Abstract:

Big Muddy Creek-Little Muddy River drainage divide area location map

Figure 1: Big Muddy Creek-Little Muddy 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 Big Muddy Creek-Little Muddy River drainage divide area location map. The Canada-United States border is shown. Saskatchewan is the Canadian Province north of the international boundary. Montana is the state in the western half of figure 1 and North Dakota is located in the eastern half of the United States area. The northeast oriented South Saskatchewan River is located north of the figure 1 map area. This essay only addresses evidence in the United States and other essays address Canadian evidence (found under Saskatchewan on sidebar category list). The Missouri River flows east from Fort Peck Dam across northeast Montana to North Dakota. The northeast oriented Yellowstone River joins the Missouri River near the Montana-North Dakota border. West of the northeast oriented Yellowstone River is the northeast and north oriented Redwater River, which joins the east-oriented Missouri River near Poplar, Montana. Big Muddy Creek originates in Canada in the Big Muddy Lake area and flows northwest before turning southeast to flow through Big Beaver to the United States. From the border Big Muddy Creek flows southeast to Redstone, Plentywood, and Antelope, Montana and then turns to flow south-southwest to Reserve, Medicine Lake, and Homestead, and finally to the east-oriented Missouri River (near Culbertson, Montana). The Little Muddy River originates south of Wildrose, North Dakota and flows west to near Zahl, North Dakota, where the Little Muddy River turns to flow south to join the Missouri River at  Williston, North Dakota. This essay uses greatly reduced topographic maps to illustrate how two large valleys probably eroded headward across a decaying ice sheet remnant to capture southeast oriented flood waters southwest of the ice sheet margin. Evidence presented in other essays (for example see northeast end Redwater River-Yellowstone River drainage divide essay and the Poplar River-Big Muddy Creek drainage divide drainage divide area essay and Missouri River-Yellowstone River drainage divide essay) provide evidence present day valleys eroded headward in sequence to capture an immense southeast-oriented flood. The Big Muddy Creek-Little Muddy River drainage divide area is interpreted in a similar manner. [Other mentioned essays can be found under MT Missouri River on sidebar category list, with Redwater River essays also being under Redwater River and the Yellowstone River essays under Yellowstone River, etc.]

• Detailed maps illustrated below provide evidence of a large northeast oriented through valley extending from the present day Missouri River valley near Poplar, Montana through the Medicine Lake area to the Crosby, North Dakota area. East of the Medicine Lake area the through valley appears to have eroded across a stagnant ice sheet remnant from a lowland located north and east of the Missouri Escarpment. Also on topographic maps the through valley appears to have been subsequently blocked by glacial deposits. A second through valley extends north from the Little Muddy River elbow of capture (near Zahl, North Dakota) and also appears to have been blocked by subsequent glacial deposits. Prior to being blocked the Medicine Lake through valley would have provided a northeast oriented flood flow route linking the Missouri River drainage basin upstream from Poplar, Montana with a lower elevation region at the base of the northwest-southeast oriented Missouri Escarpment. Also prior to being blocked the second through valley, combined with the present day south-oriented Little Muddy River valley segment and the northeast oriented Missouri River valley segment upstream from Williston, would have provided a northeast- and north-oriented flood flow route to that same lower elevation region at the Missouri Escarpment base. That lowland at the Missouri Escarpment base in other essays is referred to as the Midcontinent Trench and the Missouri Escarpment will be interpreted to have been formed as the Midcontinent Trench southwest and west wall. The Midcontinent Trench will be interpreted to have been eroded by the Midcontinent River, which eroded an ice-walled and bedrock-floored valley. What is important to this essay is the two through valleys lead to a much larger southeast-oriented lowland located in northwest North Dakota. Also of importance is the through valleys appear to have been blocked by glacial deposits, although the “glaciation” does not appear to have significantly eroded or otherwise altered the regional landscape. Before leaving figure 1 note the southeast-oriented Souris River and Des Lacs River (which is a Souris River tributary). While not at the Missouri Escarpment base in Divide County, North Dakota the southeast-oriented Souris River and/or southeast-oriented Souris River tributaries, such as the Des Lacs River, are located along the Missouri Escarpment base both northwest and southeast of Divide County.

