valley of a southeast-oriented Towhead Gulch tributary. The through valley floor elevation at the drainage divide is between 5880 and 5920 feet. Note in section 1 to the west a hill with an elevation of 6724 feet and in section 32 to the north and east a hill with an elevation of 6326 feet. These higher elevations provide a measure of the through valley depth, which probably at the time it was eroded was at least 400 feet deep. The through valley was eroded as a south-oriented flood flow channel supplying flood waters to what was then the actively eroding south-oriented Towhead Gulch valley. Headward erosion of the much deeper north-northeast oriented Little Prickly Pear Creek valley beheaded the south-oriented flood flow channel and flood waters on the north-northwest end of the beheaded flood flow channel reversed flow direction to erode the north-oriented Sheep Creek valley. Captured flood waters from west of the actively eroding Little Prickly Pear valley head and the actively eroding Towhead Gulch valley flowed in an east direction to help erode the deep north-oriented Sheep Creek valley and flood flow routes are recorded by other much more subtle saddles or through valleys across drainage divides west of the Sheep Creek headwaters area. An obvious north-south oriented through valley can be seen in section 32 linking the north-oriented Falls Gulch valley with the south-oriented Towhead Gulch valley. The elevation at the drainage divide is between 5720 and 5760 feet and provides evidence of a south-oriented flood channel beheaded by headward erosion of the deep Missouri River valley (probably after it was beheaded and reversed). Crustal warping of the region as flood waters flowed across the area may also have aided in eroding the deep valleys.

Figure 5: Little Prickly Pear Creek-Missouri River drainage divide area at Hilger Valley. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

Figure 5 illustrates the Little Prickly Pear Creek-Missouri River drainage divide area at Hilger Valley. The Missouri River flows in a north direction near the figure 5 east edge. Little Prickly Pear Creek flows in an east direction from the figure 5 west edge (south half) and then almost immediately is joined by north-oriented Willow Creek and turns to flow in a north direction to the figure 5 west center edge area where it turns to flow in a north-northeast direction to Steben and then to the figure 5 north edge (west half). Hilger Valley is a large northwest-southeast oriented through valley linking the north-northeast oriented Little Prickly Pear Creek valley with the north-oriented Missouri River valley. Note how the Missouri River upstream from Upper Holter Lake has the same alignment as the Hilger Valley alignment. Hilger Valley is probably related to one or more underlying geologic structures, although it is also an erosional landform and is viewed as such in this essay. Towhead Gulch drains in a south-southwest direction from the figure 5 north edge (east of center) to Hilger Valley and then drains the Hilger Valley southeast end to Upper Holter Lake. Clark Creek is the northwest-oriented Little Prickly Pear Creek tributary originating in the uplands south of Hilger Valley and draining the Hilger Valley northwest end. What makes Hilger Valley intriguing is both Little Prickly Pear Creek and the Missouri River have eroded deep valleys into the adjacent mountainous regions rather than taking advantage of the Hilger Valley route, which means Hilger Valley, the north-northeast-oriented Little Prickly Pear Creek valley north of Hilger Valley, and the Missouri River valley north of Upper Holter Lake, all had to be eroded at approximately the same time. At that time crustal warping, probably related to the nearby decaying thick North American ice sheet, was probably raising the mountainous regions adjacent to the Hilger Valley. The north-oriented Missouri River valley and at least segments of the north-oriented Little Prickly Pear Creek valley were eroded by reversals of flood flow on north ends of south-oriented flood flow channels beheaded by headward erosion of a much deeper north-northeast oriented Missouri River-Little Prickly Pear Creek valley. Prior to the flood flow reversal Hilger Valley was eroded as a southeast-oriented flood flow channel moving water to what was then a south-oriented flood flow channel on the Missouri River valley alignment in what was at that time a south-oriented anastomosing channel complex. Headward erosion of the deeper north-northeast oriented Little Prickly Pear Creek valley beheaded and reversed the southeast-oriented flood flow to create the present day Little Prickly Pear Creek-Missouri River drainage divide in the Hilger Valley. Note just east of Baldy Mountain are headwaters of south-, west, and north-oriented Willow Creek, which makes its U-turn to join north and north-northeast oriented Little Prickly Pear Creek (seen again in figures 6, 7, and 8).

