SURFACE GEOLOGY. 209 
effect of waves everywhere on a sloping shore composed of 
loose material; storms driving up sand and gravel to form a 
ridge which ultimately acts as a barrier to the waves that 
built it. Winds, also, often assist in building up, and some- 
times 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 
produced; and many of these now stand as enduring and 
indisputable monuments of a sea whose waves, perhapé for 
ages, beat against them. Such cliffs may be observed on Little 
Mountains: in Lake county, in the valley of the Cuyahoga, in 
Medina and Lorain county, Ohio, along the outcrops of 
the Carboniferous conglomerate and Wait sandstone. 
In all the changes through which the valley of the Missis- 
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 modifica- 
tions, and such as deeply affected its fitness for human occu- 
pation. 
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 notv 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When the waters were withdrawn, the rivers again began 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 accom- 
plished, and in many instances not half done. Numbers of 
the old channels were wholly filled and obliterated, and the 
streams that once traversed them were compelled to find 
quarters elsewhere. Examples of this kind have been al- 
ready cited, and they could be multiplied indefinitely. 
AMER. NATURALIST, VOL. IV. 27 