Big Muddy Creek-Little Muddy River drainage divide area detailed location map

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

Figure 2 provides a more detailed map of the Big Muddy Creek-Little Muddy River drainage divide area. Sheridan, and Roosevelt Counties are located in Montana. Divide and Williams County are located in North Dakota. The north-south Montana-North Dakota state line is located near the figure 2 east edge. The Fort Peck Indian Reservation land area is identified by red shading. The west to east oriented Canada-United States boundary is located just north of the figure 2 north edge. The Missouri River is located along the figure 2 south edge. The northeast-oriented Yellowstone River joins the Missouri River near the Montana-North Dakota state line just south of the figure 2 map area. From its confluence with the Yellowstone River the Missouri River flows northeast to Williston, North Dakota. The south-oriented Little Muddy River joins the southeast and northeast-oriented Missouri River at Williston and is located in the south end of a large north-south through valley (now blocked by glacial deposits) linking the northeast-oriented Yellowstone River and Missouri River valley segment (southwest of Williston) with the lowland at the Missouri Escarpment base. Big Muddy Creek flows southeast from the northeast corner of Sheridan County to Redstone and Plentywood and then turns to flow south-southwest and southeast along the Fort Peck Indian Reservation eastern boundary.

• Figure 2 does not show topography, but if it did there would be a large northeast-oriented through valley extending northeast from the Missouri River valley near Poplar, Montana through the Medicine Lake area in southern Sheridan County. Evidence of that through valley is illustrated in topographic maps shown below. The through valley extends into northwest North Dakota as described in the figure 1 discussion. The through valley is much larger than the present day Missouri River valley located east of Poplar. West of Poplar the present day Missouri River valley is much larger than it is east of Poplar, suggesting the northeast-oriented through valley and the Missouri River valley west of Poplar were originally formed as one valley, with the Missouri River valley east of Poplar being formed by a somewhat different mechanism. The Missouri River-Yellowstone River drainage divide and the Poplar River-Big Muddy Creek drainage divide essays illustrate the Missouri River valley east and southeast of Poplar and discuss its origin. The Yellowstone River-Little Missouri River, Missouri River-Yellowstone River, and Redwater River-Yellowstone River drainage divide essays, among others describe the Yellowstone River valley and discuss its origin. This essay attempts to explain the origin of the two through valleys linking the present day Missouri River and Yellowstone River valleys with the lowland located along the Missouri Escarpment base. Detailed maps below begin with a look at the Missouri Escarpment in Divide County, North Dakota and proceed south looking first at evidence of the large through valley extending north from Williston now used by south-oriented Little Muddy River. Next the detailed maps illustrate the northeast-oriented Missouri River valley segment linking the Little Muddy River through valley with the northeast-oriented Yellowstone River valley. The essay concludes with a look at the northeast-oriented Medicine Lake valley, which is today crossed by south-oriented Big Muddy Creek.

Missouri Escarpment east of Crosby, North Dakota

Figure 3: Missouri Escarpment east of Crosby, North Dakota. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 3 illustrates the Missouri Escarpment east of Crosby, North Dakota. The Canada-United States border is immediately north of the figure 3 north edge. Crosby is the town located in the figure 3 northwest quadrant. The red shaded areas represent former strip mine locations. Noonan is the town located at the highway junction (at the west end of the strip mine areas). The Missouri Escarpment is the slope immediately south of Noonan. Elevations north of Noonan average approximately 120 meters lower than elevations at the Missouri Escarpment crest. The hummocky topography at the Missouri Escarpment crest and to the south is the Missouri Coteau. The numerous small lakes and the hummocky topography suggest the Missouri Coteau is a region covered with glacial deposits typical of those that might be left by a decaying ice sheet mass. The Missouri Escarpment can be traced in a southeast direction into central North Dakota and then south into southern South Dakota. Generally for that entire distance the Missouri Escarpment is a well-defined topographic feature with the Missouri Coteau located to the southwest and west. Northeast and east of the Missouri Escarpment the landscape is often similar to what is seen north of the Missouri Escarpment in figure 3. The hummocky topography and numerous small lakes are not present, suggesting an absence of glacial deposits (or if glacial deposits are present that the deposits are relatively thin). In Canada, north of the region discussed in this essay, the Missouri Escarpment has a similar appearance and is oriented in a northwest-southeast direction. Southwest of the Escarpment crest is an area of hummocky topography similar to the Missouri Coteau and northeast of the Escarpment slope is a landscape similar to what is seen in figure 3. The Escarpment can be traced northwest from Divide County, North Dakota into Alberta. Note the large northeast-oriented valley located south and southwest of Crosby. That is the north end of the through valley drained further south by the south-oriented Little Muddy River (see figures 4, 5, and 6 below). Figure 3 illustrates how that through valley has been eroded across the Missouri Escarpment and the Missouri Coteau, suggesting that both the Missouri Escarpment and the Missouri Coteau in some way predate the northeast-oriented valley. Another large valley is located in the figure 3 northwest corner and is better seen in figure 4 below.