Figure 6: Detailed map of Clark Creek-Willow 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 Clark Creek-Willow Creek drainage divide area east of Baldy Mountain, which was seen in less detail in figure 5 above. Clark Creek originates in the section 2 northeast corner and flows in a north-northwest direction to the figure 6 north center edge. As seen in figure 5 north of the figure 6 map area Clark Creek enters the Hilger Valley and flows in a northwest direction to join north-northeast oriented Little Prickly Pear Creek. Note the north-northeast oriented Clark Creek tributary that originates near the section 4 center and which joins Clark Creek just south of the figure 6 north center edge. The south-oriented oriented stream originating in section 4 (just south of that Clark Creek tributary’s headwaters) is a tributary to southwest-oriented Willow Creek, which originates in section 10 and which flows to the figure 6 south edge (west half). South of figure 6 Willow Creek turns to flow in a west and then a north direction to join north-oriented Little Prickly Pear Creek (see figure 7 below). Note the well-defined north-south oriented through valley linking the north-northeast oriented Clark Creek tributary valley with the south-oriented Willow Creek tributary valley. The figure 6 map contour interval is 40 feet and the through valley floor elevation at the drainage divide is between 5160 and 5200 feet. Baldy Mountain to the west rises to 6128 feet while an elevation of 5528 feet can be found in section 2 to the east. Based on the eastern elevation the through valley is more than 300 feet deep. The through valley was eroded by a south-oriented flood flow channel to what was then the actively eroding south-oriented Willow Creek valley. At that time there was no deep Hilger Valley north of the figure 6 map area and flood waters could freely flow into and across the figure 6 map area. Other higher level through valleys also link the north-oriented Clark Creek valley with the south-oriented Willow Creek valley. In addition there are northwest-southeast oriented through valleys linking the northwest-oriented Clark Creek valley with the southeast-oriented Rattlesnake Gulch valley (draining to the figure 6 east edge, south half) which drains to the north-oriented Missouri River and also with the southeast-oriented McLeod Gulch valley, which originates in the south half of section 2 and the section 1 southeast corner. South and east of figure 6 both Rattlesnake Gulch and McLeod Gulch turn from draining in southeast directions to draining in northeast directions to reach the north-oriented Missouri River (see figure 5).

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

 

Figure 7 illustrates the Willow Creek-Silver Creek drainage divide area south and west of the figure 5 map area and includes overlap areas with figure 5. Silver City is the town near the figure 7 center and Marysville is the town south of the Edward Mountain in the figure 7 west center edge region. Little Prickly Pear Creek flows in an east direction from the figure 7 west edge (north half) to join north-oriented Willow Creek and then to flow in a north direction to the figure 7 north edge (west of center). Willow Creek originates in the figure 7 northeast quadrant (west of the southeast-oriented McLeod Gulch headwaters) and flows in a southwest and west direction before making an abrupt turn to flow in a north direction to join Little Prickly Pear, which then flows in a north direction to the figure 7 north edge. Silver Creek originates south and west of Marysville (near the figure 7 west edge) and flows in a northeast direction to Marysville and then in an east and southeast direction to the figure 7 south edge (just west of southeast corner). South and east of the figure 7 map area Silver Creek flows to the north-oriented Missouri River valley. Note how in the figure 7 southeast quadrant, near the small town of Gearing, how Silver Creek has eroded a water gap across a north-south oriented ridge. The Willow Creek-Silver Creek drainage divide in the Silver City region is located on the floor of a deep basin surrounded by mountains and/or ridges in all directions. Yet Willow Creek makes a U-turn to flow in a north direction to join north-oriented Little Prickly Pear Creek, which has eroded a deep valley through mountainous areas to the north and Silver Creek has eroded a southeast-oriented valley across a north-south oriented ridge to flow to what is today a north-oriented river. These drainage features can best be explained in the context of massive reversals of flood flow as headward erosion of the much deeper north-northeast oriented Missouri River-Little Prickly Pear Creek valley systematically (from east to west) beheaded and reversed individual flood flow channels in what had been a south-oriented anastomosing channel complex. The southeast-oriented headwaters of streams flowing to the figure 7 east edge (and then to the north-oriented Missouri River) provide evidence of former south-oriented flood flow channels as do south-oriented Willow Creek headwaters. Just as Willow Creek makes a U-turn to flow in a north direction, Rattlesnake Gulch, McLeod Gulch, and Silver Creek also make U-turns to flow in a north directions as they approach the north-oriented Missouri River. These U-turns are evidence of the massive flood flow reversals that took place.

