PAPERS ON BIOLOGY AND AGRICULTURE 153 



ROCK RIVER 



The Rock River, rising in southeastern Wisconsin, has a 

 course of some 300 miles, flowing southwestward to empty 

 into the Mississippi River below Rock Island, Illinois. It 

 has a drainage area of about 11,000 square miles, half of 

 which is in Wisconsin in the Wisconsin glacial drift, afford- 

 ing poor drainage with the occurrence of many swamps, 

 and half in Illinois in the lowan glacial drift, affording good 

 drainage with no swamps. 



The course of this river has been greatly altered since 

 the Pleistocene ice age. Its former valley is much to the 

 east of the present one in Ogle County. The preglacial 

 valley is departed from in Winnebago County before reach- 

 ing Ogle County, but in Ogle the river takes its way 

 along the valleys of certain of the preglacial tributaries and 

 in large part along a postglacial course. Thus the Rock 

 follows the preglacial valley of the Leaf River for a few 

 miles in the vicinity of Byron in the north of the county but 

 in the reverse direction from that of the preglacial Leaf, 

 and uses as well some of the small preglacial tributaries. 

 Farther south Kyte River flows northwestward into the 

 Rock below the town of Oregon in the valley of a pre- 

 glacial western tributary' of the Rock. The head of this is 

 in the hills back of the town of Oregon, the present Rock 

 cutting off only the headwaters portion of the preglacial 

 valley. Several smaller streams also had preglacial courses 

 cutting across the present Rock River which now intersects 

 several of them midway of their course and diverts them 

 westward into the Mississippi River by way of the Rock. 

 From not far south of the Kyte the Rock appears to follow 

 the line of a small preglacial stream as far as the mouth of 

 Pine Creek, in a valley varying from about one quarter of 

 a mile to as much as a mile in width at Grand Detour where 

 the river makes its big bend. The course of the Rock, then, 

 from where it turns away from its broad preglacial valley 

 in southern Winnebago County, is in Ogle County south- 

 westward through a much narrower valley, a valley that 

 is postglacial except where the Rock occupies the valleys 

 of preglacial streams. In this postglacial course the river 

 is about 500 feet wide in a valley varying from 1000 feet 



