Introduction to Life of the Lakes. 101 



the remaining third, or northern portion, of 

 these waters. 



It will be remembered that the ocean bar- 

 rier was so far elevated at the close of the Cre- 

 taceous period as to entirely exclude the sea 

 from the interior. Thereafter the fossil remains 

 buried in these waters would be fresh water 

 remains, and these alone are found there. The 

 contrast between these two kinds of sediment 

 is striking. One of its many instances can be 

 observed on Bridge creek, in Morrow county, 

 near the town of Mitchell. Here one can see 

 an extensive table land covered with a thin 

 soil scarcely concealing the rock below it, and 

 this rock abounding in sea shells Trigonia, 

 Actionella and Ammonites all plainly Cre- 

 taceous, while from the plateau on which they 

 occur one may look to the westward a mile 

 or two over a lower level filled with later sedi- 

 ments. These one soon learns to distinguish 

 as Miocene fresh water deposits of the Bridge 

 creek Oreodon beds. Here are brought into 

 sharp contrast the old basement floor of the 

 Cretaceous seas and the later deposits of the 



