Introduction:

• The purpose of this essay is to use topographic map interpretation methods to explore Missouri River-Missouri River drainage divide area landform origins north of the Highwood Mountains region, 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 Missouri River drainage basin landform origins research project essays is a thick North American ice sheet, comparable in thickness to the Antarctic ice sheet, occupied the North American region usually recognized to have been glaciated, and through its weight and erosive actions created a deep North American “hole”. The southwestern rim of that deep “hole” is today preserved in the high Rocky Mountains. The ice sheet through its weight and deep erosion (and perhaps deposition) along major south-oriented melt water flow routes caused significant crustal warping and tectonic change, through its action of melting fast produced immense floods that flowed across the continent, and through its action of melting fast systematically opened up space in the ice sheet created “hole” so headward erosion of newly developed north-oriented drainage systems captured immense south-oriented melt water floods and diverted immense melt water floods north into space the ice sheet had once occupied.

• If this previously unexplored paradigm is correct the geographic region explored by this essay should contain evidence of immense floods that were captured by headward erosion of new valley systems so as to cause the floods to flow in a different direction. Ability of this previously unexplored paradigm to explain Missouri River-Missouri River drainage divide area landform evidence north of the Highwood Mountains will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm. This essay is included in the Missouri River drainage basin landform origins research project essay collection.

Missouri River-Missouri River drainage divide area location map

Figure 1: Missouri 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 Missouri River-Missouri River drainage divide area north of the Highwood Mountains location map and illustrates a region in central Montana. The Highwood Mountains are not labeled, but Highwood Baldy, the highest point in the Highwood Mountains, is labeled and is located east of Great Falls. The Missouri River flows northwest in the figure 1 southwest quadrant area through Canyon Ferry Lake and Holter Lake and then turns northeast from Wolf Creek to flow to Great Falls, Fort Benton, and Loma before turning southeast, east-northeast, and southeast to flow to Fort Peck Lake (located along the figure 1 east edge). The Missouri River-Missouri River drainage divide area considered in this essay is located between the northeast-oriented Missouri River (flowing through Great Falls, Fort Benton and Loma) and the southeast-oriented Missouri River east of Loma. Included in the Missouri River-Missouri River drainage divide area considered here are areas in the northern Highwood Mountains. Towns located in the Missouri River-Missouri River drainage divide area include Highwood, Shonkin, Montague, and Geraldine. Kingsbury Lake and Big Lake are also located in the Missouri River-Missouri River drainage divide area discussed in this essay.

• Landform evidence in this essay is interpreted in the context of an immense southeast-oriented flood that crossed the entire figure 1 map area. Prior to Missouri River valley headward erosion the deep Musselshell River valley eroded headward into the figure 1 map area to capture southeast-oriented flood waters that had been moving to the newly eroded deep Yellowstone River valley (located south of the figure 1 map area) and to divert the captured flood waters further to the north and northeast. Initially the Highwood Mountains were not obstacles to southeast-oriented flood flow (the Highwood Mountains may have been surrounded by easily eroded sediments and/or ice, which flood waters removed, or the Highwood Mountains may have been uplifted above the surrounding region as flood waters eroded the area). As flood waters deeply eroded the figure 1 map area the Highwood Mountains emerged as significant obstacles to flood movements, especially after the deep east-oriented Missouri River valley eroded headward into the region north of the present day Judith Basin. At first southeast-oriented flood waters eroded valleys into the emerging Highwood Mountains. Later flood waters were channeled around the Highwood Mountains. Headward erosion of northeast-oriented Missouri River valley subsequently captured all southeast-oriented flood waters.

