Great Lal\s, and the Valley of the Mississippi. 229 



also, often assist in building up, and sometimes alone form these 

 ridges, by transporting inland the beach sand. 



In other localities, where hard rock masses formed the shore 

 of our inland sea, perpendicular wave-worn cliffs were pro- 

 duced; and many of these now stand as enduring and indis- 

 putable monuments of a sea whose waves, perhaps, for ages 

 beat against them. Such cliffs may be observed od Little 

 Mountain, in Lake county, in the valley of the Cnyahoga, 

 in Medina and Lorain county, Ohio, along the outcrops of the 

 Carboniferous conglomerate and Waverley Bandstone. 



In all the changes through which the valley of the Mis 

 sippi passed during the "Drift Period," its general structure 

 and main topographical features remained the same. Yet the 

 character of its surface suffered very important modifications, 

 and such as deeply afi'ected its fitness for human occupation. 



As we have seen, the glacial epoch was marked by erosion 

 on a grand scale. 



Then, our river valleys and some of our lakes — though 

 mapped out long before — were excavated to a much greater 

 depth than they now have. 



During their subsequent submergence, these valleys and 

 lakes were partially or perfectly filled with the drift deposits 

 which covered all the surface like a deep fall of snow, rounded 

 its outlines and softened all its asperities. 



When the waters were withdrawn, the rivers again began 



clearing their obstructed channels; a work not yet ac< i- 



plished, and in many instance- not half done. Numbers of the 

 old channels were wholly tilled and obliterated, and the streams 

 that once traversed them were compelled to lind quartet 

 where. Examples of this kind have been already cited, and 

 they could be multiplied indefinitely. 



ORloix or 'i nr: OBE \ i LAX! 9. 



The question of tie in of our lake- i- one that requii 



more observation and study than have yet been given to it be- 



