490 THE ILLINOIS GLACIAL LOBE. 



the estimated average yearly flow past the city of Rockford. 1 It is not 

 known whether the present flow differs widely from that of the stream 

 which produced this rock excavation, for the excavation appears to have 

 been practically completed prior to the Wisconsin stage of glaciatlon. The 

 drainage area and the rate of discharge may have been altered somewhat 

 as a result of the Wisconsin invasion. Also a part of the rock cutting may 

 have been produced by a glacial stream at the time of deflection. It is 

 thought, however, that the glacial stream would have expended its energies 

 at the rock divides, and that changes in drainage area are of minor conse • 

 quence, so that the excavation may be assumed to have been chiefly accom- 

 plished by a stream similar in size to the present Rock River. 



This narrow section of the Rock River Valley carries deposits of 

 glacial gravel which appear to be the continuation of the broad gravel 

 plain that leads down the preglacial Rock River Valley from the Kettle 

 moraine of the Green Bay lobe. This gravel plain stands about 50 feet 

 above the present level of Rock River at Rockford and southward from that 

 city to the point where the preglacial and present valleys of Rock River 

 part company. It has no continuation southward along the preglacial 

 valley, but passes instead down the narrow valley of the present stream. 

 Its altitude at Byron, as shown by the railway survey, is 50 to 55 feet 

 above the low-water level of the stream at that point. At Oregon its ele- 

 vation is about 40 feet, and it maintains an elevation of nearly 40 feet from 

 there to the point where it emerges into the Green River Basin above Ster- 

 ling. This filling apparently began a few feet below the preseut river level, 

 since the wells made along the flood plain encounter gravel to a depth of 

 10 to 20 feet or more below the stream. However, in places on the borders 

 of the valley it rests upon rock at a level slightly above the present 

 stream. It may not, therefore, amount to more than would be necessary 

 to build the valley up from its present level to a height of 50 feet. Assum- 

 ing this to be the case, the filling will equal about four-fifths of the amount 

 of rock removed from the channel prior to its deposition, or to about 0.16 

 of a cubic mile. The amount deposited in the 50 miles embraced in the 

 section of the preglacial valley between the Kettle moraine and the head of 

 this narrow section is much greater. It is at least 50 feet and may amount 

 to 75 feet in depth. The width being 2J miles or more, it follows that 



■ Seventeenth Ann. Kept. U. S. Geol. Survey, Part II, 1896, pp. 733, 734. 



