THE TOPOGRAPHY AND HYDROGRAPHY OF ILLINOIS XXlii 



lakes is very largely due the maintenance of a comparatively uni- 

 form flow in dry and severely cold seasons. 



In Wisconsin the banks of Rock River are quite low and rolling, 

 but at Janesville the river enters a wide preglacial valley which it 

 follows to a point a little below Rockford, 111. The stream then 

 turns abruptly westward while the valley continues southward to- 

 ward the Illinois River, the valley of which it enters at Hennepin. 

 This changing of the river course is doubtless due to the Wisconsin 

 moraine which was left across its path when the glacier retreated. 

 The water then found an easier outlet through the preglacial chan- 

 nels of some of its former tributaries. The valley averages about 3 

 miles in width, although in places it reaches a width of 5 miles. Most 

 of the way the river follows the western edge of the valley, although 

 just above Rockford it crosses to the eastern side and then back 

 again. Thus, the eastern banks are usually low while those on the 

 west are high and steep, in some places rising 75 feet above the water. 

 When the stream turns westward below Rockford, it runs for SO 

 miles through a narrow valley to a point a few miles below Dixon. 

 For a large part of this distance it flows through the preglacial chan- 

 nels spoken of above. Through this part of its course the stream 

 maintains a width of 500 feet, but its valley varies in width from 1 ,000 

 feet to fully 1 mile (Leverett). It forms long undulating curves, ex- 

 cept at Grand Detour, where it doubles upon itself in short, abrupt 

 bends. The face of the country along the river is rough, broken, and 

 timbered. The prairie extends to the water's edge in only a few 

 places. The bluffs approaching closely to the river are bold, rocky, 

 and precipitous, rising abruptly at times to a height of 125 feet. 

 The little streams on either side have cut deep ravines in the 

 banks, often exposing the several formations of the Lower Silurian. 

 The result is certainly very picturesque and somewhat awe-inspir- 

 ing. Below Dixon the bluffs gradually recede and grow lower until, 

 at Sterling, Rock River begins to flow through a sandy plain known 

 as the Green River basin, a plain which lies 25-40 feet above the 

 stream. Here the course of the river is entirely independent of pre- 

 glacial lines, and its current is broad and swift. The bluffs of the 

 Mississippi strike Rock River at Milan and for several miles above 

 this point they rise on either side abruptly, in some places towering 

 •150 feet above the water. They then break a'way and the river flows 

 in an alluvial plain of good farming land. This plain is about 5 miles 

 wide. Near the mouth of Rock River there -are several small islands 

 which divert the river into three channels. Two of these branches 

 meet again near Milan, flowing into the Mississippi two and a half 



