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 South Fork Dearborn River-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 then leaving a comment here with a link.

• 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 South Fork Dearborn River-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 header for link to essay). This essay is included in the Missouri River drainage basin landform origins research project essay collection.

Figure 1: South Fork Dearborn River-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 South Fork Dearborn River-Missouri River drainage divide area location map and illustrates a region in west-central Montana. The Montana-Idaho border is in the figure 1 southwest corner. The green shaded region along the figure 1 northwest quadrant north edge is the south half of Glacier National Park. The North American east-west continental divide extends in a south-southeast direction from the figure 1 north edge (about midway in Glacier National Park) through Marias Pass and along or near the Lewis and Clark Range crest and then to the figure 1 south center edge (just west of Basin). Figure 1 regions east of the continental divide drain to the Missouri River with water eventually reaching the Gulf of Mexico. Montana regions in figure 1 map area west of the continental divide drain to Clark Fork with water eventually reaching the Pacific Ocean. The Missouri River flows in a north-northwest direction from the figure 1 south edge (just west of the Big Belt Mountains) to Wolf Creek and then turns to flow in a northeast direction through Craig, Great Falls, Fort Benton, and Loma before turning to flow in a south-southeast and east direction to the figure 1 east edge (north half). Tributaries from the west (from south to north) include the southeast, east, southeast, east, and southeast-oriented Dearborn River (which joins the Missouri River near Craig); the east-oriented Sun River (which joins the Missouri River at Great Falls); the east-oriented Teton River (which joins the Missouri River near Loma); and the east-southeast and south oriented Marias River (which also joins the Missouri River near Loma). Note north and north-northwest oriented tributaries to the northeast-oriented Missouri River segment including the north-northwest oriented Smith River. The South Fork Dearborn River is not shown on figure 1, but is a north-northeast oriented tributary originating near the south end of the Lewis and Clark Range (west of Wolf Creek) and flowing south and east of highway 200 to reach the Dearborn River. The South Fork Dearborn River-Missouri River drainage divide area investigated in this essay is located in the region north and west of the northeast-oriented Missouri River segment extending downstream from Wolf Creek to the Dearborn River. Essays describing other Dearborn River drainage divide areas can be found by selecting the Dearborn River category from the sidebar category list.

• Based on hundreds of previously written essays for the Missouri River drainage basin landform origins research project the figure 1 map area, including the east-west continental divide, was eroded by headward erosion of deep valleys from the east and the west to capture immense south and southeast-oriented melt water floods, which were flowing from a rapidly melting thick North American ice sheet. The thick ice sheet was located in a deep “hole” and the Missouri River drainage basin in Montana and northern Wyoming is the deep “hole’s” deeply eroded southwest wall. The Canadian Rocky Mountains and mountains in western Montana are located along what was the deep “hole’s” west and southwest rim. The deep “hole” did not exist when the ice sheet formed, but was created by deep glacial erosion and by crustal warping of adjacent continental areas, which resulted in the uplift of major mountain ranges and of present day high plateau areas. Huge ice-marginal melt water floods flowed from the rapidly melting ice sheet in south and southeast directions from the ice sheet’s western margin (in Canada) into the figure 1 map area on an erosion surface equivalent in elevation to the highest figure 1 mountain ranges today (the mountain ranges did not stand high at that time). Flood waters flowed across western Montana and into Wyoming (and then further south) along routes roughly corresponding with and parallel to the present day continental divide and which today are crests of high mountain ranges. Uplift of mountain ranges, including the Rocky Mountains, occurred as gigantic melt water floods flowed across them. The east-west continental divide was carved from south to north as deep valleys eroded headward from the Gulf of Mexico in the east and from the Pacific Ocean in the west to capture these immense melt water floods. Headward erosion of each new valley beheaded south and southeast-oriented flood flow routes to what was then the newly eroded valley immediately to the south. Frequently flood waters on north ends of beheaded flood flow routes would reverse flow direction to erode north-oriented valleys.

