Geology . the sub-Aftonian and Aftonian stages isreached. The ice-sheet spread from the Keewatin and Labradorean centers tothe approximate limit of the known drift in the Mississippi valley, and depositeda typical sheet of bowlder clay (sub-Aftonian) and also gave rise to great valleytrains of glacio-fluvial material that stretched from the drift border to the Gulf,filling the low-gradient valleys of the time to depths of 30 to 50 feet near thedrift border, and of 200 feet near the Gulf (Natchez formation). The invasion :;ss GEOLOGY of the ice blocked up many northward trending valleys and cau
Geology . the sub-Aftonian and Aftonian stages isreached. The ice-sheet spread from the Keewatin and Labradorean centers tothe approximate limit of the known drift in the Mississippi valley, and depositeda typical sheet of bowlder clay (sub-Aftonian) and also gave rise to great valleytrains of glacio-fluvial material that stretched from the drift border to the Gulf,filling the low-gradient valleys of the time to depths of 30 to 50 feet near thedrift border, and of 200 feet near the Gulf (Natchez formation). The invasion :;ss GEOLOGY of the ice blocked up many northward trending valleys and caused their streamsto find new courses along tin* ice border. The present Ohio and Allegheny riversseem to have been formed by the union of several streams that previously flowed into the Erie basin. The Missouri liver seems t<» have been formed by a similarCombination Of many streams thai previously flowed northerly and easterlybut some part of this readjustment of t he drainage seems to have been later. Fig. 513.—The unconformity between the Natchez above and the Lafayette line of contact is indicated by the dotted line. than this stage. Including these later changes, the Ohio and Missouri riversmay be pictured as two great drainage arms embracing the border of the ancientice-sheet and carrying away its waters. Rather low gradients and a lowr ele-vation in the lower Mississippi seem thus to be indicated. III. The Kansan glacial stage.—As defined by Calvin, Bain, andothers who have specially studied it,1 the Kansan stage is representedby a typical sheet of till occupying a large surface area in Kansas,Missouri, Iowa, and Nebraska (Fig. 470), and theoretically extending 1 Reports of the Iowa Geol. Survey. THE PLEISTOCENE OR GLACIAL PERIOD. 389 under the later glacial formations to the northward as far back as theKeewatin center of radiation. Much of this sheet of drift, as originallydeveloped, has probably been rubbed away by later glaciations. Pre-sumably a sim
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