COASTAL PLAINS 393 



Usually the dry land of the coastal plain has appeared 

 very gradually, with long periods when there was no gain 

 in its extent. Sometimes the waste brought to the ocean 

 was of a different kind from what it was at other 

 times. Thus the character and condition of the material 

 composing the plain vary considerably, but all the strata 

 are usually inclined slightly toward the sea. The bounda- 

 ries of the different kinds of hard and soft material com- 

 posing the plain are approximately parallel to the old 

 shore line. The plain will thus become a belted plain. 



As streams wear back faster in the soft than in the hard 

 material, the side streams become longer in the soft layers 

 than in the hard, and in time streams of considerable 

 length are found running in a direction nearly parallel to 

 the old coast. These have their outlets through streams 

 which run down from the old land across the plain, so 

 that the general appearance of the drainage is something 

 like a lengthwise cross section through the trunk and 

 limbs of an oak. 



When mixtures of different materials are deposited in 

 water, the coarsest sinks first and the finest last (Exp. 130). 

 We should thus expect that of the material brought down 

 by the river the coarser layers would lie back from the 

 coast. This is often true, although there is frequently 

 uncovered near the border of the old land back from the 

 coast a belt of easily eroded material, and a lowland of 

 erosion is formed in this by the streams. The Delaware 

 River from Trenton to Wilmington and the Alabama River 

 between Montgomery and Selma flow through such low- 

 lands. These regions are called inner lowlands and possess 

 a fertile, fine-textured soil, generally the best to be found 

 in -the coastal plain area. 



This inner lowland is bordered on the landward side by 

 the old land, usually composed of firmly compacted rocks 



