The surficial deposits of Cochrane District consist mainly of three sheetlike stratigraphic units: lower till, which records regional glaciation during the Wisconsin stage, overlies Precambrian bedrock, and is overlain by mostly varved sediment of the Barlow–Ojibway Formation , which was deposited in glacial Lake Barlow–Ojibway during northward retreat of the ice margin. Till and associated glaciolacustrine sediments of the Cochrane Formation overlie the Barlow–Ojibway Formation in the northern part of the area, recording a late Wisconsin ice advance. Other deposits, of discontinuous distribution, include glaciofluvial deposits, mainly in the form of esker complexes, eolian deposits, fluviatile deposits, and organic deposits of widespread bogs. It is inferred from radiocarbon dates from the area and from James Bay lowland that the late Wisconsin advance which deposited till of the Cochrane Formation attained its maximum advance at least 8275 years ago. About 900 of some 2075 varves which comprise the varved clay unit of the Barlow–Ojibway formation were measured. Varve diagrams prepared from the measurements agree well with diagrams made by Antevs (1925 Antevs (1928), confirming his counting and correlation of the varves. Agreement of the varve diagrams indicates that personal judgment in determining upper and lower limits of individual varves of the Barlow–Ojibway formation does not affect seriously the measurements on which the diagrams are based.