DRAINAGE SYSTEMS AND THE GLACIAL PERIOD. 229 



Ancient River Terraces. 



The interest of the whole inquiry respecting the rela- 

 tion of man to the Glacial period in America concentrates 

 upon these temporary lines of southern drainage. Wher- 

 ever they existed, the swollen floods of the Glacial period 

 have left their permanent marks in the deposition of ex- 

 tensive gravel terraces. The material thus distributed is 

 derived largely from the glacial deposits through which 

 they run and out of which they emerge. While the height 

 of the terraces depended upon various conditions which 

 must be studied in detail, in general it may be said that it 

 corresponds pretty closely with the extent of the area 

 whose drainage was turned through the channel during 

 the prevalence of the ice. The height of the terraces 

 and the coarseness of the material seem also to have been 

 somewhat dependent upon the proximity of their val- 

 leys to the areas of most vigorous ice-action, and this, in 

 turn, seems to lie in the rear of the moraines which Presi- 

 dent Chamberlin has attributed to the second Glacial 

 epoch. Southward from this belt of moraines the ter- 

 races uniformly and gradually diminish both in height 

 and in the coarseness of their gravel, until they finally 

 disappear in the present flood-plain of the Mississippi 

 River. 



Fig. 60.— Ideal section across a river-bed in drift region : b b b, old river-bed ; 

 B, tbe present river ; £ t, upper or older terraces ; ft', lower terraces. 



An interesting illustration of this principle is to be 

 observed in the continuous valley of the Alleghany and 

 Ohio Rivers. The trough of this valley was reached by 

 the continental glacier at only a few points, the ice barely 



