Pleistocene Deposits of Illinois. — Jlershey. 291 
borne over the lake, gradually melting and dropping an 
occasional pebble or small boulder which sank to the bottom 
and became incorporated with the laminated clays. There is 
a total absence of fossils of any kind, which testifies to the 
arctic severity of the climate during the advance of the ice- 
sheet. 
At the type locality the laminated clays, here situated aBout 
60 feet above the present stream level, are terminated in the 
end of the cutting by a slight but sharp ridge of Galena 
limestone, to which they owe their preservation. But inter¬ 
vening between the clay and the rock there is a bed of bright 
red and yellow fine-grained sand, which seems to be the shore 
deposit of the ancient lake. These beach sands were derived 
largely from the neighboring shore land. Among all the 
different deposits of Quaternary age developed in northern 
Illinois, the variegated clays under discussion present the 
finest lamination. The surprising feature of it is its regularity. 
From top to bottom of the formation, the laminae vary but 
little from a thickness of slightly less than a fourth of an 
inch. What caused the lamination? Apparently the layer 
of clay belongs to a time of constantly muddy water, which 
we may attribute to melting ice. The interval between the 
layers seems to indicate a quiescence of the mud deposition, 
due apparently to a failure of the ice-sheet to supply the 
material, which of course means that the ice had ceased to 
melt to any great amount. If we assume that we have here 
evidence of periodic changes in the condition of the advancing 
Kansan ice-sheet, the question still remains as to the cause of 
these changes and the length of the periods. Evidently 
climatic change is the only cause capable of inducing periods 
of melting ice to recur regularly, divided by intervals when 
little or no ice melted. 
Now, if we assume that these periods were similar to the 
periodic changes of some years’ extent in the present climate, 
as indicated by a study of the behavior of alpine glaciers, we 
find ourselves obliged to accept an enormous length for the 
Glacial period. The deposit, at the type locality amounting 
to a thickness of at least 15 feet, began to form when the 
advancing ice obstructed the lower valley; and its formation 
ceased when the ice reached it and pushed back the lake bor- 
