Seismic profiler transects indicate that much of Lake Melville is underlain by sedimentary strata, which are probably an extension of the (Proterozoic? Paleozoic?) Double Mer Formation, exposed onshore to the northwest. The southeastern limit of these strata in Lake Melville is inferred to lie beneath the linear zone of deep water along the southeast shore. Unconsolidated sediments beneath this linear deep attain apparent thicknesses of nearly 400 m, so that the underlying bedrock surface is at least 600 m below the present sea level. The Lake Melville depression is thus overdeepened more than one-half a kilometre with respect to its outlet east to the Labrador Sea.