Marias River-Big Sandy Creek drainage divide area landform origins, Chouteau County, Montana, USA

Authors

Abstract:

Topographic map interpretation methods are used to determine landform origins for the Marias River-Big Sandy Creek drainage divide area in northern Chouteau County, Montana. The study region is bounded on the west by the east and south oriented Marias River, the south by the northeast-oriented Missouri River, the east by the north-northeast oriented Missouri River-Big Sandy Creek through valley and Big Sandy Creek valley, and the north by the Chouteau-Hill County line. The Marias River today flows in an east and south direction to join the northeast-oriented Missouri River, which east of the study regions flows in a south and east direction around the Bears Paw Mountains mountain mass. Big Sandy Creek flows in a north-northeast direction along the Bears Paw Mountains west flank to join the Milk River, which flows in an east direction north of the Bears Paw Mountains. A large north-northeast oriented through valley links the northeast-oriented Missouri River with north-northeast oriented Big Sandy Creek valley and a large west to east oriented through valley links the east-oriented Marias River with north-northeast oriented Big Sandy Creek. South of the west to east oriented Marias River-Big Sandy Creek through valley are smaller north-south oriented through valleys linking north-oriented Big Sandy Creek tributary valleys with south oriented Marias River and Missouri River tributary valleys. Evidence is interpreted to indicate the region was crossed by a south and/or southeast oriented anastomosing channel complex as immense ice-marginal melt water floods flowed across the region. Melt water floods were derived from a rapidly melting thick North American ice sheet which had been located in a deep “hole.” The study region was deeply eroded as melt water floods eroded the deep “hole’s” southwest wall, which is today the Montana and northern Wyoming upper Missouri River drainage basin. Regional erosion of 100 meters or more can be documented in many study region areas and it is possible earlier melt water flood erosion events lowered the regional landscape significantly more.

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 Marias River-Big Sandy Creek drainage divide area landform origins in Chouteau 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 by leaving a link to those essays in a comment here.
  • This essay is also exploring a new geomorphology paradigm in which erosional landforms are interpreted as evidence left by immense glacial melt water floods. Implied in that interpretation is the immense floods were derived from a thick North American ice sheet that created a deep “hole” in the North American continent and also melted fast. The previously unexplored paradigm being tested in this and other 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 Marias River-Big Sandy Creek drainage divide area landform evidence in Chouteau County, Montana will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm (see 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.

Marias River-Big Sandy Creek drainage divide area location map

Figure 1: Marias River-Big Sandy Creek drainage divide area location map (select and click on maps to enlarge). National Geographic Society map digitally presented using National Geographic Society TOPO software.

 

