The clay deposits of Northwestern Quebec (from Matagami to Rupert House) were formed during the retreat of the Laurentian ice sheet. The oldest deposits are varved clays formed when the region was submerged by glacial Lake Barlow – Ojibway; the varved clays have been traced northward to Rupert House. A minor glacial surge (Cochrane stadial) extending southward to Matagami, partly covered and reworked the lacustrine clays; evidences of this glacial readvance were observed on the left bank of Rupert River near Rupert House. Following this glacial stage, the land was invaded by sea water (Tyrrell Sea) from which were deposited the silty clay that underlies much of the Hudson Bay Lowlands.The mineralogical composition of the marine clay differs from that of the lacustrine clay by its carbonate content: 30% against 2% or less for the lacustrine clay. The clay fraction of the two types of clays is made up of similar minerals which, in order of decreasing importance are: illite, chlorite, vermiculite, and kaolinite. The marine clay is coarser grained than the lacustrine clay and, locally, gives rise to important flow-slides. The slides are subcircular in shape and have a typical bottle-neck opening; they have never been observed to develop in the varved clays. Their occurrence in the marine clays would seem to depend principally on the microstructure of the material, a silty clay that is fissured at the surface. The flow-slides occur specially during the spring thaw when the ground is oversaturated with water.