298 HISTORICAL GEOLOGY 



a considerable angle across the old Taconic folds above the High- 

 lands, after which it passes through a deep gorge which it has cut 

 through the hard granites and other rocks of the Highlands. The 

 simple explanation is that the Hudson had its course determined 

 upon the surface of the upraised Cretaceous peneplain, and that 

 it has been able to keep that course in spite of the discordant struc- 

 tures of the underlying rocks. The seemingly anomalous courses 

 of the Delaware, Potomac, Susquehanna, etc., are to be similarly 

 explained. 



But while the great master streams were thus cutting deep 

 trenches in hard and soft rock alike, numerous side streams or 

 tributaries came into existence and naturally developed along the 

 belts of weak rock and in harmony with the geologic structures. 

 This principle is especially well illustrated by all of the streams 

 now occupying the valleys between the Appalachian ridges. 



After the uplift of the peneplain, the larger streams cut down 

 their channels most rapidly and were the first to reach " grade," 

 that is a condition in which, because of low velocity, they could no 

 longer cut down their channels, though the widening process could 

 continue because of side cutting due to meandering of the streams 

 back and forth from one side to the other of the channels. The 

 commonly occurring, deep, broad-bottomed, stream-cut valleys, in 

 the area under discussion, show that many of the streams had 

 reached graded, or nearly graded, condition even by the close of the 

 Tertiary. In the northern Appalachian district, at least, we have 

 evidence to show that after the streams had reached grade there 

 was an appreciable renewed uplift of the land which again revived 

 the activity of the streams. Thus the broad Hudson Valley, with 

 minor hills rising above its surface, was produced when the Hudson 

 was well along toward a graded condition and then, as a result of 

 this late Tertiary uplift of the land, the present narrow and fairly 

 deep inner channel of the Hudson was formed. The Hudson did 

 not reach grade in this inner channel, its work having been inter- 

 rupted by both the subsidence of the land and the spreading of 

 the great ice sheet over the region. 



This inner channel of the Hudson has been traced for fully 100 

 miles eastward beyond the mouth of the present river. The 

 Coast and Geodetic Survey has made a detailed map of the 

 ocean bottom near New York City, and the submerged chan- 

 nel of the Hudson River is clearly shown as a distinct trench 