  • Figure 1 provides a location map for the Marias River-Big Sandy Creek drainage divide area in Chouteau County, Montana and illustrates a region in north central Montana just south of the Canadian border. The Missouri River flows in a northeast direction from the figure 1 south edge (near Great Falls) to Fort Benton and Loma before turning to flow in a southeast and east-northeast direction south of the Bears Paw Mountains and then to the figure 1 east edge (south half). The Marias River flows in an east direction from the figure 1 west edge to Lake Elwell and from Tiber Dam it continues in an east and then south direction to join the Missouri River near Loma. The Milk River flows in a southeast direction from the figure 1 north center edge to Havre and then continues in an east direction north of the Bears Paw Mountains to the figure 1 east edge (north half). East of the figure 1 map area the Milk River eventually joins the Missouri River in northeast Montana. Big Sandy Creek originates in the high Bears Paw Mountains and flows in a west, southwest, and northwest direction to Big Sandy (the town). From Big Sandy (the town) Big Sandy Creek flows in a north-northeast direction along the Bears Paw Mountains west flank to join the Milk River a short distance west of Havre. The Marias River-Big Sandy Creek drainage divide are in Chouteau County investigated in this essay is located east of the south-oriented Marias River segment, north of the Missouri River, and west of the north-northeast Big Sandy Creek segment. The Cottonwood Creek-Sage Creek drainage divide landform origins essays describes the region immediately to the north, the Big Sandy Creek-Birch Creek drainage divide area landform origins essay describes the region immediately to the east, and the Missouri River-Missouri River drainage divide area landform origins north of the Highwood Mountains essay describes the region to the south. Essays can be found under Milk River or Marias River on the sidebar category list.
  • Water in Big Sandy Creek flows north of the Bears Paw Mountains while water in the Marias River flows south of the Bears Mountains, which today are fundamentally different flow routes. Yet the Marias River appears to be flowing toward the Big Sandy Creek headwaters area before it turns to flow in a south direction to join the Missouri River. And the northeast-oriented Missouri River appears to be flowing toward the north-northeast Big Sandy Creek segment before it turns to flow south of the Bears Paw Mountains. Detailed topographic maps, examples of which are illustrated in this essay, show multiple through valleys linking the Marias River and Big Sandy Creek drainage basins. These through valleys provide evidence of anastomosing flood channels used by immense ice-marginal melt water floods, which flowed both north and south of the Bears Paw Mountains.
  • What caused these immense melt water floods and how did the flood waters reach the Marias River-Big Sandy Creek drainage divide area? The melt water floods were from a rapidly melting thick North American ice sheet which had been located in a deep “hole.” The ice sheet had been large, probably comparable in size to the present day Antarctic Ice Sheet. The deep “hole” in which the ice sheet was located had been formed by deep glacial erosion and also by crustal warping caused by the ice sheet’s great weight. When fully developed the ice sheet had stood high above the surrounding continental surface, but also had “roots” that extended far below that surrounding continental surface. The initial continental surface probably no longer exists, however to visualize the deep “holes” magnitude there is evidence massive melt water floods flowed across what are today high level Rocky Mountain erosion surfaces, which suggests the entire upper Missouri River drainage basin in Montana and northern Wyoming eroded the deep “hole’s” southwest wall as huge north and northeast oriented valleys eroded headward from deep ice-walled canyons carved by supra-glacial melt water rivers into the decaying ice sheet’s surface. One these giant ice-walled canyons extended in a southeast and south direction across Saskatchewan, North Dakota, and South Dakota and in time became an ice-walled and bedrock-floored canyon, which detached the ice sheet’s southwest margin. Today the northeast and east-facing Missouri Escarpment is what remains of that huge ice-walled and bedrock-floored canyon’s southwest and west wall. The west-facing Prairie Coteau Escarpment in South Dakota is an east wall remnant while the Turtle Mountain southwest escarpment is a northeast wall remnant in North Dakota and the Moose Mountain southwest escarpment is a Saskatchewan northeast wall remnant.
  • When ice sheet melting began the ice sheet stood high above the surrounding continental surface and massive melt water floods flowed in whatever direction the topography permitted with significant south and southeast-oriented flood flow across the present day Rocky Mountain region and along the ice sheet’s southwest margin. Rocky Mountain uplift occurred as immense south and southeast-oriented melt water floods flowed across the region with uplift occurring first in the south. In time Rocky Mountain uplift blocked south and southeast-oriented flood flow routes in northwest Wyoming and southwest Montana while ice sheet melting lowered the decaying ice sheet surface. Immense south-oriented ice-marginal melt water floods from Alberta, which were trapped between rising Canadian Rocky Mountain ranges flowed across western Montana and into southwest Montana where rising mountain ranges channeled flood waters in a north direction into north central Montana. At the same time southeast-oriented ice-marginal melt water floods trapped by the Alberta Rocky Mountain front and the ice sheet’s southwest margin also flowed into north central Montana.  These immense melt water floods converged as a large east-oriented valley and its tributary valleys eroded headward from the giant bedrock-floored and ice-walled canyon located in northwest North Dakota. West of Poplar, Montana this large valley is today used by the Missouri River, which flows south of the Bears Paw Mountains, with the Milk River valley being a major tributary valley located north of the Bears Paw Mountains.  Headward erosion of the deep Missouri River valley in the figure 1 map area initially captured southeast-oriented melt water floods moving between the Rocky Mountain front to the west and the ice sheet’s southwest while headward erosion of the deep Milk River valley subsequently captured those southeast-oriented flood flow routes in sequence from east to west. At the same time headward erosion of the deep Missouri River valley beheaded and reversed south-oriented flood flow routes east of the Rocky Mountain front. These reversed flood flow routes then captured in southwest Montana south-oriented melt water flood flow routes trapped between Rocky Mountain ranges further to the west. Continued uplift in southwest Montana then reversed those western south-oriented flood flow routes diverting that melt water flood flow westward to the Columbia River drainage basin and the Pacific Ocean.