Figure 8: Detailed map of Willow Creek-Silver 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 Willow Creek-Silver Creek drainage divide area near Silver City, which was seen in less detail in figure 7 above. Silver City is located in the figure 8 south center region. Willow Creek flows in a southwest, west, and northwest direction near the figure 8 north edge, just north of sections 30, 25, and 26. Intermittent southwest and northwest oriented drainage routes in sections 29, 30, 25, 26 and the northwest corner of section 31 and northeast corner of section 36 flow to north-oriented Willow Creek. Silver Creek flows in an east and southeast direction in the figure 8 southwest quadrant from the figure 8 west edge (south half) to the figure 8 south edge (west of center). Intermittent southwest-oriented drainage routes in sections 32 and most of section 1 flow to southeast-oriented Silver Creek. There is no easily identifiable drainage divide ridge separating the Willow Creek drainage basin from the Silver Creek drainage basin. A large flood in one drainage basin could easily spill over into the adjacent drainage basin, yet water in the two drainage basin takes very different routes using deep valleys eroded across mountainous areas before finally converging again where Little Prickly Pear Creek enters the Missouri River (see figures 1, 2, and 3). The Silver City basin seen in figure 7 and 8 probably is a structural basin, although it also is an erosional landform. The Willow Creek-Silver Creek drainage divide seen in this basin can best be explained in the context of a massive flood flow reversal where south and southeast-oriented flood flow moving to what was at that time the actively eroding southeast-oriented Silver Creek valley was beheaded by a much deeper valley to the north. The much deeper valley to the north had to be the north-northeast oriented Little Prickly Pear Creek valley, which was eroding headward from what was at that time the newly eroded north-northeast oriented Missouri River valley. The south and southeast-oriented flood flow had to be coming from north and west of the present day east-west continental divide (see figures 1 and 2), which means the deep west-oriented Blackfoot River valley had not yet eroded headward into that region. Flood waters on the north end of the south and southeast-oriented flood flow channel beheaded by Little Prickly Pear Creek valley headward erosion reversed flow direction and captured southwest-oriented flood flow channels to create the Willow Creek (and tributaries) U-turn and to create the Willow Creek-Silver Creek drainage divide.

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

 

Figure 9 illustrates the Little Prickly Pear Creek-Silver Creek drainage divide area west of the figure 7 map area and includes overlap areas with figure 7. Little Prickly Pear Creek flows from the figure 9 west edge (just north of center) in an east-northeast and east direction to near the figure 9 northeast corner where it turns to flow in a north direction to the figure 9 north edge. Note how Little Prickly Pear Creek has southeast-oriented tributaries from the north and west. The southeast-oriented tributary valleys were eroded as southeast-oriented flood flow channels captured by headward erosion of the much deeper east-northeast and east oriented Little Prickly Pear Creek valley. The southeast-oriented tributary valleys originate along the east-west continental divide (north and west of figure 9), which means the southeast-oriented flood flow was coming from north and west of the continental divide. Today the region west and north of the continental divide is drained by west-oriented Blackfoot River headwaters and tributaries, however at the time the deep Little Prickly Pear Creek valley was eroded those west-oriented Blackfoot River tributary valleys did not exist. At that time southeast-oriented flood waters could freely flow into the figure 9 map area to the actively eroding Little Prickly Pear Creek valley.

