Pleistocene Rock Gorges. — Hershey. 319 
time occupied in the erosion of the Pleistocene rock gorges 
under discussion was apparently not less than ten times as 
great as on this newer drift, 70,000 years is probably the 
minimum length of time in which they may have been formed. 
Portion of gorge erosion effected during different divisions 
or sub-epochs of the Glacial period. — For the present purpose 
it will only be necessary to divide this erosion into two por- 
tions, that which preceded the deposition of the loess, and 
that which has been accomplished since. 
1. Many of the present streams of northwestern Illinois, 
although very muddy, derive their sediment from the loess 
of the uplands and from the alluvium along their banks. 
Very little is derived from the rock bluffs at the foot of which 
they flow through small parts of their course. They are deep 
and incapable of carrying (except by floating ice), even in 
flood time, anything larger than fine sand, and throughout the 
greater part of the year they can only move the finest -ilt par- 
ticles. Hence there is no sediment to grind away the rock, 
and weathering and solution of the limestone — a slow process 
— is the chief way in which the valleys are being enlarged. It 
seems not improbable that the formation of the rock gorges 
under discussion was practically completed at the close of the 
Glacial period. 
2. While the loess was in process of formation no erosion 
was being carried on within the valleys, and subsequently the 
streams had to remove the Nor 10 feet of stiff yellow lo - 
clay before they could again attack the rock. This may have 
taken the greater part of the remainder of the Glacial period, 
although there is evidence that a considerable amount of rock- 
lias been removed from the majority of the gorges since the 
loess was laid down. A portion of this evidence will be pre- 
sented farther on. and I will dismiss the subject here with the 
statement that the amount of rock so removed is apparently 
less than one-fifth of the cubic contents of the gorge. 
Were there no other evidence bearing on the subject, for tin- 
above reasons alone it would he considered probable that a 
comparatively small amount of rock excavation has been ac- 
complished since the deposition of the loess. 
3. The following is a cross-section of a gorge and a portion 
