EOOK RIVER DRAINAGE BASIN. 491 



there was not less than a cubic mile deposited in this section of the pre- 

 glacial valley, and the amount may possibly approach 2 cubic miles. 



The amount of excavation accomplished since the deposition of the 

 gravel is remarkably small. The valley formed subsequent to this gravel 

 deposition varies but little in breadth in the 100 miles from the Kettle 

 moraine to the lower end of the narrow section. It is usually about 1,500 

 feet in width, and seldom exceeds 2,500 feet. Its flood plain is 30 to 40 

 feet below the level of the gravel plain and its stream bed about 50 feet. 

 The stream occupies about one-third the width of this postglacial valley. 

 The amount of excavation accomplished in the narrow section under discus- 

 sion is estimated to be about 650 mile-feet, or slightly more than half the 

 amount of rock excavation accomplished prior to the deposition of the 

 Wisconsin gravel. 



It is a matter of much interest to ascertain the relative lengths of time 

 involved in the interglacial rock excavation and the postglacial gravel 

 excavation. This, however, must be left to more refined methods than are 

 now at command. It seems safe to assert that the time involved in the 

 rock excavation was longer than that involved in the gravel excavation, 

 though it must be borne in mind that water may find passage through or 

 over gravel without disturbing it, where it might be cutting rapidly in the 

 soft St. Peters sandstone which forms a portion of the new channel of Rock 

 River. But the greater part of the rock excavation has been in limestone, 

 which would perhaps offer fully as much resistance to erosion as the gravel 

 deposits which now line the valley. 



The deflection of Rock River into this new course certainly preceded 

 the Wisconsin stage of glaciation, and probably preceded the Iowan. The 

 completeness of the removal of rock barriers along its new course and the 

 general character of the channeling seem to ally it with certain rock gorges 

 found in portions of northwestern Illinois and southern Wisconsin, west of 

 Rock River, which, as determined by Hershey, were largely excavated 

 prior to the Iowan stage of glaciation. 1 These gorges are discussed below 

 and their relation to the Iowan deposits are set forth. Furthermore, the 

 work accomplished elsewhere between the Iowan and Wisconsin stages of 

 glaciation throws doubt upon the view that this channel was entirely 

 excavated after the Iowan stage of glaciation. The work involved in the 



1 Pleistocene rock gorges of northwestern Illinois, by Oscar H. Hershey : Am. Geologist, Vol. 

 XII, 1893, pp. 314-323. 



