THE TERTIARY PERIOD 299 



cut into the continental shelf. Even in the Hudson Valley above 

 New York City, the narrow inner rock channel has a depth of 

 hundreds of feet and is mostly submerged below tide water. 

 Without question, this submerged Hudson channel was cut when 

 the region was dry land, and thus we have positive proof that, 

 late in the Tertiary and possibly extending into the early Quater- 

 nary, the region of southeastern New York was notably higher 

 than it is today. Conservative estimates place the amount of 

 elevation greater then than now at not less than 2000 feet because 

 the end of the Hudson channel is submerged to that extent. 1 The 

 coast was then at what is now the edge of the continental shelf or 

 platform about 100 miles east of the present coast line. That this 

 greater altitude was before the Ice age is proved by the fact that 

 the inner Hudson channel now contains much glacial debris 

 filling. 



By similar reasoning, based upon the drowned valleys of the 

 Maine coast and the lower St. Lawrence, we know that all of the 

 middle Atlantic sea-board region, at least, was notably higher in 

 late Tertiary time than now. 



The Mississippi Valley area also appears to have been notably 

 elevated during late Tertiary time, and hence the major (erosion) 

 relief features of that great area, as we now know them, have been 

 produced by the dissection of that upraised area by streams. 



Mountain Making and Development of Relief in the West. 2 — 

 In North America, as well as other continents, the Tertiary was a 

 period of unusual mountain-making activity. Many of the present 

 great mountain ranges of the earth actually had their birth and 

 principal development during this period, while others were re- 

 juvenated and brought essentially to their present form and 

 altitudes. 



Coast Ranges. — This belt was somewhat affected by deforma- 

 tive movements toward the close of the Jurassic, as we have already 



1 It has been suggested by Chamberlin and Salisbury (Geology, Vol. 1, 

 p. 529) that the very end of the Hudson, and other submerged channels, 

 may have been deepened by tidal scouring and, if so, the figure (2000 feet) 

 generally given may be too high. At any rate the Hudson channel at the 

 Highlands is submerged nearly 800 feet, which certainly implies an altitude 

 of more than 1000 feet greater than now when the river was there actively 

 eroding. 



2 The topographic influence of Tertiary vulcanism in the west will be 

 described under another heading. 



