General studies : November, 1967 / by John C. Ayers, Joseph C. K. Huang.

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The lake-bottom topography (bathymetry) has been subject to
the smoothing erosional and depositional actions of the lake
water since recession of the last glacier.. This smoothing action
has been last to happen after the gouging and scouring actions of
the glacier itself and after the water-laid deposition of sediments
released from the glacier ice during melting and recession of the
ice. From the sediment-sorting actions of the water there has
further followed the development of a much more regular distribution
of sediment types than is the case on land.
Bathymetry
The basin of Lake Michigan as a whole is divided by an incomplete sill of resistant materials extending from the region of
Milwaukee, Wisc., toward Muskegon, Mich.; this incomplete sill
establishes two sub-basins called herein the northern and southern
basins. While both basins show the smoothing actions of the lake
water, the northern basin is deeper, more elongate and of more
irregular bathymetry while the southern basin is shallower,
rounder and of more regular bathymetry. Figure 1 is a recent
chart of the bathymetry of Lake Michigan.
Sediment Types
In both basins of Lake Michigan the basic progression of
bottom surface-sediment types is from sands or sand and gravel in
a relatively wide belt along the beaches, to silty sands in a
narrower more irregular belt in shallow intermediate depths, to
sandy silts in a variable discontinuous band in deeper intermediate
depths, to soft combinations of silts and clays and loam (sandy