• Important to understanding what happened are northwest-oriented Missouri River tributaries west of the Highwood Mountains. These northwest-oriented Missouri River tributary valleys were initiated as southeast-oriented flood flow routes that supplied significant water to help erode the northeast and north-oriented Missouri River tributary valleys east of the Highwood Mountains. Southeast-oriented flood flow in those southeast-oriented valleys was reversed when headward erosion of the northeast-oriented Missouri River valley beheaded southeast-oriented flood flow routes across the Highwood Mountains region. Reversal of flow may have also been aided by Highwood Mountains uplift as flood waters were eroding the figure 1 map area. The Missouri River-Arrow Creek drainage divide area essay, the Arrow Creek-Judith River drainage divide area essay, the North Fork Smith River-North Fork Musselshell River drainage divide area essay, and the Big Sandy Creek-Birch Creek drainage divide area essay, describe drainage divides located near the Missouri River-Missouri River drainage divide area discussed here. Essays can be located under appropriate river names on the sidebar category list (e.g. MT Missouri River, Judith River, Smith River, Musselshell River, and Milk River, with Big Sandy Creek being a Milk River tributary).

Missouri River-Missouri River drainage divide area detailed location map

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

Figure 2 illustrates a somewhat more detailed map of the Missouri River-Missouri River drainage divide area discussed in this essay. Cascade, Chouteau, and Fergus Counties are located in Montana. The Missouri River flows northeast from the figure 2 west edge to Great Falls and Fort Benton and then almost to the figure 2 north edge. At the figure 2 north edge the Missouri River turns to flow southeast and upon reaching the Chouteau County-Fergus County boundary to flow east to the figure 2 east edge. Green areas are National Forest lands, which usually are located in mountainous areas. The green area in the figure 2 south center area is located in the Highwood Mountains. Arrow Creek originates in the Highwood Mountains and flows southeast and then turns east, northeast, and north to flow to the Missouri River. The northeast-oriented Chouteau County-Fergus County boundary line follows the Arrow Creek valley. Highwood Creek originates in the Highwood Mountains and flows northwest through Highwood to join the northeast-oriented Missouri River midway between Great Falls and Fort Benton. Also of concern here is Shonkin Creek, which originates in the Highwood Mountains, north of Highwood Creek, and flows north-northwest through Shonkin (near the figure 2 center) before joining the northeast-oriented Missouri River near Fort Benton. Flat Creek is a Missouri Creek tributary, which flows in a northeast direction from the Highwood Mountains near Geraldine and then north before turning to flow east to join the Missouri River just before the Missouri River becomes east-oriented.

• Evidence presented in this essay suggests the Highwood Creek and Shonkin Creek valley alignments originated as southeast-oriented flood flow channels that were first captured by headward erosion of what was then an actively eroding northeast and north-oriented Arrow Creek drainage basin. Evidence presented here demonstrates some of this southeast-oriented flood flow moved across the present day Highwood Mountains. Based on this evidence the Highwood Mountains are interpreted to have emerged as flood waters eroded the region. Emergence of the Highwood Mountains may have occurred as flood waters removed easily eroded sediments and/or ice that had buried the Highwood Mountains and/or as the Highwood Mountains were uplifted while flood waters eroded the region. In either case southeast-oriented flood flow to the northeast and north-oriented Arrow Creek valley ended when headward erosion of the northeast-oriented Missouri River valley west of the Highwood Mountains captured the southeast-oriented flood flow routes.

North end of the Missouri River-Missouri River drainage divide area

Figure 3: North end of the Missouri River-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 Missouri River-Missouri River drainage divide area north of the Highwood Mountains. The Missouri River flows northeast from the figure 3 southwest corner almost to the figure 3 north edge and then turns abruptly to flow south-southeast to the figure 3 east center edge. Just west of Loma (located along the figure 3 west edge) is the southeast-oriented Marias River, which joins the northeast-oriented Missouri River at Loma (see figures 1 and 2). An interesting through valley links the northeast-oriented Missouri River in the figure 3 southwest corner area with the south-southeast oriented Missouri River in the figure 3 northeast corner area. The southwest end of that through valley is drained by west-oriented Rowe Coulee. A middle section of the through valley is drained by northwest, north, and northwest oriented Crow Coulee and its northwest and west-oriented tributary Homestead Coulee. The northeast section of the through valley is labeled The Sag and drains to north-northwest and north-northeast oriented Rattlesnake Coulee. The through valley provides evidence water once flowed through it and also provides an interesting puzzle to unravel.