Detailed location map for Marias River-Big Sandy Creek drainage divide area

Figure 2: Detailed location map for Marias River-Big Sandy Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

  • Figure 2 provides a detailed location map for the Marias River-Big Sandy Creek drainage divide area in Chouteau County, Montana. County boundaries and some county names are shown. Hill County is north of Chouteau County. The Missouri River flows in a northeast direction from the figure 2 south edge (west half) to Fort Benton, Loma, and Virgelle. Near Virgelle the Missouri River makes an abrupt turn to flow in a south-southeast, south, and southeast direction before flowing in an east-northeast direction to the figure 2 east edge (the east-northeast Missouri River segment also serves as a county boundary). Note how the Missouri River flows toward the Bears Paw Mountains and then as it nears the Mountains it turns to flow in a south direction so as to flow south of the Bears Paw Mountains. Also note how most tributaries to the northeast-oriented Missouri River segment are oriented in southeast directions and enter the Missouri River as barbed tributaries. The Marias River flows from Lake Elwell (Tiber Reservoir) in an east-southeast and south direction to join the Missouri River near Loma. Note how the east-southeast oriented Marias River segment has a well-developed network of south- and southeast-oriented tributaries from the north. The Milk River flows in a southeast direction from the figure 2 north edge (east half) to Havre and then flows in an east direction to the figure 2 northeast corner. Note how the southeast-oriented Milk River flows toward the Bears Paw Mountains and turns in an east direction so as to flow north of the Bears Paw Mountains. Big Sandy Creek originates near Baldy Mountain, which has an elevation of 6916 feet and is the Bears Paw Mountains highest point. From the Baldy Mountain area Big Sandy Creek flows in a west and southwest direction before turning to flow in a northwest and north direction to the town of Big Sandy in northern Chouteau County and then Big Sandy Creek flows in a north-northeast direction along the Bears Paw Mountains west flank to join the Milk River just west of Havre. Big Sandy Creek tributaries in northern Chouteau County include northeast-oriented Sixmile Coulee and Twelvemile Coulee, which flow to Lonesome Lake, with Lonesome Lake appearing to drain in an east direction to north-northeast oriented Big Sandy Creek. Note how Sixmile Coulee originates very near the south-oriented Marias River segment suggesting an asymmetric drainage divide. North of the Hill-Chouteau County line southeast-oriented Sage Creek enters north-northeast oriented Big Sandy Creek as a barbed tributary. Another interesting feature is south of the northwest-oriented Big Sandy Creek segment are the northwest-oriented headwaters of Little Sandy Creek, which south of the town of Big Sandy turn to flow in a south direction to join the Missouri River. The region investigated in this essay is east of the south-oriented Marias River, south of the Hill-Chouteau County line, north of the northeast-oriented Missouri River, and west of south-oriented Little Sandy Creek and of north-northeast oriented Big Sandy Creek.

