Pleistocene Deposits of Illinois. — Hershey. 
293 
under the influence of a warm climate, as immense esker de¬ 
posits indicate, and its retreat may well have been five or ten 
times more rapid than its previous advance over the same re¬ 
gion. In short, I do not consider the period of 750 years for 
the twenty-five miles of advance as at all improbable, and, in 
the absence of evidence to the contrary, shall henceforth as¬ 
sume it to be approximately correct. 
An especially notable feature of the laminated clays under 
discussion is the effect produced by the passage over them of 
a heavy mass of forwardly moving ice. The upper portion of 
the deposit rarely remains in an undisturbed position. At the 
type locality occurs the most magnificent faulting and folding, 
on a small scale, that I have ever beheld. The effect is as 
though a very heavy advancing body had, in passing over an¬ 
other semi-plastic body, pushed the latter forward and down¬ 
ward, causing the upper layers to become closely appressed 
and even overturned, with innumerable very small faults. 
Some of the fault planes are regularly tilted, only a quarter of 
an inch apart, and have a downthrow of about a quarter of 
an inch, being traceable from a few inches to three feet. The 
disturbance extends to depths between 5 and 10 feet below 
the surface of the deposit. Doubtless much of the upper por¬ 
tion has been removed and incorporated with the till. In ad¬ 
dition to this evidence of forward movement of the ice, a mass 
of till has been forcibly injected into the side of an esker de¬ 
posit. Evidence of considerable strength in the advancing 
ice'is found all over this region. The laminated clays, wher¬ 
ever exposed, are much disturbed at their top, although below 
sometimes showing only a gentle undulation of the strata. 
At one place, about six miles farther west, the laminated clays 
appear to have been frozen, and the advancing ice broke them 
up into irregular masses, which were redeposited, along with 
sand and similar boulder-like masses of till, in a confused 
manner, at first sight constituting a very puzzling section. 
Usually these clays are not in sight, either lying deeply buried 
under drift, or having been removed during the subsequent 
glaciation of the country. 
These variegated, laminated, unfossiliferous clays of lacus¬ 
trine origin in the Pecatonica basin, belonging to the Kansan 
epoch, are of sufficient scientific, if not economical, importance 
