The topography of the lake-bottom between St. Mary's River and Michipicoten Harbour (Lake Superior, Ontario), consists of a lake-shelf, 2 to 4 miles (3.2–6.4 km) wide, and a series of north–south aligned topographic deeps and highs, 3 to 4 miles (4.8–6.4 km) wide, lakeward. This topographical trend is in marked contrast to the east–northeast and east–southeast trends of the onshore Precambrian rocks.The Holocene sediments of the lake-bottom consist of sand, which covers the lake-shelf and topographical highs, and clay-silt which covers the topographical deeps. The sands of the lake-shelf area can be differentiated from the sands of the topographical highs on the basis of grain-size parameters and mineral composition. The clay-silt sediments of the topographical deeps form the following consistent stratigraphic sequence: an upper, thin veneer of dark yellowish brown silt; an intermediate unit of olive gray, fine-grained silt to coarse-grained clay; and a lower unit of dark yellowish brown silt. These Holocene sediments, which may range from 2 to greater than 180 cm in thickness, are underlain by varved sediments of Pleistocene age.