• Sometimes flood flow captures and/or flood flow reversals would significantly alter what had been an evolving valley development pattern. Perhaps the most significant change in the valley development pattern occurred when uplift of mountains raised the deep “hole’s” southwest rim to elevations higher than low points on the decaying ice sheet’s surface. Lowest points on the ice sheet’s surface were deep south oriented ice-walled canyons being carved by large south-oriented supra-glacial melt water rivers, which at that time were supplying vast quantities of melt water to the actively eroding Mississippi and Missouri River valleys and their tributary valleys in regions south and east of Montana. Of particular importance to the figure 1 map area was a giant southeast and south-oriented ice-walled canyon located in what is now Saskatchewan, North Dakota, and South Dakota, and which over time became an ice-walled and bedrock-floored canyon which detached the ice sheet’s southwest margin. The northeast and east facing Missouri Escarpment in Saskatchewan, North Dakota, and South Dakota is what remains of that ice-walled and bedrock-floored canyon’s southwest and west wall. Deep east and northeast oriented valleys eroded headward from that giant southeast and south-oriented ice-walled and bedrock-floored canyon into Montana and northern Wyoming to capture massive southeast and south-oriented ice-marginal melt water floods flowing from the decaying ice sheet’s western margin in Canada. These deep northeast- and east-oriented valleys eroded headward in sequence with each valley beheading flood flow routes to what was then the newly eroded valley immediately to the south or to the southeast. The deep east-oriented Missouri River valley in northern Montana was one of the deep valleys that eroded headward from the giant ice-walled canyon in the North Dakota northwest corner to the figure 1 map area. This deep Missouri River valley was significantly deeper than the upland surface on which the southeast and south-oriented floods were flowing. As this deep Missouri River valley beheaded south- and southeast-oriented flood flow routes flood waters on north ends of the beheaded flood flow routes reversed flow direction to erode north and northwest-oriented tributary valleys (e.g. Smith River valley and Missouri River valley upstream from Wolf Creek). At the same time deep east-oriented tributary valleys eroded headward from the newly eroded Missouri River valley, with each east-oriented valley beheading flood flow routes to what was then the newly eroded east-oriented valley immediately to the south or southeast.

Figure 2: Detailed location map for South Fork Dearborn River-Missouri River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

• Figure 2 provides a detailed location map for the South Fork Dearborn River-Missouri River drainage divide area in Lewis and Clark County, Montana. Green shaded areas are National Forest lands, which are generally located in mountain regions. The east-west continental divide is marked and is labeled in the figure 2 southwest quadrant. Rogers Pass in the figure 2 southwest quadrant is where highway 200 crosses the continental divide. The Lewis and Clark-Cascade County line is marked and extends from the figure 2 north edge (east of center) to the figure 2 east edge (just north of southeast corner) with Lewis and Clark County located west of the line. The Missouri River flows in a north-northwest direction from the figure 2 south edge (east of center) to Holter Lake and Dam and then turns to flow in a northeast direction to the figure 2 northeast corner. Note how the Missouri River is joined by north-northeast oriented Little Prickly Pear Creek just north of Holter Dam. The Little Prickly Pear Creek alignment is a southwest extension of the northeast-oriented Missouri River valley segment. Headward erosion of what was a deep northeast-oriented Missouri River-Little Prickly Pear Creek valley beheaded multiple southeast-oriented flood flow routes and flood waters on the northwest ends of the beheaded flood flow routes reversed flow direction to erode northwest-oriented valleys. The southeast-oriented tributaries provide evidence of captured southeast-oriented flood flow routes while the north-northwest oriented Missouri River valley segment through Holter Lake provides evidence of a beheaded and reversed flood flow route. While evidence is not seen in figure 2 the flood flow reversal that eroded the north-northwest oriented Missouri River valley segment was extremely successful in capturing yet to be beheaded flood flow still moving south and west of the figure 2 map area.