• The Missouri Escarpment is here interpreted to have been eroded by an immense southeast and south-oriented meltwater river that originally flowed across a decaying ice sheet surface and later eroded a deep ice-walled and bedrock-floored valley into the ice sheet surface. This immense southeast and south-oriented melt water river is here named the Midcontinent River. Other North Dakota and South Dakota essays (published on this website) illustrate and describe Midcontinent River segments and will also illustrate and describe how the Midcontinent River was dismembered to create present day north-oriented drainage networks. Evidence presented in those other essays suggest the Midcontinent River, as it sliced its deep southeast and south-oriented ice-walled and bedrock-floored valley into the decaying ice sheet mass, for all practical purposes detached the ice sheet’s southwest margin. The Missouri Coteau represents the area where the ice sheet’s detached southwest margin was located and subsequently melted. As the southeast and south-oriented Midcontinent River sliced its ice-walled and bedrock-floored valley into the decaying ice sheet immense southeast oriented floods also flowed southeast along the ice sheet’s original southwest margin (evidence of which has probably been completely eroded away). Where possible these ice-marginal southeast oriented floods spilled in an east or northeast direction across the ice sheet margin to the deeper southeast and south oriented Midcontinent River valley.

• Flood waters moving from west and southwest of the ice sheet margin across the ice sheet margin to the southeast and south-oriented Midcontinent River sometimes eroded deep ice-walled and bedrock-floored valleys across the detached ice sheet southwest margin. The northeast-oriented valley south and southwest of Crosby is one such valley and the valley northwest and west of Crosby is another such valley. Once southwest and west of the decaying ice sheet mass these valleys often eroded headward to capture significant southeast-oriented flood flow. In the case of the valley south and southwest of Crosby it is linked to the Yellowstone River valley further to the southwest. In the case of the valley northwest of Crosby it is linked to the Missouri River valley further to the west and southwest. Landforms seen in this essay were formed very late during the ice sheet decay process. The original ice sheet was comparable in thickness to the present day Antarctic Ice Sheet, which in some locations is thousands of meters thick. The North American ice sheet thickness in northwest North Dakota cannot be determined, but it almost certainly was at least hundred meters thick, maybe much more. Ice-walled and bedrock-floored valleys carved into the decaying ice sheet were preceded by ice-walled and ice-floored valleys, which sliced deeper and deeper into the decaying ice sheet surface. What is important to remember when viewing evidence in this essay is the Yellowstone and Missouri River valleys originally eroded headward from ice-walled and ice-floored valleys at much higher elevations than the valleys seen in this essay.

