Geology of Upper Illinois. 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, 1 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- 

 pressed 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 

 swells of land on the rolling prairie, and more particularly in the 



* How much farther inland these formations extend, I cannot say; but I should 

 not be surprised to learn, that they prevail under various modifications, quite back 

 to the summit level which turns the waters of the country into the Kankakee, a 

 distance of tifleen or twenty miles. 



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