• The Dearborn River is labeled near the figure 2 northwest corner and originates west and north of figure 2 and flows in an east, northeast, and southeast direction in the figure 2 northwest quadrant until it is joined by the north-northeast oriented South Fork Dearborn River. After being joined by the north-northeast oriented South Fork Dearborn River the Dearborn River flows in a northeast direction to be joined by southeast-oriented Auchard Creek and then the Dearborn River flows in a southeast, northeast, and southeast direction to join the northeast-oriented Missouri River along the Lewis and Clark-Cascade County line. The Middle Fork Dearborn River originates near Rogers Pass (across the continental divide) and flows in a northeast direction to join the southeast-oriented Dearborn River. The South Fork Dearborn River originates south and slightly east of Rogers Pass, which means it also originates in a high mountain region adjacent to the continental divide. South and east of South Fork Dearborn River headwaters are headwaters of southeast-oriented Lyons Creek, which flows to northeast-oriented Little Prickly Pear Creek. North and east of Lyons Creek is southeast-oriented Wolf Creek, which also originates near the northeast-oriented South Fork Dearborn River and which flows to join Little Prickly Pear Creek near the town of Wolf Creek. Rock Creek is a southeast-oriented Missouri River tributary located east and north of Wolf Creek, and also has headwaters near the northeast-oriented South Fork Dearborn River. The southeast-oriented Lyons Creek, Wolf Creek, and Rock Creek alignments suggest their valleys were eroded as southeast-oriented flood flow channels, which were then beheaded by headward erosion of the north-northeast oriented South Fork Dearborn River valley. While not seen in figure 2 this flood origin interpretation for figure 2 landforms predicts topographic maps should illustrate other types of evidence for former southeast-oriented flood flow channels which once crossed the South Fork Dearborn River drainage divides with southeast-oriented Rock Creek, Wolf Creek, and Lyons Creek and elsewhere along the South Fork Dearborn River-Missouri River drainage divide.

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

 

 

• Figure 3 illustrates the Dearborn River-Rock Creek drainage divide area. The north-northeast oriented Missouri River flows across the figure 3 southeast corner. The South Fork Dearborn River flows in a north-northeast direction from the figure 3 west edge (south of center) to join the southeast-oriented Dearborn River, which flows from the figure 3 northwest corner and which then turns to flow in a northeast direction to near the figure 3 north edge (west of the north center edge area). Once near the figure 3 north edge the Dearborn River then turns to flow in a southeast, northeast, and southeast direction (with numerous incised meanders) to join the Missouri River just east of the figure 3 east edge (just south of center). The Reef is a northwest-southeast oriented ridge in the figure 3 south center region and appears to be related to an underlying geologic structure (perhaps a plunging syncline or anticline), although my concern in this essay is with erosional landforms. On the southwest side of The Reef is southeast oriented Rock Creek, which flows to the figure 3 south center edge. Note how northwest-oriented Pruden Creek flows to the north-northeast oriented South Fork Dearborn River on the same alignment as the southeast-oriented Rock Creek valley. Also note the northwest-southeast oriented through valley linking the northwest-oriented Pruden Creek valley with the southeast-oriented Rock Creek valley. This northwest-southeast oriented through valley was eroded as a southeast-oriented flood flow channel prior to headward erosion of the north-northeast oriented South Fork Dearborn River valley. Headward erosion of the deep north-northeast oriented South Fork Dearborn River valley beheaded the southeast-oriented flood flow channel and flood waters on the northwest end reversed flow direction to erode the northwest-oriented Pruden Creek valley. North and east of Pruden Creek is the Nicholas Basin. Note several unnamed northwest-oriented South Fork Dearborn River tributaries in the Nicholas Basin. South of the headwaters of these northwest-oriented streams are northwest-oriented headwaters of northeast-oriented Deadman Creek, which flows to the Dearborn River. The northwest-oriented Deadman Creek tributaries are located in valleys eroded by reversals of southeast-oriented flood flow caused by headward erosion of the deep northeast-oriented Deadman Creek valley, which captured the southeast-oriented flood flow. Headward erosion of the north-northeast oriented South Fork Dearborn River valley next beheaded the southeast-oriented flood flow to the newly eroded northeast-oriented Deadman Creek valley. Also note southeast-oriented Dog Creek in the figure 3 southeast quadrant and how west of Coburn Mountain a northwest-southeast oriented through valley links the southeast-oriented Dog Creek valley with a north-oriented Deadman Creek tributary valley. The figure 3 map contour interval  is 50 meters and the through valley floor elevation at the drainage divide is between 1250 and 1300 meters. Coburn Mountain rises to 1582 meters and Electric Mountain on The Reef rises to 1619 meters. In other words the through valley is approximately 300 meters deep. The through valley was eroded by southeast-oriented flood flow prior to headward erosion of the northeast-oriented Deadman Creek valley, which beheaded and reversed the flood flow.

