1897.] The Inferior Boundary of the Quaternary Era. 113 
far the longer. This is graphically illustrated by the vast dif- 
ference in the amount of valley erosion accomplished during 
the two periods. Although the glacial epochs are usually con- 
sidered to have been accompanied by increased precipitation 
and other favorable conditions for rapid erosion, the valleys 
then excavated are quite insignificant as compared with the 
] 
3 
2 Modern Epoch—Flood plain deposition. 
D 
= 
g 
S 
Pe Terrace Epoch—Valley erosion. 
Wisconsin Epoch—Drift deposition. 
z 
— m Toronton ? Epoch— Valley erosion. 
i E 
> ` ; e 
3 z 3 Iowan Epoch—Drift deposition. 
eee ee 3 
S| See i ; : 
© © Aftonian Epoch—Valley erosion. 
=] : 
Q 
8 
2 Kansan Epoch—Drift deposition. 
A 
28 
fz 2 Ozarkian Epoch—Valley erosion. 
as 
Ozarkian valleys. More erosion was accomplished in certain 
districts during the Aftonian interglacial epoch than during 
the combined Kansan, Iowa, Toronton ? and Wisconsin epochs. 
Yet the Aftonian rock gorges never approach in size the 
‘Ozarkian valleys of the same region. In fact, any one who 
has studied the Ozarkian valleys of the southern Appalachian, 
the Ozark and the upper Mississippi regions, and compared 
them with the results of post-Kansan erosion in such districts 
as western Illinois and northern Missouri cannot fail to be im- 
pressed with the fact that the time which has elapsed between 