Big Sandy Creek-Little Sandy Creek drainage divide area

Figure 3: Big Sandy Creek-Little Sandy Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

  • Figure 3 illustrates the Big Sandy Creek-Little Sandy Creek drainage divide area south of the town of Big Sandy, Montana. The Missouri River meanders in a northeast direction near the south margin of the figure 3 southwest quadrant and east of the figure 3 south center edge turns to flow in a south direction to the figure 3 south edge. Spring Coulee is the southeast-oriented Missouri River tributary just barely seen in the figure 3 southwest corner. Coal Banks Coulee is the south-southeast oriented Missouri River tributary north of the Coal Banks Landing State Recreation Area with Dry Fork Coulee being a south-oriented Coal Banks Coulee tributary. Big Sandy Creek flows in a west-northwest and north-northwest direction from the figure 3 east edge (north of center) to the town of Big Sandy and then flows in a north-northeast direction to the figure 3 north edge and joins the east-oriented Milk River north of the Bears Paw Mountains. Coal Mine Coulee is the northwest-oriented Big Sandy Creek tributary draining across the figure 3 northeast corner. North of figure 3 map Big Sandy Creek has other northwest-oriented tributaries. Little Sandy Creek flows in a northwest direction from the figure 3 east edge (south of center) and then turns to flow in a south-direction to join the Missouri River where the Missouri River turns to flow in a south direction. Note the south-southeast oriented Little Sandy Creek tributary flowing near the small town of Verona. South of northwest-oriented Little Sandy Creek is northwest-oriented Alkali Coulee, which like Little Sandy Creek turns to flow in a south direction, but then turns to flow in a northwest direction to meet the southeast-oriented Missouri River as a barbed tributary. Probably one of the most obvious figure 3 landscape features is the north-northeast oriented through valley linking the Missouri River valley with the Big Sandy Creek valley. While it is tempting to say this large through valley was an early Missouri River flow route the figure 3 evidence documents the primary melt water flood flow direction across the figure 3 map area was in a southeast direction and was captured by headward erosion of a deep north-northeast oriented Big Sandy Creek valley (which may at one time have included segments of the northeast-oriented Missouri River valley to the southwest) and which also captured massive southeast-oriented melt water flood flow. Flood waters on northwest ends of beheaded flood flow routes reversed flow direction to erode the northwest-oriented Big Sandy Creek tributary valleys. This reversal of flood flow may have been aided by Bears Paw Mountains regional uplift. Headward erosion of the deep east-northeast oriented Missouri River valley south of the figure 3 map area captured the southwest end of this northeast-oriented Big Sandy Creek valley (now the northeast-oriented Missouri River valley segment south and west of figure 3) when southeast-oriented flood waters overwhelmed the northeast-oriented Big Sandy Creek valley and spilled in a southeast and south direction to what was then the actively eroding and deep east-oriented Missouri River valley.

Twelvemile Coulee-Little Sandy Creek drainage divide area

Figure 4: Twelvemile Coulee-Little Sandy Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

  • Figure 4 illustrates the Twelvemile Coulee-Little Sandy Creek drainage divide area north and west of the figure 3 map area and includes overlap areas with figure 3. Lonesome Lake is located at the figure 4 north center edge. Twelvemile Coulee drains in a north-northeast direction across the figure 4 northwest corner and north of figure 4 turns to drain in an east direction to Lonesome Lake. Big Sandy Creek flows in a north-northwest direction from the figure 4 east edge (near southeast corner) to the town of Big Sandy and then in a north and north-northeast direction to the figure 4 northeast corner. Lonesome Lake has no outlet shown on the map (north of figure 4) and no drainage route is shown to east-northeast oriented Lonesome Coulee, which joins Big Sandy Creek near the figure 4 northeast corner. However a shallow north-south oriented through valley links the Lonesome Lake Basin with the Lonesome Coulee valley and should Lonesome Lake ever overflow the water would flow to Lonesome Coulee and then to Big Sandy Creek. Note the small town of Verona near the figure 4 south center edge. A southeast and south oriented Little Sandy Creek tributary flows near Verona to the figure 4 south edge. Note how that south oriented tributary is linked by a through valley to the Lonesome Prairie region, which today is drained by Lonesome Coulee. West of Verona headwaters of south oriented Dry Fork Coulee drain to the figure 4 south edge (west half). Remember Dry Fork Coulee drains to Coal Banks Coulee which drains to the Missouri River and that Little Sandy Creek flows to the Missouri River. Note how the south oriented Dry Fork Coulee valley is also linked by a shallow north-south oriented through valley with the Lonesome Prairie region. Other more subtle north-south oriented through valleys can also be seen crossing the Lonesome Prairie-Missouri River drainage divide. These south oriented through valleys, which join the northeast-oriented Missouri River-Big Sandy Creek through valley as barbed tributaries, provide evidence the northeast-oriented Missouri River-Big Sandy Creek through valley was eroded by headward erosion of a deep northeast-oriented valley across south and southeast oriented flood flow and was not eroded prior to the massive melt water flood events. The multiple south and southeast oriented flood flow routes documented by the figure 3 and 4 map evidence suggest the northeast-oriented Big Sandy Creek valley and (a short time later) what was probably a north-northeast oriented tributary valley (the Twelvemile Coulee valley) eroded headward across a south- and/or southeast-oriented anastomosing channel complex.