Figure 4: Detailed map of South Fork Dearborn River-Dog Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

 

• Figure 4 provides a detailed map of the South Fork Dearborn River-Dog Creek drainage divide area seen in less detail in figure 3 above. The Reef is labeled and is the northwest-southeast oriented ridge located near the figure 4 southwest corner. Coburn Mountain is also labeled and is located in the figure 4 southeast quadrant. Dog Creek originates in the figure 4 south center region (between the Reef and Coburn Mountain) and flows in a southeast direction to the figure 4 south edge (east half). South and east of the figure 4 map area Dog Creek joins the north-northeast oriented Missouri River as a barbed tributary (see figure 3). North-oriented streams flowing to the figure 4 north edge are tributaries to the east and southeast-oriented Dearborn River (just barely seen in figure 4 northeast corner), which also flows to the north-northeast oriented Missouri River as a barbed tributary. Of particular interest in this discussion is the north-oriented stream in section 14. Note how that section 14 north-oriented stream is linked by a through valley with east and southeast-oriented Dog Creek headwaters. Sevenmile School is located in the northwest-southeast oriented through valley which has an elevation at the drainage divide of between 4160 and 4200 feet (the figure 4 map contour interval is 40 feet). The high point on Coburn Mountain is shown as 5189 feet while the high point on the Reef (along figure 4 south edge) is shown as 5241 feet. In other words this northwest-southeast oriented through valley is approximately 1000 feet deep. The Sevenmile School through valley is actually the deepest channel eroded into the floor of a broad northwest-southeast oriented through valley linking the deep Dearborn River valley to the north with the southeast-oriented Dog Creek valley to the south. Other channels have higher valley floor elevations and link other north-oriented Dearborn River tributary valleys with the southeast-oriented Dog Creek valley, The board 1000-foot deep northwest-southeast oriented through valley between the Reef and Coburn Mountain was eroded by massive southeast-oriented flood flow moving to what was then the newly eroded and deep north-northeast oriented Missouri River valley, which captured the southeast-oriented flood flow and diverted the flood waters in a northeast and east direction to the floor of the decaying ice sheet (north and east of Montana). Once on the ice sheet floor the flood waters flowed on the floor of a giant ice-walled and bedrock-floored canyon in a southeast and south direction until headward erosion of deep north-oriented ice-walled and bedrock-floored canyons intersected with the south-oriented canyons and reversed the flood flow so the flood waters then flowed in a north direction across the decaying ice sheet floor. What is important in the figure 4 map is to recognize the depth of erosion that occurred. Flood waters scoured at least 1000 feet of bedrock material from the region between the Reef and Coburn Mountain, which suggests the deep Missouri River valley was at least 1000 feet deep as it eroded headward across this region of Montana.

