ii4 



PHYSICAL GEOLOGY 



FlG. ioi. — Drowned river valleys. 

 Chesapeake and Delaware bays and Albe- 

 marle Sound were formed by a lowering of 

 tin laud which permitted the sea to fill the 

 valleys. 



Canyon, more than 2000 feet 

 deep, was excavated. The Un- 

 compahgre River, which joins 

 the Gunnison after flowing in 

 approximately the same direc- 

 tion for some distance, was born 

 at the same time. It has flowed, 

 however, over soft material which 

 could be readily eroded, and has 

 been able to excavate a valley 

 several miles in width which in 

 one place is separated from the 

 narrow Black Canyon by a 

 narrow ridge of granite. 



If a region is depressed, the 

 velocity of the streams will be 

 lessened, and the condition of old 

 age will be hastened. Drowned 

 river valleys (p. 227), such as 

 the Delaware, the St. Lawrence, 

 and Chesapeake Bay (Fig. 101), 

 are the result of the sinking of 

 the land in the lower courses of 

 the rivers. 



Peneplanation 



When a base-leveled region 

 (peneplain) has later been up- 

 lifted and dissected by erosion, 

 tin evidence of the former base-leveled condition is to be seen in 

 the horizontal sky line presented by the higher hills (Fig. 102). The 

 effect of erosion on an elevated peneplain under different conditions 

 of rock structure is shown by a study of (1) southern New England, 

 (2) the Appalachian region, and (3) eastern Canada. 



(1) The Peneplain of Southern New England. — Southern New 



and is underlain on each side of the broad Connecticut valley 



by hard, crystalline rocks; the valley is in part composed of sand- 



stonc and in part of lava. Before the present elevation took place, 



n had been active for so long that even the lavas, granites, 



