EOCK GORGES OF NORTHWESTERN ILLINOIS. 493 



The several headwater streams descend rapidly to the sandy plain outside 

 the moraine. They there soon enter a wet prairie — "Inlet Swamp" — whose 

 altitude is about 775 feet above tide and which covers perhaps 16 square 

 miles. No channel is maintained across this prairie, but from the west end 

 of the prairie a stream with a well-defined channel leads westward about 15 

 miles to another wet prairie — "Winnebago Swamp" — making a descent of 

 nearly 100 feet. In this second wet prairie, which is fully 10 miles in 

 length, the stream has only a poorly-defined channel, but apparently makes 

 a descent of several feet. The western part of the prairie is estimated by 

 Rolfe to stand only 660 feet above tide. In the next 25 miles, to the Gross- 

 ing of the Bureau-Henry county line, the stream has a poorly-defined 

 channel, meandering about through a series of marshes among sand hills, 

 but making a descent of 60 feet (Rolfe). In the remaining 35 or 40 miles 

 to its mouth the stream falls about 40 feet and maintains a well-defined 

 channel. In the lower 18 or 20 miles (below Geneseo) it has excavated a 

 valley fully 20 feet in average depth and nearly one-half mile in width. In 

 this section of its course it is bordered by uplands which are far less sandy 

 than the lowland plain known as the Green River Basin. 



The sand which covers the Green River Basin, as previously indicated, 

 appears to be largely an outwash from the Wisconsin moraine. The chan- 

 nel of Green River is, therefore, of post- Wisconsin age, although traversing 

 a district which stood outside the limits of the ice sheet at the Wisconsin 

 stage of glaciation. The trench which it has cut below Geneseo is entirely 

 in deposits of silt which are also of apparently Wisconsin age. They have 

 an elevation corresponding closely to that of the Wisconsin terrace on 

 neighboring portions of the Mississippi, and it is thought that they are 

 slack-water deposits connected with the swollen Mississippi during the time 

 in which it was building up the terrace. 



ROCK GORGES OF NORTHWESTERN ILLINOIS. 



In the portion of northwestern Illinois lying between Rock River and 

 the border of the Driftless Area the drift is generally so thin that the streams 

 follow in large part the preglacial lines. There are, however, several 

 instances of the deflection of streams into the edge of the bluff or across a 

 projecting point on the border of a valley. Such deflections usually occur 

 where a cluster of knolls or ridges of drift greatly obstruct the valley, and 

 they are usually only of sufficient length to pass the obstruction. In some 



