The Pleistocene detritus in Ryedale above Helmsley is described. A study of its distribution leads to the conclusion that at one time the surface of “Lake Pickering” stood at between 300 and 400 feet O.D. If this were so, the water must have overtopped a large part of the Howardian Hills, and evidence derived from these hills has proved this inference to be correct. Erratics of easterly derivation, now found at the same altitude on the western side of the Vale of York, have been known for a considerable time, and support the conclusion that “Lake Pickering” and “Lake Humber” formed one sheet of water. Lacustrine deposits in South Durham, of which those representing the deeper water facies extend to a height of 250 feet O.D., are described in detail. As the result of this work, a number of conclusions have been reached, including the following: (1) a “Lake Humber”, the surface of which stood at a height of over 300 and not far short of 400 feet O.D., existed in late Glacial times; (2) even after the retreat of the Stainmore glacier from the Vale of York and the Tees basin this lake was still in existence, and its surface still at about the same level; (3) the land ice was in retreat before the North Sea ice-sheet had begun to withdraw. The hypothesis of glacial lateral drainage, as it has been applied in the Vale of York, is briefly discussed in the light of the above conclusions. A note is added on the character aqnd distribution of the Hessle Clay.