• Some clues to unraveling this puzzle are provided the north-northwest oriented headwaters of Rattlesnake Coulee and their alignment with the south-southeast oriented headwaters of Coal Mine Coulee in the figure 3 southeast quadrant. Also, the northwest orientation of Crow Coulee (in figure 3 center area) and of Homestead Coulee headwaters appears aligned with southeast-oriented Sherry Coulee in the figure 3 southeast corner. Northwest-oriented Crow Coulee headwaters are also linked by a through valley (south of figure 3) with a south-southeast oriented Flat Creek tributary (see figure 5 below). These clues suggest the northeast-oriented through valley was eroded across multiple southeast-oriented flood flow routes such as might be found in a southeast-oriented anastomosing channel complex, The present day south-southeast oriented Missouri River valley alignment in figure 3 probably originated as one of those southeast-oriented flood flow channels. Apparently as the deep south-southeast oriented Missouri River valley eroded headward into the figure 3 map area it beheaded and reversed a south-southeast oriented flood flow channel using the present day Rattlesnake Coulee headwaters and Coal Mine Coulee alignment. The reversed flood flow eroded the north-northeast oriented Rattlesnake Coulee north end and northeast-oriented Sag valley eroded headward to capture southeast-oriented flood flow further to the southwest. Headward erosion of that northeast-oriented Sag valley beheaded and reversed flood waters on the northwest end of what was then a southeast-oriented flood flow channel using the Homestead Coulee headwaters-Sherry Coulee alignment. As headward erosion of the northeast-oriented valley progressed west it also captured southeast-oriented flood flow on the present day Marias River alignment. The northeast-oriented Missouri River valley then eroded southwest to behead and reverse flood flow routes to the Rowe Coulee-Crow Coulee-Homestead Coulee-The Sag-Rattlesnake Coulee valley resulting in the present day dismembered drainage routes in that through valley.

Highwood Creek-Shonkin Creek drainage divide area

Figure 4: Highwood Creek-Shonkin Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 4 illustrates the Highwood Creek-Shonkin Creek drainage divide southwest of the figure 3 map area. The Missouri River flows northeast from the figure 4 west center edge area to the figure 4 north center edge. Fort Benton is the town located just north of the figure 4 north center edge. The north edge of the Highwood Mountains can be seen along the figure 4 south edge (east half). Shonkin Creek is the north-northwest and north-northeast oriented stream flowing from the Highwood Mountains in the figure 4 southeast quadrant to join the Missouri River just north of the figure 4 north center edge (near Fort Benton). Highwood Creek is the northwest-oriented stream flowing from the figure 4 south edge by the town of Highwood to join the northeast-oriented Missouri at the figure 4 west enter edge. Note how the northwest-oriented Highwood Creek valley and north-northwest and north-northeast oriented Shonkin Creek valley are linked along the Highwood Mountains north flank by a deep through valley named The Sag. Also note how that through valley continues east of the Shonkin Creek valley through Shonkin Lake, White Lake, Big Lake, and Kingsbury Lake (in figure 4 southeast corner). Figure 5 below illustrates how the through valley continues in a southeast direction until it joins the northeast-oriented Arrow Creek valley, which the through valley crosses to join the north-oriented Judith River valley, which drains to the east-oriented Missouri River valley east of the Highwood Mountains (see figures 1 and 2). This through valley has been interpreted in the geomorphology literature as having been formed when “ice crowded the [Missouri] river southward nearly to the Highwood Mountains, compelling it to cut a new channel (Shonkin Sag) which it abandoned when ice departed and opened up a lower way for the water to escape.”[1] While Fennaman’s interpretation (which was based on earlier interpretations) does explain some evidence, it does not explain all of the evidence illustrated in the Missouri River-Missouri River drainage divide area and in adjacent drainage divide areas (e.g. see Missouri River-Arrow Creek drainage divide area essay). Evidence is better explained by “the thick ice sheet that melted fast paradigm” used by Missouri River drainage basin landform origins research project essays to interpret detailed topographic map evidence. Figure 4 evidence is interpreted in a similar manner as the figure 3 evidence is interpreted. The southeast-oriented through valley from the White Lake area to the figure 4 southeast corner was initiated as just one several southeast-oriented flood flow channels which eroded northwest from a deep northeast-oriented valley that eroded southwest from what was then the newly eroded east-oriented Missouri River valley (east of the Highwood Mountains). The northeast-oriented valley from Highwood to White Lake was eroded headward from the southeast-oriented valley to capture additional southeast-oriented flood flow routes. Headward erosion of the northeast-oriented Missouri River valley then beheaded and reversed the southeast-oriented flood flow routes, which dismembered flood flow in the “Shonkin Sag” through valley and created the present day drainage system.

