THE TOPOGRAPHY AND HYDROGRAPHY OF ILLINOIS XXix 



larger. Hills of sand rise in chains and clusters from the midst 

 of them. These hills were originally heaped up by the winds 

 from the sands of the old lake-bed. Some of them are 40-50 

 feet high and are covered with a scattering and stunted growth 

 of trees. The intervening swamps are fringed with bands of 

 thick-growing swamp grass on a miry, mucky soil. Within 

 these are inner fringes of dense cane-like rushes and cattails 

 growing so thick and tall that it is almost impossible to pene- 

 trate them. Then come stretches of clear water with hard 

 sand bottoms. In the next 25 miles, to the crossing 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. In the remaining 

 35 to 40 miles to its mouth, the stream falls about 40 feet and 

 maintains a well-defined channel. In the lower 18 to 20 miles, 

 below Geneseo, it has excavated a valley fully 20 feet in average 

 depth and nearly half a mile in width. In this section of its 

 course its uplands are far less sandy. 



Along the whole course of Green River, there are no bold 

 bluffs except at Lee Center, where some low outcrops of Galena 

 dolomite are quarried. 



THE NORTHWESTERN AREA 



The waters of extreme northwestern Illinois differ suffi- 

 ciently in condition and surroundings from those of the smaller 

 tributaries of the Mississippi farther south to warrant their 

 separate discussion in this report. The surface drained by them 

 is the southernmost part of a tract known to geologists as the 

 Wisconsin driftless area, a region not covered by ice during the 

 glacial period, and consequently wholly destitute of glacial 

 drift. Because of its prolonged exposure to erosion its streams 

 have reached the limit of their development, and run usually 

 through deep valleys with rather a swift current, mostly unob- 

 structed by rapids or falls. As a consequence of this perfect 

 drainage and rapid flow, the surface waters quickly escape to 

 the Mississippi; but as the streams are fed to a considerable 

 extent by springs flowing from the limestone rocks, they rarely 

 are completely dry. There are no lakes, swamps, or other 

 reservoirs for the sedimentation of the surface waters, and the 

 streams are consequently easily roiled by storms, in the intervals 

 of which, however, the water is comparatively clear. 



