488 



THE ILLINOIS GLACIAL LOBE. 



N° I. PREGLAGIAL AND PRESENT VALLEY 

 OF ROCK RIVER AT ROCKFORD, ILLINOIS. 



N92PR0FILE AT 

 COL CROSSBD 

 BY ROCK RIVER 

 AT'A'PLATEXII 



ridges the rock surface descends to only 20 or 25 feet above the river. The 

 exeavation in this ridge is, therefore, not more than 15 mile-feet. 1 



In the three miles along the preglacial Bluff Creek the amount of cut- 

 ting ranges in depth from 25 to nearly 100 feet and in width from 1,500 to 



2,000 feet. The level at which the pre- 

 glacial stream had cut is marked by a 

 change in the angle of slope (see fig. 6, 

 No. 3), that below being much steeper 

 than that above the preglacial level. It 

 seems safe to assume 100 mile-feet of 

 excavation. 



On the divide between Bluff Creek 

 and the north tributary of the preglacial 

 Leaf River the cutting is estimated to be 

 double that ' at the neighboring rock 

 divide on the east, or 30 mile-feet (see 

 fig. 6, No. 4) . It has a depth of 80 to 1 10 

 feet and a width of 1,200 to 2,000 feet. 



Along the north tributary of the 

 preglacial Leaf River the excavation de- 

 creases from about 80 feet practically to 

 zero. The width of excavation is about 

 2,000 feet, widening to nearly 2,500 feet 

 at the border of the old Leaf River Val- 

 ley. The excavation is estimated to be 

 60 mile-feet, 



In the 7 miles along the preglacial 

 Leaf River Valley the excavation is con- 

 fined to a slight cutting of perhaps 25 



N93 ROCK RIVER 

 IN PREGLACIAL 

 BLUFFCREEK 

 VALLEY 



n94-. profile 

 at col crossed 

 by rock river 

 at'b'platexh 



N°5.ROCK RIVER IN THE 

 PREGLACIAL LEAF RIVER 

 VALLEY AT BYRON.ILLI N OIS. 



N96 PROFILE AT 

 COL CROSSED 

 BY ROCK RIVER 

 AT'C' PLATE XII 



N°7 ROCKRIVER 

 INTHEPRECLACIAL 

 KITERIVER VALLEY 

 ATOREGON.I 



NPaPROFILEAT 

 COLIDCROSSED 

 BY ROCK RIVER 

 AT"o" PLATEXII 



FlG.6.-Sectious across Eock River Valley, in northern feet at the bordei'S of the preglacial Val- 

 ley, which diminishes to zero before 

 reaching the middle of the valley. The valley here is a mile wide, with 

 gradual rise to either bluff, as shown in fig. 6, No. 5. The excavation is 

 estimated to be but 50 mile-feet, which is. proportionately less than along 

 any other part of this narrow portion of the valley. 



1 A mile-foot embraces a block 1 mile square and 1 foot thick. 



