82 
HAROLD’S DISCUSSIONS. 
When the continent was submerged during the 
warm period, the ice was melted or broken up into 
icebergs, as is now the case on the shores of Green¬ 
land. The sea extended as bays far up into the rivers 
in the Mississippi valley perhaps as far north as Iowa. 
The greatly shortened rivers deposited their debris 
into these quiet bays. When the land was again 
Fig. 48. —Glacial flutings of bed-rock, near Burlington, Iowa. 
raised, the rivers flowed in broad stream over this 
sediment, gradually wearing it down. As the land 
rose higher, the rivers narrowed and cut smaller and 
deeper channels. In this way the terraces along rivers 
have been formed. 
On the high banks of Lake George, and on other 
bluffs, marine shells can be found. Some of these 