Missouri Coteau region southwest of Crosby, North Dakota

Figure 4: Missouri Coteau region southwest of Crosby, North Dakota. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 4 illustrates the Missouri Coteau region southwest of Crosby, North Dakota and includes overlap areas with figure 3. Crosby is located in the figure 4 northeast quadrant. The northeast-oriented valley in the figure 4 east half is the north end of the through valley drained further south by south-oriented Little Muddy River. Note how that through valley has been partially filled with what appear to be glacial moraines of some type. The northeast-oriented valley in the figure 4 northwest quadrant is the northeast end of the Medicine Lake through valley illustrated in figures 8, 9, and 10 below. This valley also appears to be partially filled with some type of glacial moraine material. Between the two valleys is a Missouri Coteau remnant and the Missouri Coteau can be seen in the figure 4 southeast and northwest corners. The presence of glacial moraines in the through valleys provides evidence of some type of glaciation event following erosion of the valleys. That subsequent glaciation was significant enough that it was able to partially fill the through valleys with glacial moraines, yet the subsequent glaciation was not significant enough to deeply erode or otherwise alter the regional landscape (e.g. the northeast ends of the through valleys are still identifiable).

• Why would a minor glacial event follow the rapid melting of a thick North American ice sheet, especially a thick ice sheet located in a deep “hole”? While evidence presented in this essay is not adequate to answer the question, evidence presented in the entire Missouri River drainage basin landform origins research project essay series (published on this website) does provide an answer. Briefly, the answer is found in essays describing dismemberment of the southeast and south-oriented Midcontinent River (see essays under ND Missouri River and James River on sidebar category list). The Midcontinent River developed initially as a significant supra-glacial melt water flood flow route carrying melt water off the rapidly melting thick ice sheet surface. Flood flow was to the south (or Gulf of Mexico) even after the Midcontinent River sliced its ice-walled and bedrock-floored valley headward into the decaying ice sheet. Flood flow on the decaying ice sheet surface moved to the deep Midcontinent River valley and carved a network of ice walled and bedrock floored tributary valleys. In time these tributary valleys intersected with each other to create an ever-changing south-oriented anastomosing complex of ice-walled and bedrock-floored valleys. As long as immense melt water floods moved south to the Gulf of Mexico warm tropical waters were pushed north into the North Atlantic Ocean and probably contributed to the warming that was causing the ice sheet to rapidly melt. However, when headward erosion of the south-oriented Midcontinent River ice-walled and bedrock-floored valley complex intersected with north-oriented ice-walled and bedrock-floored valleys being sliced in the decaying ice sheet surface by north-oriented melt water flood flow a drastic change took place. The north-oriented valleys had shorter routes to the ocean and step by step they captured the south-oriented flood flow. When the immense melt water floods stopped moving south and instead moved north there no longer was a mechanism to push warm tropical waters north in the Atlantic Ocean. At the same time there was a mechanism to push cold Arctic waters south. The effect was a rapid climate change. Melt water floods froze on the former ice sheet floor and created a thin ice sheet with a wet bottom. That thin ice sheet subsequently melted slowly without creating the immense floods the thick ice sheet had created.

Little Muddy River valley in the Zahl, North Dakota area

Figure 5: Little Muddy River valley in the Zahl, North Dakota area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 5 illustrates the large Little Muddy River valley south of Zahl, North Dakota. The Little Muddy River originates near the figure 5 east center edge (north of the east-west highway) and flows west-southwest to Cottonwood Lake and Alamo (located just south of the words “Rock Island”) and then to just east of the north-south highway to enter what appears to be a large south-oriented valley. Once in the valley the Little Muddy River crosses the highway and flows south to the figure 5 south edge. The south-oriented valley is the south section of the partially blocked through valley seen southwest and south of Crosby in figures 3 and 4. Figure 5 evidence suggests there is considerable glacially deposited material present in the former north-oriented flood flow valley. Much of that glacially deposited sediment probably was picked up locally and moved by relatively short ice thrust movements facilitated by the wet-based thin ice sheet that covered the former thick ice sheet floor. The Missouri Coteau region is present on both sides of the former north-oriented flood flow route with northern areas appearing to have more glacially deposited sediments than areas further to the south. Southern areas may have been more intensely eroded by southeast oriented melt water floods at about the time of the sudden climate change. A case can be made for intense flood erosion of the southern areas, although I will not try do so with this figure (my goal with this figure is to illustrate where and how headward erosion of the northeast-oriented Yellowstone River valley originated). The figure 6 discussion describes flood erosion of the southern areas. Also, northern areas of figure 5 may have been more affected by ice thrusting associated with the wet-based thin ice sheet that developed after the rapid climate change.

