242 llie American Geologist April, i8!<7 
the deposit of the single stage of the destruetioti of the gla- 
cier, while the "special area" indicates the position of the 
mouth of the tunnel. 
I shall next attempt an explanation of another ])henomenon 
connected with these stratified drift deposits, which has been 
observed in many other districts. It is that the beds of coarse 
gravel, cobbles, and small boulders, are almost invariably at 
the top of the deposit, contrary to what we usually find in the 
product of ordinary stream action. They cap the mounds in 
the "special areas," and, by protecting them from erosion, are 
largelj'- responsible for the present topography. It is evident 
that some special cause must have operated in this case. This 
cause I would find in periodic changes of climate, such !»s are 
affecting the Alpine glaciers at the present time. The ice- 
front having remained nearly stationary during some j'^ears or 
possibly decades, very suddenly retreated to a distance of sev- 
eral miles. This sudden recession corresponded to a mild 
period or a series of warmer summers. Melting of the ice was 
excessive, and hydrostatic pressure very great, so that the 
streams, hy their increase of power and lower position, due to 
the decay of the ice, rapidly eroded the rock ridges with which 
they came in contact, hurried the material forward, and hurled 
it over the banks of sand at their mouths. It was their last 
effort, for after a few seasons the ice-front had established a 
new terminal line, and a new series of " special areas '' was 
formed, to be capped in turn by a thick stratum of coarse 
gravel. I'his hypothesis is, as far as I can see, the only one 
which will satisfactorily account for the persistent association 
of beds of coarse gravel with the upper portions of the depos- 
its in the " special ai-eas." 
None of the ridges are terminated at the west end by a well 
marked delta plain or a gravel plateau, such as are common 
in some of the more eastern districts. The nearest approach 
to a delta is the sand plain which terminates the western end 
of the Pecatonica belt. But instead of sloping gradually from 
the end of the ridged deposit to the outer edge of the plain, it 
is simply a nearly flat bed of sand lying at the foot of a series 
of abruptly ended ridges. This abrupt westerly termination 
of the several ridges is characteristic of this district, and at 
first thought might seem to be a very puzzling phenomenon. 