Figure 5: Detailed map of South Fork Dearborn River-Deadman Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

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• Figure 5 illustrates a detailed topographic map of the South Fork Dearborn River-Deadman Creek drainage divide area seen in less detail in figure 3 above. The South Fork Dearborn River flows in a northeast direction across the figure 5 northwest corner. Deadman Creek originates near the figure 5 south center edge and flows in a northeast and north-northeast direction to the figure 5 northeast corner. As seen in figure 3 Deadman Creek is a Dearborn River tributary. Note how Deadman Creek has eroded a deep water gap across the northeast arm of the Reef, which suggests initially Deadman Creek flowed on an erosional surface as high as the top of the Reef today. The Nicholas Basin is labeled and is mostly drained by northwest-oriented South Fork Dearborn River tributaries, although one southeast-oriented Deadman Creek tributary is also shown. The figure 5 contour interval is 40 feet and the South Fork Dearborn River-Deadman Creek drainage divide in the Nicholas Basin has an elevation of between 4280 and 4320 feet. Elevations on the southwest arm of the Reef rise to 6085 feet, although most elevations are lower. Elevations on the northeast arm of the Reef seen in figure 5 are less than 5000 feet with 4953 feet being the high point, although as seen in figure 4 elevations south of figure 5 rise to 5247 feet. These elevations suggest the Nicholas Basin through valley records approximately 700 feet of erosion, if not more. The figure 5 drainage history can be described as beginning with massive southeast-oriented flood flow moving across the entire figure 5 map area on an erosional surface at least as high as the highest figure 5 ridges today (and possibly as high or higher than the highest figure 5 elevations today). Flood waters were probably moving to what was then the actively eroding and deep north-northeast oriented Missouri River valley. Headward erosion of the deep northeast-oriented Deadman Creek valley (from the what was then the actively eroding and deep Dearborn River valley, which was eroding headward from the newly eroded Missouri River valley) captured the southeast-oriented flood flow. Flood waters on the northwest end of the beheaded flood flow routes reversed flow direction to erode what is today the northwest-oriented Deadman River tributary valley between the southwest and northeast arms of the Reef. This deep erosion was probably aided by captured flood flow from southeast-oriented flood flow routes located west of the Deadman Creek valley head. Next headward erosion of the deep northeast-oriented South Fork Dearborn River valley beheaded the southeast-oriented flood flow to the newly eroded Deadman Creek valley. Flood waters on northwest ends of the beheaded flood flow routes reversed flow direction to erode the Nicholas Basin and the northwest-oriented South Fork Dearborn River tributary valleys. Again yet to be beheaded flood waters from west of the actively eroding South Fork Dearborn River valley head probably were captured (west of figure 5) and aided in the deep erosion of the Nicholas Basin.

Figure 6: Rock Creek-Wolf Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

 