Flat Creek-Arrow Creek drainage divide area

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

Figure 5 illustrates the Flat Creek-Arrow Creek drainage divide area east and south of figure 4 and includes overlap areas with figure 4. The southeast and east-oriented Missouri River is located in the figure 5 northeast corner area. Arrow Creek flows northeast from the figure 5 south enter edge and then turns north to flow to the east-southeast oriented Missouri River. Flat Creek flows northeast from the figure 5 southwest corner area to the highway and then turns to flow north and north-northeast to Dammel Reservoir. At Dammel Reservoir Flat Creek turns to flow east-southeast and northeast to the east-southeast oriented Missouri River. A southeast-oriented Flat Creek tributary joins Flat Creek at Dammel Reservoir (and is linked by a through valley with northwest-oriented headwaters of northwest-oriented Crow Coulee). The Shonkin Sag through valley seen in figure 4 is continues in figure 5 from Kingsbury Lake (along the figure 5 west edge) to the Arrow Creek valley in the figure 5 south center edge area and then northeast from the Arrow Creek valley in the figure 5 northeast corner area to join the north-oriented Judith River valley (located just east of the figure 5 map area-see Arrow Creek-Judith River drainage divide area essay). The Shonkin Sag through valley is just one of several northwest-southeast oriented valleys located in the figure 5 map area. The Crow Coulee-Flat Creek through valley and the present day Missouri River provide obvious evidence of at least two other parallel valleys. Closer investigation of the figures 3, 4, and 5 map area evidence reveals these valleys are interconnected in the same manner flood flow channels in an anastomosing channel complex are interconnected. For example, in figure 5 the Crow Coulee-Flat Creek through valley is linked to the Missouri River by the present day Flat Creek valley east of Dammel Reservoir and is also linked to the Missouri River valley by the Flat Creek-Arrow Creek through valley and also by the Arrow Creek-Judith River through valley. These through valleys provide evidence of an anastomosing channel complex that once carried flood waters across the figure 5 map area. The present day drainage system evolved as the deep Missouri River valley eroded northwest and then southwest to behead and reverse flood flow routes using competing channels.