Little Muddy River valley north of Williston, North Dakota

Figure 6: Little Muddy River valley north of Williston, North Dakota. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 6 illustrates the south-oriented Little Muddy River valley north of Williston, North Dakota and includes overlap areas with figure 5. The large south-oriented Little Muddy River valley is located in the figure 6 center. Williston is the town located along the figure 6 south center edge. Southeast-oriented tributary valleys eroded into the Little Muddy River valley west wall were probably eroded by southeast oriented flood flow before headward erosion of northeast-oriented Medicine Lake valley (seen in figures 8, 9, and 10 below) beheaded that flood flow. In the figure 6 northeast quadrant is the northwest and southwest-oriented East Fork Little Muddy River. The East Fork headwaters are a network of northwest-oriented tributaries, which have been captured by headward erosion of the southwest-oriented East Fork valley. This complex of northwest-oriented tributaries provides evidence headward erosion of the southwest-oriented East Fork valley beheaded multiple southeast oriented flood flow routes, such as might be found in a southeast oriented anastomosing channel complex. Flood waters on the northwest ends of the beheaded flood flow channels reversed direction to flow northwest to the newly eroded southwest-oriented East Fork valley. Similar evidence can also be seen in the headwaters of southwest-oriented Spring Brook, which is the major southwest-oriented stream flowing to the Little Muddy River at Williston. Headward erosion of these southwest-oriented valleys were some of the final events to significantly affect the figure 6 landscape and may have occurred when the large north-oriented valley was initially blocked. At that time large quantities of flood waters from the southwest would still have been moving northeast to this figure 6 map area. Those flood waters would have been dammed by freezing of flood water further north and a large short-lived lake may have covered much of the region. The lake would have rapidly drained when headward erosion of the deep Missouri River valley east of Williston reached the Williston area. But, prior to that time the lake may have drained in a southeast direction across the figure 6 east edge area to the rapidly eroding southeast and northeast-oriented Missouri River valley located south and southeast of the figure 6 map area. Once headward erosion of the southeast and northeast oriented Missouri River valley reached the Williston area southeast-oriented drainage across the upland surface ceased and drainage was south in what is today the Little Muddy River valley. That south-oriented drainage was responsible for headward erosion of the southwest oriented Spring Brook and East Fork valleys and beheading and reversal of southeast oriented flood flow routes located on the figure 6 east half upland surface.

Missouri River-Yellowstone River confluence area

Figure 7: Missouri River-Yellowstone River confluence area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 7 illustrates the Missouri River-Yellowstone River confluence area southwest of Williston, North Dakota. Williston is located in the figure 7 northeast corner and the south-oriented Little Muddy River valley can be seen north of Williston. The Yellowstone River flows north to join the southeast and northeast oriented Missouri River in the figure 7 south center area. The north-south Montana-North Dakota border is located just west of the Yellowstone River. Note how the Missouri River valley downstream from the confluence area is much large than it is upstream from the confluence area. The northeast oriented valley segment between the confluence area and Williston existed before the  oriented valley segment upstream from the confluence area. During final stages of the thick North American ice sheet’s rapid melt down significant volumes of flood water flowed northeast from the Yellowstone River valley to the present day Little Muddy River valley and then north to the Midcontinent River northeast of the Crosby area. These final stages of the immense flood probably occurred at a time when the Midcontinent River was being dismembered. Prior to that time there was northeast flood flow to the southeast and south-oriented Midcontinent River (which may not have yet eroded a bedrock-floored valley) in a progression of higher level northeast and east-oriented valleys. Flood waters moving in those higher level valleys probably flowed in ice-walled and ice-floored valleys once the flood flow arrived on the decaying ice sheet surface. The southeast-oriented Missouri River valley upstream from the confluence area was probably eroded when the climate suddenly changed and the northeast and north-oriented floods became blocked by freezing of flood waters further to the north. Large volumes of flood waters were moving north in the large Medicine Lake valley illustrated in figures 8, 9, and 10 below. Unable to move north that flood water finally spilled over drainage divides into the already existing northeast-oriented Yellowstone River-Little Muddy River valley. Multiple southeast-oriented valleys eroded headward (see dry valley in figure 7 northwest corner area). The Missouri River valley east of Williston was eroded at the same time (as were other Missouri River valley segments further downstream). Erosion of these Missouri River valley segments at that time enabled the remaining flood waters southwest of the ice sheet margin to drain from the region. Subsequent to that time the thin ice sheet melted, but at a much slower pace than the thick ice sheet and immense melt water floods did not occur (although much smaller floods probably did occur).