• Figure 6 illustrates the Rock Creek-Wolf Creek drainage divide area south and west of the figure 3 map area and includes overlap areas with figure 3. The Missouri River flows in a north direction in the figure 6 southeast corner. The northeast-oriented stream joining the Missouri River in the figure 6 southeast corner is Little Prickly Pear Creek and the southeast oriented stream joining Little Prickly Pear Creek at the town of Wolf Creek (near figure 6 south edge) is Wolf Creek. Wolf Creek originates in the figure 6 west center south of Roberts Mountain and flows in a southeast direction to join Little Prickly Pear Creek at the town of Wolf Creek. The north and north-northeast oriented stream west of the Wolf Creek headwaters is the South Fork Dearborn River, which originates near the figure 6 southwest corner. The southeast oriented stream originating in the figure 6 southwest corner, just east and south of the South Fork Dearborn River headwaters, is the North Fork Lyons Creek, which flows to southeast-oriented Lyons Creek, which in turn flows to north-northeast oriented Little Prickly Pear Creek. Roberts Creek is the northwest-oriented South Dearborn River tributary located between Roberts Mountain and Limekiln Mountain in the figure 6 northwest quadrant. The Reef is labeled and extends in a southeast and south-southeast direction from the figure 6 north center. Rock Creek is the southeast-oriented stream originating near Roberts Mountain and flowing along the southwest margin of the Reef to join the north-oriented Missouri River downstream from where Little Prickly Pear Creek joins the Missouri River. Coburn Mountain is labeled and is located in the figure 6 northeast corner. Southeast-oriented Dog Creek flows between Coburn Mountain and the Reef. Note how the Rock Creek headwaters are linked by a deep east-oriented through valley (south of Limekiln Mountain) with the northwest-oriented Roberts Creek valley. This through valley was eroded by southeast- and east-oriented flood flow moving to what was then the actively eroding southeast-oriented Rock Creek valley prior to headward erosion of the deep north-northeast oriented South Dearborn River valley. This flood flow route illustrates how yet to be beheaded flood flow west of an actively eroding valley head could be captured and diverted to a previously beheaded flood flow channel (in this case the southeast-oriented Rock Creek valley) or to a beheaded and reversed flood flow channel (in this case the northwest-oriented Pruden Creek valley just north of figure 6). When South Dearborn River valley headward erosion did behead the southeast- and east-oriented flood flow channel flood waters on the northwest end of the beheaded flood flow channel reversed flow direction to erode the northwest-oriented Roberts Creek valley. North and east of Limekiln Mountain a northwest-southeast oriented through valley links the northwest-oriented Pruden Creek valley (north of figure 6) with the southeast-oriented Rock Creek valley and provides evidence of converging flood flow channels.

Figure 7: Detailed map of Roberts Creek-Rock Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

 

• Figure 7 provides a detailed topographic map of the Roberts Creek-Rock Creek drainage divide area seen in less detail in figure 6 above. The South Fork Dearborn River flows in a north-northeast direction from the figure 7 west edge (south of center) to the figure 7 north edge (west half). Roberts Creek originates in section 27 south of Limekiln Mountain and flows in a northwest direction to join the South Fork Dearborn River in the section 21 southwest quadrant. Rock Creek flows in an east-northeast and east direction across the section 23 and 24 south halves to the figure 7 east edge (north half) and then turns to flow in a southeast direction as seen in figure 6. The highway in the figure 7 northeast corner is located in the northwest-southeast oriented through valley linking the northwest-oriented Pruden Creek valley (north of figure 7) with the southeast oriented Rock Creek valley (east of figure 7). Note in the section 27 northeast quadrant (south of Limekiln Mountain) a southwest-northeast oriented through valley linking the Roberts Creek headwaters valley with the Rock Creek headwaters valley. The figure 7 map contour interval is 40 feet and the through valley floor elevation at the drainage divide appears to be between 5000 and 5040 feet (the contour lines are very faint in that area so it might be higher). Limekiln Mountain to the north rises to 5375 feet while Roberts Mountain just south of the figure 7 south edge rises to 6431 feet. Depending on which elevations are chosen the Roberts Creek-Rock Creek through valley is at least 300 feet. The Roberts Creek-Rock Creek through valley was eroded by southeast- and east-oriented flood flow from west of what was then the actively eroding north-northeast oriented and deep South Fork Dearborn River valley head. The flood flow had been captured by headward erosion of the deep southeast-oriented Rock Creek valley east of the figure 7 map area. Headward erosion of the deep north-northeast oriented South Fork Dearborn River valley beheaded southeast-oriented flood flow to the actively eroding Rock Creek valley and flood waters on the northwest end of the beheaded flood flow channel reversed flow direction to erode the northwest-oriented Pruden Creek valley. However, flood waters from further west were still able to reach the beheaded southeast-oriented Rock Creek valley by flowing south of Limekiln Mountain on the Roberts Creek-Rock Creek through valley route and eroded a deep flood flow channel there. Continued headward erosion of the deeper South Fork Dearborn River valley next beheaded the Roberts Creek-Rock Creek flood flow channel with flood waters on the northwest end of the beheaded flood flow channel reversing flow direction to erode the northwest-oriented Roberts Creek valley.