Twelvemile Coulee-Missouri River drainage divide area

Figure 5: Twelvemile Coulee-Missouri River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

  • Figure 5 illustrates the Twelvemile Coulee-Missouri River drainage divide area south and west of the figure 4 map area and includes overlap areas with figure 4. The Maias River flows in a south-southwest direction near the west edge of the figure 5 northwest quadrant and joins the Missouri River south of the figure 5 map area. Note there are no significant Marias River tributaries from the east. The Missouri River can be just barely seen flowing in a northeast direction in the figure 5 southeast corner. Dry Fork Coulee drains in a south direction from the figure 5 east center area to south-southeast oriented Coal Banks Coulee, which joins the Missouri River near the figure 5 southeast corner. Spring Coulee drains in a southeast direction from near the figure 5 center to the figure 5 south edge (east half) and joins the Missouri River south of the figure 5 map area. Twelvemile Coulee drains in a north-northeast direction west of the Spring Coulee headwaters and then turns to drain in a northeast direction to the figure 5 north edge (east half). Sixmile Coulee is located between Twelvemile Coulee and the Marias River and drains in a north-northeast direction to the figure 5 north edge (west half). North of the figure 5 map area Sixmile Coulee and Twelvemile Coulee drain to Lonesome Lake, which if it ever overflows would drain to north-northeast oriented Big Sandy Creek and the Milk River. Note how Sixmile Coulee is draining in a north-northeast direction while the adjacent Marias River is flowing in a south-southwest direction. This situation developed when headward erosion of the deep east-oriented Milk River valley (to the north and east of figure 5) captured south- and southeast-oriented flood flow routes causing flood waters on north ends of beheaded flood flow routes to reverse flow direction and to erode north-oriented Milk River tributary valleys. The north-oriented Sixmile Coulee valley represents the westernmost of such north-oriented valleys in the figure 5 map area while the Marias River valley was eroded on the easternmost of the unaffected south-oriented flood flow routes. Note the shallow north-south oriented through valleys linking the northeast-oriented Twelvemile Coulee valley with the southeast-oriented Spring Coulee valley. These through valleys were eroded by southeast-oriented flood flow moving to the northeast-oriented Missouri River valley (which at that time may have been a southwest extension of the Big Sandy Creek valley). Figure 6 below illustrates those through valley in greater detail.

Detailed map of Twelvemile Coulee-Spring Coulee drainage divide area

Figure 6: Detailed map of Twelvemile Coulee-Spring Coulee drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

