. Bulletin of the Geological Society of America. Geology. 34 J. E. TODD—HYDROGRAPHIC HISTORY OF SOUTH DAKOTA If our conclusion is right, we may believe that the ice of the Kansan epoch, coming from the northeast into the Minnesota valley, was pre- vented, by the abruptness of its western side, the height and unbroken character of the plateau west of it, from entering the James River valley, although it succeeded in overriding the eastern edge and entering the valley of the Big Sioux. Glacial striae in that valley are found in such relations as to indicate that it was occupied by a narrow lobe
. Bulletin of the Geological Society of America. Geology. 34 J. E. TODD—HYDROGRAPHIC HISTORY OF SOUTH DAKOTA If our conclusion is right, we may believe that the ice of the Kansan epoch, coming from the northeast into the Minnesota valley, was pre- vented, by the abruptness of its western side, the height and unbroken character of the plateau west of it, from entering the James River valley, although it succeeded in overriding the eastern edge and entering the valley of the Big Sioux. Glacial striae in that valley are found in such relations as to indicate that it was occupied by a narrow lobe pushing down the Big Sioux, west of a small area, which is comparatively drift- less, north of Garretson. South Dakota. When the ice-sheet crowded against this plateau of the Coteau des Prairies the whole drainage from its western border farther north must. Figure 2.—Hydrographic Conditions in South Dakota in pre-Wisconsin Pleistocene. have found its way over the divide between the James and Red rivers, we may suppose, at some lower and narrower portion, such as we may readily suppose, from the present conditions, may have been a little north of our state, as is represented on the map. There may have been first formed a temporary lake east of the ridge, and from it there must have been a rapid descent to the level of the James River valley. As the materials of the ridge were mostly clay and sand, such a barrier as that could not long resist the eroding influence of a stream which was fed by the draining from the whole western slope of the ice-sheet farther north. We can scarcely doubt that in a comparatively short time, it would cut down a canyon through this ridge to the level of the James river, which. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Geological Society of America. [New York : The Societ
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