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

RESULTS AND DISCUSSION

The basic purpose of this report is to present the tabulated
data of the Appendix, but in order that these data may be visualized Figure 3 has been prepared.
Comparisons between Figures 2 and 3 are the present best
available means of determining whether the intervening period of
30 years has produced differences in the composition of the surficial sediments.
In one sense these figures are only partially comparable
Figure 2 is based upon the results of particle-size analyses
augmented by field descriptions obtained from U. S. Lake Survey
charts; Figure 3 is based only upon field descriptions. Incomparability becomes greatest in cases where a thin layer of a sediment of one type overlies a thick layer of a sediment of a different type. Partical-size analysis of a split "representative
sample" gives most statistical weight to the thicker second layer
and is not apt to detect the environmental change represented by
the thin surficial layer. Plots of field descriptions of the
uppermost layer, however thin it may be, take no account of the
underlying layer(s) and are not based upon the concept of the
"representative sample" or aliquot thereof.
Fortunately, the numbers of cases where greatly different
sediment types occur in layered configurations are not numerous.
For this reason there is a degree of Justification for crosscomparisons between Figures 2 and 3.
I
The primary re s ult of c ompari son of t he se f igure s i s t ha t

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