Shonkin Creek headwaters area

Figure 6: Shonkin Creek headwaters area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 6 illustrates the Shonkin Creek headwaters area and is located south of the figure 4 map area (and includes overlap areas with figure 4). Northwest-oriented Highwood Creek and North Fork Highwood Creek are located in the figure 6 southwest quadrant. Shonkin Creek originates in the figure 6 south center south of Mount Kennon and flows northwest, north, west, and northwest to the figure 6 north edge. Note how Shonkin Creek headwaters are linked by a high level through valley with southeast-oriented Squaw Creek, which flows to the figure 6 south edge and then to southeast-oriented Cottonwood Creek, which flows to northeast-oriented Arrow Creek. Northeast of Mount Kennon is Postill Creek, which is a northwest oriented Shonkin Creek tributary. Postill Creek is also linked by a high level through valley (or saddle notched in what is today a high ridge) to southeast-oriented Squaw Creek. Shonkin Creek as seen in figure 4 flows northwest to the “Shonkin Sag” through valley and then after flowing for a short distance in the through valley continues northwest to the northeast-oriented Missouri River. In addition, note how southeast of Carter Mountain (in the figure 6 center north area) the northwest oriented Shonkin Creek valley is linked by a through valley with headwaters of northeast-oriented Lepleys Creek. Lepleys Creek today flows to the “Shonkin Sag” valley and then northwest, north, and northeast to Big Lake, which does not appear to have an outlet. Note also east of Mount Kennon northwest- and northeast-oriented Alder Creek, which is also linked by a high level valley with southeast-oriented drainage to southeast-oriented Cottonwood Creek and also to southeast-oriented headwaters of northeast-oriented Flat Creek. Alder Creek flows to Kingsbury Lake located in the “Shonkin Sag” through valley (which today has no outlet) and Flat Creek as seen in figure 5 flows northeast into the “Shonkin Sag” through valley and then flows north to Dammel Reservoir before flowing east and northeast to the Missouri River (see figure 5). Any explanation for the “Shonkin Sag” valley must also explain these other valleys as well. The interpretation provided here is the high level through valleys were eroded by multiple southeast-oriented flood flow channels being eroded into the emerging Highwood Mountains. Southeast-oriented flood flow across the Highwood Mountains was then systematically beheaded and reversed as the deep east-oriented Missouri River valley eroded west and northwest into the region. The southeast-oriented “Shonkin Sag” through valley segment seen in figure 5 was initiated as one of several anastomosing flood flow channels moving southeast-oriented flood water to what was then the Missouri River valley head. Subsequently, perhaps for reasons not determinable from the local region topographic maps, the present day Missouri River valley route captured all southeast-oriented flood flow from the parallel and competing flood flow channels.

Detailed map of Shonkin Creek-Lepleys Creek drainage divide area

Figure 7 illustrates the Shonkin Creek-Lepleys Creek drainage divide area seen in less detail in figure 6 above. Shonkin Creek flows northwest from the figure 7 south center edge to the figure 7 northwest corner. Lepleys Creek originates in section 30 and flows northeast through sections 29 and 20 to the figure 7 north edge. A well-defined through valley links the northwest-oriented Shonkin Creek valley with the northeast-oriented Lepleys Creek valley (the northeast-oriented road from section 36 to sections 31, 30, 29, and 20 uses the through valley. The through valley is a water eroded feature and must be explained by any interpretation of Highwood Mountains area drainage history. The interpretation provided here is the Shonkin Creek valley was initiated by southeast-oriented flood flow which continued southeast across the Highwood Mountains region. At that time the Highwood Mountains had not yet emerged as a significant mountain region. Deep flood erosion of the region east of the Highwood Mountains (probably associated with headward erosion of the deep east-oriented Missouri River valley) and possibly uplift of the Highwood Mountains caused flood waters to begin to carve deep valleys in the present day Highwood Mountains. As headward erosion of the deep east-oriented Missouri River valley reached the Highwood Mountains area deep northeast-oriented tributary valleys eroded southwest to capture the southeast-oriented flood flow routes. The deep Arrow Creek valley successfully eroded into the region southeast of the Highwood Mountains to capture southeast-oriented flood flow routes moving flood waters across the emerging Highwood Mountains. As Missouri River valley headward erosion progressed additional northeast-oriented valleys eroded southwest into the Highwood Mountains region to capture the southeast-oriented flood flow. These valleys were not as successful as the Arrow Creek valley because the Highwood Mountains are composed of erosion resistant rocks. However, some of these valleys did succeed in capturing some southeast-oriented flood flow and in diverting that captured flood water northeast. Headward erosion of the northeast-oriented Lepleys Creek valley was one such valley. As seen in figure 7 the northeast-oriented Lepleys Creek valley successfully eroded headward to capture southeast-oriented flood flow in the Shonkin Creek valley and to divert that flood flow northeast to what was then the actively eroding southeast-oriented “Shonkin Sag” valley head, which was eroding headward from what was then the actively eroding Missouri River valley head. Headward erosion of the “Shonkin Sag” valley soon thereafter beheaded and reversed southeast-oriented flood flow in the figure 7 Shonkin Creek valley (although it was headward erosion of the deep northeast-oriented Missouri River valley that beheaded and reversed southeast-oriented flood flow in the Shonkin Creek valley north of the “Shonkin Sag” valley).

