Abstract
Barbed tributaries flowing in southeast directions, an asymmetric drainage divide with both the South Fork Grand River and the North Fork Moreau River, and the Jump-off escarpment-surrounded basin (interpreted here to be a large abandoned headcut) are examples of topographic map evidence suggesting the north oriented Little Missouri River valley eroded headward across a large southeast oriented anastomosing complex of ice-marginal melt water flood flow channels that once crossed Harding County, South Dakota. Additional evidence includes southeast oriented tributaries to the northeast oriented South Fork Grand River and multiple divide crossings (e.g. through valleys and wind gaps) on the Boxelder Creek-Little Missouri River divide (in eastern Montana and west of the Little Missouri River) and suggests deep regional erosion occurred as the north oriented Little Missouri River valley eroded headward into and across the region. Harding County is located south and west of the southwest limit of coarse-grained glacial erratic material and ice-marginal melt water flow routes logically should have crossed it. Deep melt water erosion of Harding County and adjacent eastern Montana regions to the west is not consistent with many previous drainage history and glacial history interpretations, but is consistent with deep erosion by continental ice sheets.
Highlights
Additional evidence includes southeast oriented tributaries to the northeast oriented South Fork Grand River and multiple divide crossings on the Boxelder Creek-Little Missouri River divide and suggests deep regional erosion occurred as the north oriented Little Missouri River valley eroded headward into and across the region
Headward erosion of the deep O’Fallon Creek and the even deeper Yellowstone-Powder River valleys occurred in the same progressive sequence from the southeast to the northwest that was seen with North Fork Moreau River, South Fork Grand River, North Fork Grand River, Little Missouri River, Boxelder Creek, and Little Beaver Creek valleys
Highest Boxelder Creek-Little Missouri River drainage divide elevations are similar to highest elevations in the Short Pine Hills and in the Slim Buttes area, and to highest elevations along the Little Beaver Creek-Boxelder Creek drainage divide all of which stand 400 - 800 feet (122 - 244 meters) above the surrounding plains while the Powder River valley floor west of Figure 2 is more than 400 feet (122 meters) lower than the Little Beaver Creek valley floor
Summary
Added to these observations is the commonly made suggestion that coarse-grained alluvium found at isolated locations along or near the asymmetric Little Missouri-Missouri River drainage divide had a Black Hills source Fossils found in those discontinuous deposits have been correlated with Oligocene White River Formation sediments found in the South Dakota Badlands located east of the Black Hills (e.g. Lillegraven [3] and Clark [4]). Ice-dammed glacial lakes are sometimes thought to have filled those valleys large southeast or north oriented delta deposits in the hypothesized lake basins have never been mapped, otherwise reported, or found by this author Without such evidence there is good reason to believe the north oriented Little Missouri River in Harding County and western North Dakota and the north oriented Yellowstone and Powder Rivers to the west did not exist at that time. An analysis of drainage divide evidence along the asymmetric Little Missouri-South Fork Grand River drainage divide (including the Jump-off escarpment rim) may provide clues as to how the north oriented Little Missouri River drainage route evolved
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