Little deposition takes place at the present in the St. Lawrence middle estuary. In the shallow marginal areas, broad unconformities under a thin cover of sandy muds reflect the dominance of erosional processes during the post-glacial emergence. Sands from nearby recessional moraines have been carried into the north channel troughs. Numerous sand dunes and ripples develop there. Over the central regions, a thin layer of inhomogeneous sandy sediments unconformably covers a deposit of massive grey muds, which seismic reflection data indicate to be possibly 15-30 meters thick resting over a well defined late glacial horizon. At the surface, the silt-to-clay ratio remains remarkably constant (about 3:1) in spite of highly variable proportions of sand, perhaps as a result of the continuous winnowing action of tidal currents which range between 10 cm/sec and 150 cm/sec near the bottom. In the underlying massive grey muds, X-ray radiographs reveal silty and less silty layers 2 to 3 cm thick. These muds were formed under a different sedimentation regime in which deposition from suspension must have been dominant.