Shonkin Creek-Cottonwood Creek drainage divide area

Figure 8 illustrates the Shonkin Creek-Cottonwood Creek drainage divide area east of the figure 6 map area and includes overlap areas with figure 6. Northwest-oriented Shonkin Creek flows west of Mount Kennon to the figure 8 northwest corner. Postril Creek is located east of Mount Kennon and flows northwest to join Shonkin Creek in the figure 8 northwest corner area. West of Postril Creek is northwest and northeast oriented Alder Creek. Arrow Creek is the northeast oriented stream in the figure 8 southeast corner and east and north of figure 8 turns to flow north to join the east-oriented Missouri River. Cottonwood Creek is the southeast and east-oriented stream joining Arrow Creek in the figure 8 southeast corner. Squaw Creek and Timber Creek are the two southeast-oriented Cottonwood Creek tributaries linked by high level through valleys (or saddles notched in a high ridge) with northwest-oriented Shonkin Creek headwaters. Gerard Creek is a northeast and southeast oriented Cottonwood Creek tributary located southeast of Squaw Creek and using the same northwest-southeast alignment as northwest-oriented Shonkin Creek and southeast-oriented Squaw Creek. Square Butte and Round Butte appear to be some sort of igneous intrusions (or other localized erosion resistant rock). Note multiple southeast-oriented streams and valleys in the figure 8 south center area and southeast quadrant. These multiple southeast-oriented valleys provide evidence of a southeast-oriented anastomosing channel complex that was supplying large quantities of southeast-oriented flood water to the northeast-oriented Arrow Creek valley. The flood water was coming from northwest of the present day Highwood Mountains and crossed the Highwood Mountains areas using routes now used by the northwest-oriented Shonkin Creek and Alder Creek (although the northwest-oriented valleys have since been deepened). Southeast-oriented flood flow into the deep northeast-oriented Arrow Creek valley ended as flood waters eroded deep valleys (such as the “Shonkin Sag” and present day Missouri River valleys) around the north end of the Highwood Mountains. Those valleys one by one captured the southeast-oriented flood flow routes and flood waters on the northwest ends of the beheaded flood flow routes reversed flow direction to initiate the present day northwest-oriented drainage routes and also to create the present day drainage divide. Because southeast-oriented flood flow channels were beheaded one by one and because the flood flow channels were anastomosing (or interconnected), reversed flow on newly beheaded flood flow routes often captured significant yet to be beheaded flood flow from adjacent flood flow channels. This captured yet to be beheaded flood flow helped erode the deep northwest-oriented valleys seen today.

Cottonwood Creek-Gerard Creek drainage divide area

Figure 9 illustrates the Cottonwood Creek-Gerard Creek drainage divide area seen in less detail in figure 8 above. Squaw Creek flows southeast from the figure 9 northwest corner to join east and southeast-oriented Cottonwood Creek. East of Squaw creek is southeast-oriented Tolan Creek, east of Tolan Creek is southeast-oriented Timber Creek, and east of Timber Creek is southeast-oriented Merril Creek. Gerard Creek flows northeast from the figure 9 southwest corner area to the figure 9 center south and then turns to flow southeast in a large southeast-oriented valley. Note how the southeast-oriented Gerard Creek valley is linked by a well-defined through valley with the southeast-oriented Squaw Creek valley (remember the Squaw Creek valley is aligned with the northwest-oriented Shonkin Creek valley). Also note how the southeast-oriented Gerard Creek valley is linked by a through valley with the southeast-oriented Timber Creek valley. Further, note how the southeast-oriented Gerard Creek valley is linked to the southeast-oriented Cottonwood Creek in the figure 9 southeast quadrant. These southeast-oriented valleys are providing evidence of a southeast-oriented anastomosing channel complex, where headward erosion of the Cottonwood Creek channel beheaded southeast-oriented flood flow routes to the Gerard Creek valley. The southeast-oriented flood flow was coming across the high ridge that today serves as the Shonkin Creek-Cottonwood Creek drainage divide (see figure 8). In other words, at the time the Cottonwood Creek valley eroded headward to capture southeast-oriented flood flow to the Gerard Creek valley, large quantities of southeast-oriented flood waters were still moving across the Highwood Mountains. Flood waters could not move across the Highwood Mountains with present day topography, which means at the time flood waters eroded the figure 9 map area the present day topography did not exist northwest of this figure 9 map area. There was no “Shonkin Sag” nor was there a Missouri River valley to the northwest. In fact, there was no open route between the Highwood Mountains and the Little Belt Mountains to the south as there is today. The topography at that time was such that flood waters had to flow southeast to the deep Cottonwood Creek valley, which had eroded headward from the deep Arrow Creek valley, which had eroded headward from the actively eroding and deep Missouri River valley.