  • Figure 6 is a detailed topographic map of the Twelvemile Coulee-Spring Coulee drainage divide area seen in less detail in figure 5 above. Twelvemile Coulee is labeled in section 6 (figure 6 southwest quadrant) and drains in a north-northeast and northeast direction from section 6 into section 31 and finally to the figure 6 north edge in section 29. Spring Coulee drains in a south, east, and southeast direction from the section 5 southeast corner across the section 8 northeast corner to the figure 6 south edge in section 9 (near figure 6 south center edge). Note two well-defined through valleys (one in section 5 and the other in section 4) linking the northeast-oriented Twelvemile Coulee valley with the southeast-oriented Spring Coulee valley. The map contour interval is ten feet and the floor elevation of the section 5 through valley at the drainage divide is between 2970 and 2980 feet. Elevations in the section 4 northwest quadrant rise to more than 3050 feet while elevations in the section 5 southwest quadrant rise to more than 3040 feet (and south of the figure 6 map area near the Twelvemile Creek headwaters rise to 3143 feet with an elevation of 3195 feet being located near the south-oriented Marias River valley). The through valley using just elevations seen in figure 6 is at least 60 feet deep. The section 4 through valley floor has an elevation of between 3000 and 3010 feet with elevations on either side rising to more than 3050 feet. Continuing further east past additional north-south oriented through valleys to the Verona area seen in figure 4 leads to a hill with an elevation of 3081 feet before reaching the large north-northeast oriented Missouri River-Big Sandy Creek through valley seen in figure 3. High elevations in the adjacent Marias River-Big Sandy Creek drainage divide area suggest the through valleys seen in figure 6 are simply channels eroded into the floor of much broader and deeper north-south oriented through valleys eroded by immense south-oriented floods. Based on the high elevations adjacent to the Marias River valley and Bears Paw Mountain uplift region elevations just east of the north-northeast oriented Missouri River-Big Sandy Creek through valley (seen in figure 3) more than 200 feet of melt water flood erosion can be documented in the Marias River-Big Sandy Creek divide area with the strong possibility that the flood water erosion stripped much greater thicknesses of bedrock material from the region.

Marias River-Sixmile Coulee drainage divide area

Figure 7: Marias River-Sixmile Coulee drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

  • Figure 7 illustrates the Marias River-Sixmile Coulee drainage divide area south and west of the figure 5 map area and includes overlap areas with figure 5. The Marias River flows in a south-southwest and south-southeast direction near the figure 7 west edge. Fourmile Coulee is the east-northeast Marias River tributary in the figure 7 northwest corner. Sheep Coulee is the southeast and northeast-oriented tributary north of the figure 7 west center edge area. Chip Creek is the southeast-oriented Marias River tributary in the figure 7 southwest corner. Lone Tree Coulee and Fisher Coulee are south-oriented Marias River tributaries east of the Marias River. South and southeast-oriented streams east of Fisher Coulee drain to the northeast-oriented Missouri River, which can be seen in the figure 7 southeast corner. Sixmile Coulee is not labeled on figure 7 but drains in a northeast and north-northeast direction to the figure 7 north center edge. The West Fork (Sixmile Coulee) is labeled and joins Sixmile Coulee in the figure 7 north center area. Twelvemile Coulee can be seen draining in a northeast direction to the figure 7 north edge (east half). Note how northeast-oriented West Fork (Sixmile Coulee) is linked by a through valley with the south-oriented Lone Tree Coulee valley. This through valley provides evidence of south-oriented flood flow from the present day Big Sandy Creek drainage basin to the Marias River valley. The south-oriented flood flow was captured by headward erosion of the Sixmile Creek valley, which was related to headward erosion of the deep Milk River valley to the north and east of the figure 7 map area. Elevations in the figure 7 map area are given in meters with the contour interval being 20 meters. Discovery Butte in the figure 7 northwest quadrant has an elevation of 949 meters and two areas with elevations greater than 960 meters can be seen in the figure 7 west center area. The hill west of the Marias River and south of Fourmile Coulee near the figure 7 northwest corner rises to more than 1040 meters and near the Chip Creek headwaters west of the figure 7 west edge is a hill with an elevation of 1149 meters. The Marias River crosses the 800 meter contour line a short distance upstream from where Lone Tree Coulee joins it. The Missouri River in the figure 7 southeast corner is flowing at an elevation of between 760 and 780 meters. The high elevations provide markers to document minimal amounts of bedrock erosion accomplished by the massive south-oriented melt water floods. There is no reason to believe any of the high elevations represent the pre melt water flood surface so depths of bedrock erosion probably are much greater than shown. However based on the present day high elevations up to 200 meters of bedrock erosion occurred over much of the figure 7 map area with much deeper erosion in the Marias and Missouri River valleys.