Figure 8: South Fork Dearborn-Little Prickly Pear Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

 

• Figure 8 illustrates the South Dearborn River-Little Prickly Pear Creek drainage divide area south and west of the figure 6 map area and includes significant overlap areas with figure 6. The east-west continental divide is marked with a dashed line and follows a high ridge seen near the figure 8 west edge. Areas west of the continental divide drain to the Pacific Ocean, while all areas east of the continental divide (most of the figure 8 map area) drains to Little Prickly Pear Creek and then to the Missouri River with water eventually reaching the Gulf of Mexico. Little Prickly Pear Creek flows in a north-northeast and northeast direction in the figure 8 southeast corner region. East of the figure 8 map area Little Prickly Pear Creek joins the north-northwest and north-northeast oriented Missouri River. Wolf Creek originates south of Roberts Mountain (in figure 8 northwest quadrant) and flows in a southeast direction to join northeast-oriented Little Prickly Pear Creek at the town of Wolf Creek. The south-southeast and south-southwest oriented stream originating just east of the continental divide in the figure 8 southwest quadrant and flowing to the figure 8 southwest corner is Canyon Creek and is a Little Prickly Pear Creek tributary with Canyon Creek flowing in a southeast direction and then making a U-turn south of the figure 8 map area when it joins north-northeast oriented Little Prickly Pear Creek. The north-oriented stream originating in the figure 8 west center region just east of the continental divide and flowing to the figure 8 north edge (west half) is the South Fork Dearborn River. The southeast-oriented stream originating just south and east of the South Fork Dearborn River headwaters is the North Fork Lyons Creek and joins the east-northeast oriented South Fork Lyons Creek just north of the figure 8 center to form southeast-oriented Lyons Creek, which south of the figure 8 map area joins north-northeast oriented Little Prickly Pear Creek. South-southeast and east-southeast oriented Rock Creek can be seen in the figure 8 northeast corner area and joins the Missouri River east of the figure 8 map area a short distance downstream from where Little Prickly Pear Creek joins the Missouri River. The Little Prickly Pear Creek valley eroded headward from the actively eroding Missouri River valley to capture massive southeast-oriented flood flow and as previously mentioned can be considered to be a southwest extension of the northeast-oriented Missouri River valley segment. Southeast-oriented Rock Creek, Wolf Creek, and North Fork Lyons Creek-Lyons Creek (and even the south-oriented Canyon Creek) valleys were eroded by southeast-oriented flood flow moving to the actively eroding Little Prickly Pear Creek valley. Flood waters were moving across the present day east-west continental divide to reach the actively eroding Little Prickly Pear valley and were next captured by headward erosion of the deep north-oriented South Fork Dearborn River valley. Figures 9 and 10 below provide detailed topographic maps of the South Fork Dearborn River drainage divides with Wolf Creek and with the North Fork Lyons Creek to illustrate additional evidence supporting this flood origin interpretation.

Figure 9: Detailed map of South Fork Dearborn River-Wolf Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

 

