Studies on water movements and sediments in Southern Lake Michigan : part 1[-3] ... / by John C. Ayers.

over most of the lower lake, there is no conclusive demonstration
of change in the surficial sediments since the early 1930's.
In the central portion of the basin apparent differences in
sediment types involve, in large part, those distinctions between
proportions of sand and silt or silt and clay that are least accurately made by the field descriptions. The apparent differences
are believed not to be real, but rather effects of masking (in
Fig. 2) due to the weight given to a second thicker layer by
partical-size analysis of representative samples.
Along the shores there are but minor differences between the
two figures. The differences for the most part, again, involve
the judgment of proportions of sand and silt or silt and clay.
In the area off Chicago and its environs there are some differences in position of gravel and sand in the two figures.
These sediments are quite accurately assessed by the field description; they also are sediments more apt to be non-layered
through the depths reached by a grab sampler. The positional
differences are taken to indicate that sand bars move or migrate
in the region off Chicago.
The more complete spatial coverage upon which Figure 3 is
based allows the elimination of open spaces (terrae incognitae)
that were necessary in Figure 2. It is to be noted, however,
that our coverage is not complete, and that some sediment-type
boundaries will move if more-closely spaced samples are obtained.
Samples of Special Note
In only eight of the samples were there evidences that man's
activities have resulted in modification of the native lake-bottom

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