Geology of Upper IMlinois. 137 
line at Chicago, it may very obviously be remarked, that an east- 
erly storm, (the only one that could here produce any effect,) 
acting simply on the breadth of the lake, would have very little 
power in giving rise to beaches, compared with gales traversing 
the entire length of such an immense body of water. Besides 
which, the line of coast on the western shore is so broad as to 
prevent the heaping up of the water to any extent, compared 
with what must take place at the confined extremity of the lake. 
It will be an interesting inquiry to ascertain if possible, the 
length of time requisite for forming a single beach at the head of 
the lake, since, if this could be settled, we should have the ele- 
ments for the chronological computation of all the ridges belong- 
ing to the first system, above described. Could this be satisfac- 
torily made, the era of the second series might perhaps be found 
capable of an approximative determination, as well as that of the 
third and fourth belt, both of which correspond in outline to those 
first mentioned, and are therefore plainly of lacustrine origin.* 
Before dismissing this very striking appearance of the coast 
connected with the action of the lake, [ must be permitted to ex- 
press the opinion, that a careful examination of the country bor- 
dering on the Kankakee and the Des Plaines valleys, will afford 
evidence of the occasional overflow of the lake at ancient periods, 
in those directions. It is a well known fact, that the lip of the 
lake, near its south-western extremity, is at one place so de- 
sed as to permit canoes to pass from the head waters of the 
Chicago river across to that of the Des Plaines. Nor would the 
circumstance cease to be an alarming one to the safety of this 
portion of country, except for the fact that the border to the lake 
is every where composed of a firm limestone. If then the waters 
of the lake are still capable of interlocking with those of the 
Des Plaines, it is clear that at a higher level of the lake, consid- 
erable descents of water upon the low country must have taken 
place. ‘Traces of such incursions appear to exist on the stage 
road from Chicago to Ottawa, in the general direction of the 
Jat: oF land on the nSiag prairie, and more particularly in the 
'* Heaton hh farth land these formations extend, I cannot say; but I should 
not be si rised to learn, that they p revail under yarious modifications, quite back 
to the summit level which turns the waters of the country into the Kankakee, a 
distance of fifteen or twenty miles. 
Vou. XXALV.—No. 1. 18 