Abandoned valleys northwest of Bainville, Montana

Figure 8: Abandoned valleys northwest of Bainville, Montana. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 8 is located northwest of the figure 7 map area, illustrates the relationship of the present day Missouri River valley to the northeast-oriented Medicine Lake valley, and includes overlap areas with figure 7. The east and southeast-oriented Missouri River valley is located in the figure 8 southwest quadrant. A segment of the abandoned northeast-oriented Medicine Lake through valley is visible in the figure 8 northwest quadrant (figure 8 below better illustrates the northeast-oriented Medicine Lake through valley). Big Muddy Creek is the stream flowing south-southeast and southeast from the figure 8 northwest corner to join the Missouri River in the figure 8 southwest quadrant. Note how Big Muddy Creek flows across the northeast-oriented Medicine Lake through valley and has eroded a southeast-oriented valley across the upland surface separating the northeast-oriented Medicine Lake through valley from the present day Missouri River valley. Culbertson, Montana is the town located at the highway junction east of where Big Muddy Creek joins the Missouri River. Bainville is the town located in the figure 8 southeast quadrant. Note the presence of a dry valley extending from Culbertson to Bainville and then southeast to the figure 8 southeast corner. Figure 7 above illustrated how southeast of Bainville that dry valley joins the southeast-oriented Missouri River. As previously mentioned the northeast-oriented Medicine Lake valley represents the final stage in what was probably a series of northeast-oriented valleys which moved flood waters from the present-day Missouri River drainage basin (upstream from the figure 8 map area) into the deep “hole” the rapidly melting thick North American ice sheet had occupied. Evidence for most earlier higher elevation valleys has been removed by deep flood erosion of ice margin regions and melting of the thick ice sheet, although some field evidence might be found on tops of uplands and hills in southern Alberta and Saskatchewan, southwest North Dakota, northwest South Dakota, and elsewhere. In any case, as previously mentioned flood waters moving northeast in the Medicine Lake valley became dammed (probably by freezing of flood waters further to the north and northeast) and were forced to find an alternate route along the decaying ice sheet’s southwest margin. The present day Missouri River valley and the parallel dry valley extending from Culbertson to Bainville (and then southeast) were eroded as flood waters spilled across what was then a drainage divide between the northeast-oriented Medicine Lake valley and the northeast-oriented Yellowstone River valley. The southeast-oriented Big Muddy Creek valley eroded was also eroded across the upland surface between the Medicine Lake valley and the present day Missouri River valley at that time. These valleys were eroded by the final stages of the immense floods this entire essay series describes. Once flood waters from those final stages had drained from the upstream Missouri River drainage basin no comparable flood events occurred. The Missouri River-Yellowstone River drainage divide essay illustrates and discusses some additional figure 8 evidence not addressed here.