Alder Creek-Flat Creek drainage divide area

Figure 10: Alder Creek-Flat Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 10 illustrates the Alder Creek-Flat Creek drainage divide area seen in less detail in figures 6 and 8 above. The figure 10 map area is located north and somewhat east of the figure 9 map area. The West Fork Flat Creek flows northeast in sections 14 and 13 into the figure 7 northeast corner area. Alder Creek originates in the northeast corner of section 22 and flows northwest through section 15 and then flows north to the figure 10 north edge. Through valleys between Alder Creek headwaters and Flat Creek headwaters are well-preserved. A well-defined through valley in section 23 links headwaters of northwest-oriented Alder Creek with headwaters of both southeast oriented Merril Creek (which flows to southeast oriented Cottonwood Creek as seen in figure 9 above) and southeast and northeast oriented Flat Creek. Also a well-defined through valleys link the northwest-oriented Alder Creek valley with the northeast-oriented West Fork Flat Creek valley. These through valleys provide evidence that southeast-oriented flood flow was using the present day Alder Creek alignment to move to both the Flat Creek valley and to the Cottonwood Creek and Arrow Creek valleys and also to move to the northeast-oriented West Fork Flat Creek valley. The through valley to the West Fork Flat Creek is the deeper through valley, which suggests southeast-oriented flood flow continued to move northeast to the Flat Creek valley (via the West Fork Flat Creek valley) after flood flow ceased to move southeast to the southeast-oriented Flat Creek and Merril Creek headwaters. At the time southeast-oriented flood flow was moving across the figure 10 map area there was no “Shonkin Sag” channel, nor was there a Missouri River valley northwest of the Highwood Mountains. Deep erosion of the region between the Highwood Mountains and the Bears Paw Mountains to the north (including headward erosion of “Shonkin Sag” and Missouri River valleys) ended southeast-oriented flood flow across the figure 10 map area and further caused the emergence of the Highwood Mountains as a high mountain region.

• The immense quantities of flood water required to erode the Highwood Mountains region probably came from a rapidly melting North American ice sheet located in a deep “hole”. Why would such an ice sheet generate immense southeast-oriented floods that deeply eroded central and western Montana mountain ranges? The massive southeast-oriented floods and deep erosion in central and western Montana mountain ranges makes sense if the deep “hole” was eroded into a topographic surface equivalent in elevation to the crests of highest central and western Montana mountain ranges today. In other words, central and western Montana mountain ranges were located along the deep “hole” rim and initially melt water from the rapidly melting ice sheet flowed southeast along the ice sheet margin. As the ice sheet melted flood waters were captured by headward erosion of deep northeast-oriented valleys so as to divert flood waters into space being opened up by the rapidly melting ice sheet. To visualize what happened think in terms of an Antarctica-sized ice sheet developing on a North American topographic surface at least as high as the high Rocky Mountain peneplain defined by crests of high Rocky Mountain peaks today. The ice sheet through its weight and deep glacial erosion created a deep “hole” in that topographic surface. Then the ice sheet began to melt faster than it was being formed. Essays in this Missouri River drainage basin landform origins research project building the case the Missouri River drainage basin landforms evolved during the resulting melt water floods.

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.