• Edward Mountain is located south and east of the figure 9 center and Marysville is located south of Edward Mountain. Silver Creek originates south and west of Marysville and flows in a northeast direction to Marysville and then in an east and east-southeast direction to the figure 9 east edge (south half). Note how between Edward Mountain and Belmont Mountain (west of Marysville) a through valley links the east-oriented Silver Creek valley with a northwest and north-northeast oriented Little Prickly Pear Creek tributary valley (Piegan Creek). Figure 10 provides a detailed topographic map of the Piegan Creek-Silver Creek drainage divide area. The through valley was eroded by southeast-oriented flood flow moving to what was then an actively eroding east- and southeast-oriented Silver Creek valley. At that time the deep east-northeast oriented Little Prickly Pear Creek valley had not yet been eroded and flood waters could easily flow to the actively eroding Silver Creek valley. The figure 9 map contour interval is 50 meters and the through valley floor elevation at the drainage divide is between 1900 and 1950 meters. The Little Prickly Pear Creek valley floor elevation to the north has an elevation of between 1400 and 1450 meters, meaning headward erosion of the deep Little Prickly Pear Creek valley carved a valley at least 450 meters deep. The Piegan Creek-Silver Creek through valley or flood flow channel was eroded into a high-level erosion surface now preserved by the tops of the highest figure 9 mountains today. Edward Mountain rises to an elevation between 2000 and 2050 meters while Mount Belmont rises to an elevation of between 2200 and 2250 meters. Headward erosion of deep valleys into the figure 9 map area to capture the immense southeast-oriented flood flow was responsible for lowering the figure 9 surface and creating all present day figure 9 erosional landforms.

Figure 10: Detailed map of Drinkwater Gulch-Silver 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 Drinkwater Gulch (Piegan Creek)-Silver Creek drainage divide area seen in less detail in figure 9. Marysville is the town located in section 36 slightly north and east of the figure 10 center. Silver Creek flows in a northeast and east direction in section 36 and then flows in an east direction to the figure 10 east edge (north of center). East of figure 10 Silver Creek turns to flow in a southeast direction as seen in figure 7 and eventually reaches the north-oriented Missouri River. Just north of the east-oriented Silver Creek segment is east-oriented Trinity Creek, which originates in the north half of section 31 and which flows to the figure 10 east edge (north of Silver Creek). East of the figure 10 map area Trinity Creek turns to flow in a northeast direction to join north-oriented Willow Creek, which then flows to north-oriented Little Prickly Pear Creek. Note how north-south oriented through valleys cross the Trinity Creek-Silver Creek drainage divide (e.g. west edge of section 32) and provide evidence of south-oriented flood flow channels first captured by headward erosion of the deep east-oriented Silver Creek valley and next captured by headward erosion of the deep east-oriented Trinity Creek valley (which beheaded the flood flow to the newly eroded Silver Creek valley).

• Drinkwater Gulch originates in the section 26 southwest corner area and drains in a northwest direction to the figure 10 north edge (west half). North of figure 10 Drinkwater Gulch drains to north-oriented Piegan Creek, which flows to the east-northeast oriented Little Prickly Pear Creek. A deep northwest-southeast oriented through valley links the northwest-oriented Drinkwater Gulch valley with the east-oriented Silver Creek valley. The figure 10 map contour interval is 40 feet and the through valley floor elevation at the drainage divide is between 6240 and 6280 feet. Edward Mountain to the northeast rises to 6713 feet while Mount Belmont to the southwest rises to 7331 feet. The northwest-southeast oriented through valley floor is 433 feet lower than the top of Edward Mountain and was eroded as a southeast-oriented flood flow channel moving flood water to what was then the actively eroding Silver Creek valley. At that time there was no deep Little Prickly Pear Creek valley to the north and northwest and flood waters could freely flow from north and west of the present day continental divide into the figure 10 map area and to the actively eroding Silver Creek valley. Headward erosion of the deep east-northeast oriented Little Prickly Pear Creek valley (north of figure 10) beheaded the southeast-oriented flood flow and flood waters on northwest ends of the beheaded flood flow routes reversed flow direction to erode north- and northwest-oriented valleys including the northwest-oriented Drinkwater Gulch valley and the north-oriented Piegan Creek valley (north of figure 10). Many similar through valleys can be seen throughout the Little Prickly Pear Creek-Missouri River drainage divide area and this essay has provided only a few examples to illustrate how the regional drainage history can be reconstructed.

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.