In studying the geological history of the rivers of the Don system, my attention was specially directed to the evidences of glacial action in the area, with the object of ascertaining whether glaciation had anything to do with the interesting diversions of the Don, Dearne, and Dove. Certain valleys in the area, also, attracted my attention as possessing abnormal features with respect to the present drainage of the district, and I began to inquire what their relations might be to an altered system of drainage during the Glacial Period. The present paper is an attempt to piece together the scattered glacial evidence, and to ascertain the effect that the advance of a glacier from the north and north-east would have on the drainage of this district, and how far the present valleys would help to explain the water-flow under such conditions.