62 CHICAGO ACADEMY OF SCIENCES. 



plished by the lake outlet. Being the outlet from a lake, however, 

 its waters probably carried but little sediment in this portion, a 

 feature which should be weighed in discussing the slight amount 

 of excavation. Whether a stream of nearly clear water could have 

 accomplished the slight excavation here displayed in the course of 

 a few thousand years remains to be determined. 



Turning to Niagara River, where the work accomplished in 

 a few thousand years by a clear stream is open to investigation, it 

 is found that the recession of the Horseshoe FaH is due almost 

 wholly to undermining, and that the American Fall, in which there 

 is little undermining, is nearly stationary. The cutting on the 

 rapids above theFalls is also very slow and amounts to but a few 

 feet in average depth. 



Professor Cooley has called my attention to the deposits at 

 the head of Lake St. Clair as likely to furnish an index of the 

 amount of sediment transported by the Chicago outlet. A delta, 

 with an area of several square miles, has been built in the head of 

 Lake St. Clair, which must have derived the bulk of its material 

 from southward moving littoral currents along both the borders 

 of Lake Huron. In the lake under discussion littoral currents 

 along the west border would have transported material probably 

 in as great volume as on either shore of Lake Huron, but those 

 on the east and south may have contributed less, for wind drifting 

 there is very effective. It seems legitimate to assume that at least 

 half as much sediment was being transported down the Chicago 

 outlet as is carried by the St. Clair River. The waters of the 

 Chicago outlet would be probably more turbid than the Niagara 

 and less turbid than the St. Clair. Prof. Cooley thinks the contri- 

 butions of sediment to the outlet through the Des Plaines were of 

 little consequence, for this river has, since the lake waters were 

 withdrawn, made scarcely any filling of the outlet below River- 

 side, where its delta would naturally accumulate. The accession 

 of larger tributaries below may have rendered the stream slightly 

 more turbid than on the rapids, and the rate of rock excavation 

 would have been correspondingly accelerated. 



It should not be inferred that this outlet is entirely free from 

 river debris. Beginning at the upper beach, near Summit, there 

 is for several miles a mass of coarse material, largely limestone 

 blocks, too large to have been transported by the current, covering 

 the bed of the outlet. The Drainage Canal exposes excellent sec- 

 tions of the coarse river debris from Summit to Lemont, there 

 being only limited areas in this interval, where the solid rock comes 



