Geography of Ohio 367 



Where were the land areas whose streams bore the sediments 

 from which were built the series of rocks within this state? Ob- 

 viously much of the sedimentary rock material of Ohio was trans- 

 ported here by drainage from older lands to the north, while some 

 may have been deposited by streams flowing westward from lands 

 that existed farther east. While the rocks of our state were accu- 

 mulating, the ocean occupied the region of the present Mississippi 

 valley; the Ohio area, during most of the time, was a portion 

 of this interior sea, but later it was a gulf or bay extending north- 

 eastward. 



The same agencies were active in forming the sedimentary 

 rocks in the Ohio scale as are in operation at the present time. 

 Along the rivers material is in transit. Some of this load is always 

 on the move; some is carried on in installments, a period of rest 

 following one of movement. The part of the load that reaches 

 the large bodies of water, a lake or the ocean, is the finest ; this is 

 largely deposited near the mouth of the river as a delta. In time 

 the delta grows more and more into the body of water, forming a 

 wider marsh region. But a very significant part of stream depos- 

 its is found in the flood plain that borders the river channel from 

 the sea inland; in the flood plain of any mature river we can see 

 ample deposits, enough to make hundreds of deltas such as exist 

 at the river's mouth. As the major river widens its valley, the 

 tributaries also become bordered by deep deposits which they 

 are unable to carry onward. 



So a river's history is one of constant work, and of various 

 kinds of work. At some particular point along the river's course, 

 it is doing such work as at an earlier time it did farther down 

 stream. Rivers first cut into the rock, deeper and deeper; later 

 they cut laterally, making the valley wider. But while they are 

 engaged down stream in widening a valley, upstream their busi- 

 ness is deepening the valley. So long as the stream is cutting 

 deeper, it generally carries away the products of this erosion; 

 when the work changes to valley-widening, it fails to carry off all 

 of the products of its erosion; thus flood plains are made. 



An old valley need not be of great length in order to have several 

 hundred square miles of flood plain deposits ; the Mississippi south- 

 ward from Cairo, Illinois, is an example. A large percentage of 

 the surface of a region that contains several old streams consists 

 of flood plains. Much of this flood plain material in a later 