Detailed map of Sixmile Coulee-Fisher Coulee drainage divide area

Figure 8: Detailed map of Sixmile Coulee-Fisher Coulee 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 Sixmile Creek-Fisher Coulee drainage divide area seen in less detail in figure 7 above. Sixmile Coulee is labeled and drains in a northeast direction across section 16 , the section 15 northwest corner and to the figure 8 north center edge in section 10. The West Fork Sixmile Coulee is also labeled and drains in a northeast direction from section 18  across the section 8 southeast quadrant to the figure 8 north edge in section 9. Twelvemile Coulee is also labeled and drains in a north and northeast direction toward the figure 8 northeast corner. The south-oriented stream originating in section 20 and draining across section 29 to the figure 8 south edge is the headwaters of Fisher Coulee, which drains to the Marias River. The south-oriented stream in section 26 and draining to the figure 8 south edge (east half) is the headwaters of south-oriented Dry Fork Coulee, which drains to the Missouri River (this is a different Dry Fork Coulee than the Dry Fork Coulee seen in figures 3 and 4). Note the drainage divide between the north-oriented drainage to the Big Sandy Creek and Milk River drainage basins and the south-oriented drainage to the Marias and Missouri Rivers. Starting at the figure 8 west edge in the north half of section 19 is a shallow through valley linking the northeast-oriented West Fork Sixmile Creek valley with the south-oriented Fisher Coulee valley. The map contour interval is 10 feet and the through valley has a floor elevation of between 3150 and 3160 feet. Note the high elevation of 3175 feet on the line between sections 20 and 21. In the east half of section 20 are two shallow through valleys linking the northeast-oriented Sixmile Creek valley with the Fisher Coulee valley (these valleys are defined by only one contour line on a side and have floor elevations of between 3120 and 3130 feet). In the northwest quadrant of section 22 is a deeper through valley linking the northeast-oriented Sixmile Coulee valley with the south-oriented Dry Fork Coulee valley. The floor of this through valley at the drainage divide has an elevation of between 3080 and 3090 feet. Still another through valley linking the Sixmile Coulee and Dry Fork Coulee valleys is seen in the section 23 northwest corner and has a floor elevation of between 3090 and 3100 feet. In the section 23 northeast quadrant are through valleys linking the north-oriented Twelvemile Coulee valley with the south-oriented Dry Fork Coulee valley. In section 24 is an eastern high point of 3143 feet. The southeast-oriented Spring Coulee valley is east of the figure 8 map area and elevations do not rise higher than 3143 feet. The multiple north-south oriented through valleys crossing the drainage divide in figure 8 suggest a former south-oriented anastomosing channel complex which was dismembered by headward erosion of deep valleys into the region.

Black Coulee-Sixmile Coulee drainage divide area

Figure 9: Black Coulee-Sixmile Coulee drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

