60 CHICAGO ACADEMY OF SCIENCES. . ■ 



not markedly greater than in the portion above Johet. The por- 

 tion above Johet is cut to a shght depth into the Niagara hmestone, 

 which there underhes the glacial gravel. The excavation in lime- 

 stone, however, amounts to not more than one-fourth the size of 

 the channel, for the limestone seldom rises more than 40 feet above 

 the bed of the lake outlet and in many places its surface comes 

 down nearly to the level of the vahey floor. Below Johet there 

 was even less excavation in the rock than above. It is estimated 

 that the rock excavation here does not exceed ten per cent, of the 

 total cutting. 



In the low tract at the head of the Illinois (the Morris basin) 

 the depth of excavation by the outlet is very slight, averaging prob- 

 ably less than 20 feet in the ten miles between the head of the 

 Illinois and Morris. The plain appears to have descended nearly 

 to the 520-foot contour on the borders of the river before modified 

 at all by lake or stream action. . A low bluff formed on the north 

 border of the basin has a height of 15 to 20 feet. On the south 

 border there is no bluff, that side of the basin being heavily coated 

 with sand deposits. These deposits may perhaps have been laid 

 down in part at the time the lake waters were forming the outlet, 

 but they are probably largely of earlier date. In this basin the 

 lake outlet had an average width of four or five miles. 



In the section of the lUinois, immediately below (west from) 

 this basin, erosion, prior to the opening of the Chicago outlet, may 

 have brought the level of the valley bottom down to that of the 

 upper beach line of the basin, 550 to 560 feet above tide. The bed 

 of the Chicago outlet is nearly 500 feet, thus leaving about 60 feet 

 depth of erosion. Passing westward the broad bed of the Chicago 

 outlet declines nearly 60 feet in the forty miles between the west 

 border of the basin, just mentioned, and the bend of the Illinois 

 near Hennepin. AVhether the valley had the same gradient. at the 

 time the accession of lake waters occurred is not known, but it 

 could not have been greatly different, for the glacial terrace just 

 above Hennepin stands about 30 feet lower than the beach lines 

 of the Morris basin, and this terrace was, in all probability, eroded 

 the remaining 30 to 40 feet necessary to give a similar gradient. 



The width of the outlet between Morris and Hennepin aver- 

 ages about 1 1 miles. The excavation is largely in a soft sandstone, 

 there being nearly continuous rock bluffs to a height of 60 to 75 

 feet above the level of the bed of the outlet. This sandstone 

 presents much less resistance to stream action than the firm 



