Cfl. ^'IL] 



RIVER TERRACES. 



85 



oscillations of level, I liave endeavored to show in my description of that 

 country;^ and the freshwater shells of existing species and bones of 

 land quadrupeds, partly of extinct races preserved in the terraces of flu- 

 viatile origin, attest the exclusion of the sea during the whole process of 

 filling up and partial re-excavation. 



In many cases, the alluvium in which rivers are now cutting their 

 channels, originated when the land first rose out of the sea. If, for ex- 

 ample, the emergence was caused by a gradual and uniform motion, 

 every bay and estuary, or the straits between islands, would dry up 

 slowly, and during their conversion into valleys, every part of the up- 

 heaved area would in its turn be a sea-shore, and might be strewed over 

 with littoral sand and pebbles, or each spot might be the point where a 

 delta accumulated during the retreat and exclusion of the sea. Mate- 

 rials so accumulated would conform to the general slope of a valley fi^om 

 its head to the sea-coast. 



River terraces. — AVe often observe at a short distance from the present 

 bed of a river a steep cliff a few feet or yards high, and on a level with 

 the top of it a flat terrace corresponding in appearance to the alluvial 

 plain which immediately borders the river. This terrace is again bounded 

 by another cliff, above which a second terrace sometimes occurs : and in 

 this manner two or three ranges of cliffs and terraces are occasionally 

 seen on one or both sides of the stream, the number varying, but those 

 on the opposite sides often corresponding in height. 



Fig. 102. 



Eiver Terraces and Parallel Eoads. 



These terraces are seldom continuous for great distances, and their 

 surface slopes downwards, with an inclination similar to that of the river. 

 They are readily explained if we adopt the hypothesis before suggested, 

 of a gradual rise of the land ; especially if, while rivers are shaping out 

 their beds, the uphea\'ing movement be intermittent, so that long pauses 

 shall occur, during which the stream will have time to encroach upon 

 one of its banks, so as to clear away and flatten a large space. This 



* Second Visit to the U. S., vol. ii. chap. 34. 



