THE CHICAGO OUTLET. 423 



a breadth of 1 to 1£ miles, averaging perhaps 1^ miles. Between Joliet 

 and the head of the Illinois several island-like remnants of the glacial 

 terraces are preserved in the midst of the channel, making it more difficult 

 to estimate the breadth,. but it is not markedly greater than in the portion 

 above Joliet, The portion above Joliet is cut to a slight depth into the 

 Lockport (Niagara) limestone, which there underlies the glacial gravel. The 

 excavation in limestone, 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 valley floor. Below Joliet there was even less excava- 

 tion in the rock than above. It is estimated that the rock excavation there 

 does not exceed 10 per cent of the total cutting. 



In the low tract at the head of the Illinois (the Morris Basin) the depth 

 of the excavation b) T the outlet is very slight, averaging probably less than 

 20 feet in the 10 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 has an 

 average width of 4 or 5 miles. 



In the section of the Illinois immediately below (west from) this basin, 

 erosion prior to the opening of the Chicago Outlet probably had 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 subsequent depth of erosion. Passing 

 westward the broad bed of the Chicago Outlet declines nearly 60 feet in the 

 40 miles between the west border of the basin, just mentioned, and the bend 

 of the Illinois near Hennepin. Whether 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 in all probability had been eroded the remaining 30 to 40 feet neces- 

 sary to give a similar gradient. 



