Western Lake Ontario sediments record a till–mud–till sequence over bedrock and a thick layer of partly distorted glaciolacustrine clay unconformably overlying this. Another unconformity separates this clay from younger fine (lacustrine?) deposits, which are also distorted. Modern foreset and bottomset beds of the Niagara River overlap the lacustrine and glaciolacustrine deposits.In eastern Lake Ontario, thick glaciolacustrine clays overlie bedrock and the infilling ice-marginal deposits. These glaciolacustrine clays are severely distorted, and in the deepest parts of the area patches of overlying lacustrine (?) clay are preserved. A distorted terracelike deposit is preserved at higher elevation in the St. Lawrence trough. Bedrock, lag gravels, and sandy deposits characterize shallow-water areas.We suggest that after the fall of lake levels in the Lake Ontario basin, during the post-Iroquois lake phases, glaciolacustrine clays were subject to distortion by ice action at the time of the Greatlakean stadial. This deformation was more severe at the east end of the basin. At its lowest level, the surface of early Lake Ontario following the post-Iroquois phases was below the present lakebed in much of western Lake Ontario. Ice-wedge casts formed in sediments at both ends of the basin, soon after the initiation of early Lake Ontario, and are correlated with the Algonquin (St. Narcisse) glacial phase. Whereas there are significant accumulations of modern mud in the western basin of Lake Ontario, in the Kingston basin there is little except in the St. Lawrence trough.