Introduction.-On the east valley side of the southern end of the Cayuga Lake Valley, just to the north of Ithaca, New York, occur considerable areas of lake-clay deposits laid down when the waters of the lake were ponded to higher levels by the ice barrier of the retreating front of the last advance of the continental ice sheet. These lake-clay deposits are especially well developed in the zones marginal to the outer fronts of the higher-level deltas of streams tributary to the Cayuga Valley, and undoubtedly represent, in such cases, the extension of the bottom-set beds of the delta accumulations. The particular occurrence to which reference is made in this article is found at a level of 840 feet to the south and west of the top of a notable delta deposit of Fall Creek, having an average elevation of 930 feet (the block diagram, Fig. i, illustrates the geography of the occurrence). Thus the clay may be assumed to have been laid down in water having a depth of 90 feet and removed from the nearest point of the steep front of the delta deposit by a little less than one-fourth mile. At the time when the delta and clay deposits were made the ice barrier must still have existed within the confines of the Cayuga Lake Valley, for the level of the delta top indicates that its building must have been coincident with the outflow of the lake waters across the north-south divide between Cayuga and Seneca Lake valleys, and the overflow of their combined waters was at a present elevation of 900 feet from the south end of the Seneca Lake Valley into the Chemung River and thus into the Susquehanna. On the other hand, it is unlikely that the ice barrier was immediately adjacent to the delta and lake-clay deposits, for the nearest point of the north-south divide with a low enough elevation to permit of a flow of water from the one lake valley to the other is found approximately 15 miles to the north