Large northeast-oriented abandoned valley at Medicine Lake, Montana

Figure 9: Large northeast-oriented abandoned valley at Medicine Lake, Montana. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 9 illustrates the large northeast-oriented Medicine Lake through valley at Medicine Lake, is located north of the figure 8 map area, and includes overlap areas with figure 8. The large through valley is the southwest to northeast trending lowland extending from the figure 9 southwest corner to the figure 9 northeast corner. Medicine Lake is the large lake located on the through valley floor. Other lakes in figure 9 are also located on the through valley floor. Big Muddy Creek flows south-southwest and south just west of the north-south highway. Note how Big Muddy Creek flows across the northeast-oriented through valley (see figure 8 for south half of valley). This evidence suggests flood waters were still in the region at the time the through valley became blocked and flood waters were forced to erode the present day Missouri River located south of the figure 9 map area. The northeast-oriented through valley as previously mentioned represents the final stage what almost certainly was a progressive series of lower and lower elevation valleys. Hummocky landscape located in the Missouri Coteau seen in figures 3, 4, 5, and 10 probably was deposited at least in part by what were slowly melting thick North American ice sheet remnants. Those ice sheet remnants would have been located at the ice sheet base and it is possible the ice sheet southwest margin was deeply eroded by flood water erosion (meaning the ice sheet extended further southwest than present day evidence suggests).  The “thick ice sheet that melted fast” paradigm being used in this essay series to interpret topographic map evidence also assumes the ice sheet was located in a deep “hole”. The deep “hole” was carved by ice sheet erosion and was also created as ice sheet weight caused downwarping of underlying crustal material. In other words, the thick ice sheet roots extended significantly below the level of the surrounding unglaciated regions. Ice sheet remnants in the Missouri Coteau region were almost certainly remnants of ice sheet roots, which at one time had been located significantly below the level of the surrounding unglaciated region. When the ice sheet rapidly melted at first flood waters went south because the ice sheet still stood above the surrounding unglaciated topographic surface. However, when ice sheet melting progressed to the point that space lower in elevation than the surrounding topographic surface elevation was being opened up, valleys eroded headward from the deep hole to capture the south-oriented flood waters and to divert the flood flow into the deep “hole” the rapidly melting ice sheet had once occupied. As the ice sheet continued to melt flood waters lowered the surrounding topographic surface elevation to correspond with the level of the remaining ice sheet remnants. In a short 1997 North Dakota Academy of Science communication[1] I suggested the top of the Cypress Hills (in southwest Saskatchewan and southeast Alberta) might be the floor of an early northeast-oriented flood flow valley. I also suggested that as the ice sheet melted this northeast-oriented valley migrated to the southeast and the Flaxville upland surface (in the Big Muddy Creek drainage basin northwest of figure 9) might be the floor of an intermediate level of that same valley floor and that this northeast-oriented Medicine Lake valley was the final stage of that same valley’s migration. I should state for readers unfamiliar with the regional geologic literature the Cypress Hills and Flaxville upland surfaces are covered with thick alluvium that came from the southwest, but which also contain fossils previous investigators have used to suggest a fundamentally different origin.

Glacial moraines filling abandoned valley north of Grenora, North Dakota

Figure 10: Glacial moraines filling abandoned valley north of Grenora, North Dakota. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 10 illustrates the northeast-oriented Medicine Lake through valley northeast of the figure 9 map area and includes overlap areas with figure 9. Figure 10 also includes overlap areas with figures 4 and 5. The northeast-oriented Medicine Lake through valley, like the north-oriented through valley between Williston and Crosby, North Dakota has been partially filled with what appears to be some type of glacial moraine material. The glacial event responsible for depositing the materials was significant enough that it was able to locally move and deposit sufficient materials to block the through valley. Yet, the glaciation responsible for depositing the materials was not significant enough to destroy the northeast-oriented through valley. Based on these observations I suggest the glaciation responsible for depositing the materials was triggered when ice sheet melting progressed to the point where the immense south-oriented melt water floods were captured and diverted to flow north into what is today Hudson Bay. This immense melt water flood flow direction change probably significantly altered Atlantic Ocean currents and triggered a major Northern Hemisphere (if not world-wide) cooling event. That cooling event caused flood waters on the former ice sheet floor to freeze while additional flood waters from the southwest kept flowing north and also froze. The frozen flood waters developed into a wet based thin sheet occupying the former thick ice sheet floor. Embedded in this thin ice sheet were remnants of the former thick ice sheet. Isostatic rebound, triggered by rapid melting of the thick ice sheet, progressively was raising northern areas relative to southern areas. These elevation changes caused ice thrusting which moved slabs of frozen underlying “bedrock” material that had become incorporated in the thin ice sheet ice mass. Melting of the thin ice sheet and the embedded thick ice sheet remnants does not appear to have produced the types of immense flooding that rapid melting of the thick ice sheet produced.

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.