  • Figure 9 illustrates the Black Coulee-Sixmile Coulee drainage divide area north of the figure 7 map area and north and west of the figure 4 map area and includes overlap areas with figure 4. The west-east oriented Hill-Chouteau County line is labeled and is slightly north of the figure 9 center. The Marias River flows in an east and south direction in the figure 9 southwest corner. Black Coulee drains in a south-southeast direction from the figure 9 north edge (near northwest corner) to the county line and then in a south and south-southwest direction to join the Marias River just south of the figure 9 south edge. The East Fork Black Coulee drains in a south-southwest direction from the figure 9 north edge (west of center) to join Black Coulee near the county line. Sixmile Coulee drains in a north-northeast, east, and north direction from the figure 9 south center edge almost to the county line and then turns to drain in a southeast and east direction to the figure 9 east edge (south of center). Twelvemile Coulee drains in a north-northeast direction from the figure 9 south edge (east half) to join Sixmile Coulee near the figure 9 east edge. Fourteenmile Coulee drains in a south-southwest and southeast direction from the figure 9 north edge (east half) to also join Sixmile Coulee near the figure 9 east edge. Black Butte is the high point shown on the figure 9 map with an elevation of 924 meters. With the exception of the north-south oriented ridge on which Black Butte is located all elevations except along the figure 9 north margin are lower than 900 meters. South of the figure 9 map area elevations rise to more than 900 meters except in the deep Marias River valley and Missouri River-Big Sandy Creek through valley (see figure 5 for example). North of the figure 9 map area elevations also rise significantly above 900 meters. What we are seeing in figure 9 is the west end of a west to east oriented through valley linking the east-oriented Marias River valley with the north-northeast oriented Big Sandy Creek valley. Elevations on the floor of this through valley, which extends eastward to Lonesome Lake and the Lonesome Lake Coulee valley seen in figure 4, are in the 860 to 880 meter range. High elevations both north and south of this west to east oriented through valley rise to more than 960 meters. The map grid shows squares one mile on a side indicating this west to east oriented through valley may have been tens of mile wide. This west to east oriented through valley represents a former east-oriented flood flow channel to what was then the actively eroding north-northeast oriented Big Sandy Creek valley. Flood waters were subsequently captured by headward erosion of the deep south-oriented Marias River valley segment resulting in the elbow of capture seen today.

Detailed map of Black Coulee-Sixmile Coulee drainage divide area

Figure 10: Detailed map of Black Coulee-Sixmile Coulee 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 Black Coulee-Sixmile Coulee drainage divide area seen in less detail in figure 9 above. Black Coulee is labeled and drains in a south direction near the figure 10 west edge with a turn to the west near the figure 10 southwest corner. Black Butte is located just south of the figure 10 southwest corner. Sixmile Coulee is not labeled but drains in a north-northeast direction across the figure 10 southeast corner region. At first this map looks like an uninteresting region of low relief. However a close look reveals the 2900 foot contour line defining a region of elevations greater than 2900 feet in the figure 10 southwest quadrant with highest elevations near the southwest corner. The Black Butte elevation just south of the southwest corner is 3033 feet. Continuing south of the figure 10 map area as seen in figure 7 elevations rise to more than 3100 feet. Areas higher than 2900 feet can be seen along the north edge of the figure 10 northeast quadrant. While these are isolated areas continuing north of the figure 10 map area reveals elevations do rise to more than 3100 feet. In other words figure 10 is illustrating the floor of the west to east oriented through valley linking the east-oriented Marias River valley with the north-northeast oriented Big Sandy Creek valley. This through valley probably captured massive southeast and south oriented flood flow and diverted that flood flow to the deep Milk River valley to the north. Headward erosion of the Milk River valley in southern Alberta beheaded the south and southeast oriented flood flow routes to this east-oriented flood flow channel. The fact that a deep south-oriented valley was able to capture the Marias River valley flood flow is evidence flood waters were overwhelming this 200 foot plus deep through valley and spilling in a south direction to what was at that time the actively eroding Missouri River (Big Sandy Creek?) valley. Capture of the Marias River drainage basin by the northeast-oriented Missouri River (Big Sandy Creek?) valley ended east-oriented flood flow in the through valley to the Big Sandy Creek valley.

Additional information and sources of maps studied

This essay has provided only a sample of the detailed topographic map evidence supporting the flood erosion interpretation. Many additional illustrations could be provided. Readers are encouraged to look at mosaics of detailed topographic maps to see the abundance of available data. Maps used in this study were created and published by the United States Geologic Survey and can be obtained directly from the United States Geological Survey and/or from dealers offering United States Geological Survey maps. Hard copy maps can also be observed at United States Geological Survey map depositories, which are located throughout the United States and elsewhere. Illustrations used here were created using National Geographic Society TOPO software and digital map data. TOPO software and map data can be obtained from the National Geographic Society and/or dealers offering National Geographic Society digital map data.

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