• Figure 9 illustrates a detailed topographic map of the South Fork Dearborn River-Wolf Creek drainage divide area seen in less detail in figure 8 above. The South Fork Dearborn River flows in a north and north-northeast direction from the figure 9 south edge (near southwest corner) to the figure 9 north edge (west half). Roberts Mountain is labeled and is located in the figure 9 northeast quadrant. Wolf Creek originates in the figure 9 center region and flows in a southeast direction to the figure 9 south edge (near southeast corner). Note in the section 5 southwest corner area a deep northwest-southeast oriented through valley linking a north-northwest oriented South Fork Dearborn River tributary valley with an east-oriented Wolf Creek tributary valley. A bench mark on the floor of this through valley at the drainage divide is shown as having an elevation of 5100 feet (the figure 9 map contour interval is 40 feet). The high point on Roberts Mountain to the northeast is 6431 feet and Denton Mountain to the southwest has a high point in excess of 6120 feet. In other words the through valley floor elevation is at least 1000 feet lower than adjacent mountains on either side. This deep northwest-southeast oriented through valley is actually a deep channel eroded into the floor of a much broader northwest-southeast oriented through valley eroded between Roberts Mountain and Denton Mountain. The much broader through valley was eroded by massive southeast-oriented flood flow moving to what was then the actively eroding and deep north-northeast oriented Little Prickly Pear Creek valley. At that time the deep north-oriented South Fork Dearborn River valley did not exist and flood waters were free to flow in a southeast direction across the figure 9 map area. The east-west continental divide is located a short distance west of the figure 9 map area and the southeast-oriented flood water almost certainly came from west of the continental divide. At that time flood waters were eroding deep southeast-oriented flood flow channels into a high level surface perhaps as high as the highest figure 9 elevations today. Headward erosion of the much deeper north-oriented South Fork Dearborn River valley captured the southeast-oriented flood flow supplying flood waters to the Wolf Creek flood flow channel and in doing so beheaded the southeast-oriented flood flow channel and also carved the continental divide east slope. Flood waters on the northwest end of the beheaded flood flow channel reversed flow direction to erode the north-northwest oriented South Fork Dearborn River tributary valley. At about the same time headward erosion of deep valleys west of the present day east-west continental divide beheaded the flood flow routes there resulting in flood flow reversals and captures that carved the continental divide west slope with the east-west continental divide being formed.

Figure 10: Detailed map of South Fork Dearborn River-North Fork Lyons 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 South Fork Dearborn River-North Fork Lyons Creek drainage divide area seen in less detail in figure 8 above and south and slightly west of the figure 9 map area. The east-west continental divide is marked with a dashed line and the word “DIVIDE” and is located along the high ridge seen near the figure 10 west edge. The South Fork Dearborn River originates in section 25 east of the continental divide and flows in a north-northeast and north direction to the figure 10 north edge (west half). The North Fork Lyons Creek originates in the section 36 west half and flows in a southeast direction to the figure 10 south center edge and as seen in figure 8 eventually reaches north-northeast oriented Little Prickly Pear Creek. A northwest-southeast oriented through valley links the north-oriented South Fork Dearborn River valley with the southeast-oriented North Fork Lyons Creek, although the through valley appears as a saddle near the corner of sections 25, 30, 31, and 36 on a narrow ridge separating the two valleys. The floor of the through valley has an elevation of between 6480 and 6520 feet (the figure 10 map contour interval is 40 feet). To the northeast the unnamed mountain top near the east center edge of section 30 rises to more than 7100 feet while to the southwest the unnamed mountain top near the center of section 36 rises to 7245 feet. The saddle along this narrow ridge is approximately 600 feet lower than adjacent mountain tops on either side and provides evidence of a former southeast-oriented flood flow channel to the what was then the actively eroding southeast-oriented North Fork Lyons Creek valley. Headward erosion of the much deeper north-oriented South Fork Dearborn River valley beheaded the southeast-oriented flood flow channel. Flood water supplying the southeast-oriented flood flow channel almost certainly was coming from west of the continental divide and low saddles in the continental divide ridge provide evidence of former flood flow routes, although those flood flow routes were eroding into an erosion surface as high as the present day continental divide. Headward erosion of the deep north-oriented South Fork Dearborn River valley captured that southeast-oriented flood flow (from west of the continental divide) which means deep valleys west of the continental divide had yet to be eroded. Headward erosion of deep valleys west of the continental divide subsequently captured the southeast-oriented flood flow and diverted the flood waters westward to eventually reach the Pacific Ocean while also carving the continental divide west slope and creating the east-west continental divide.

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.