196 MAN AND THE GLACIAL PERIOD. 



nel has sunk ninety feet below the general depth of the 

 water on the bank, and continues at this depth for twenty 

 miles farther. This narrow channel continues with more 

 or less variation for a distance of seventy-five miles, where 

 it suddenly enlarges to a width of three miles and to a 

 depth of 200 fathoms, or 1,200 feet, and extends for a dis- 

 tance of twenty-five miles, reaching near that point a 

 depth of 474 fathoms, or 2,844 feet. According to Mr. 

 Lindenkohl, this ravine maintains for half its length " a 

 vertical depth of more than 2,000 feet, measuring from 

 the top of its banks, and the banks have a nearly uni- 

 form slope of about 14°. The mouth of the ravine opens 

 out into the deep basin of the central Atlantic. 



With little question there is brought to light in these 

 remarkable investigations a channel- eroded by the exten- 

 sion of the Hudson River, into the bordering shelf of the 

 Atlantic basin at a time when the elevation of the conti- 

 nent was much greater than now. This is shown to have 

 occurred in late Tertiary or post-Tertiary times by the 

 fact that the strata through which it is worn are the con- 

 tinuation of the Tertiary deposits of New Jersey. The 

 subsidence to its present level has probably been gradual, 

 and, according to Professor Cook, is still continuing at 

 the rate of two feet a century. 



Similar submarine channels are found extending out 

 from the present shore-line to the margin of the narrow 

 shelf bordering the deep water of the central Atlantic 

 running from the mouth of the St. Lawrence Eiver, 

 through St. Lawrence Bay, and through Delaware and 

 Chesapeake Bays.* All these submerged fiords on the 

 Atlantic coast were probably formed during a continental 

 elevation which commenced late in the Tertiary period, 

 and reached the amount of from 2,000 to 3,000 feet in 

 the northern part of the continent. 



* See Lindenkohl in American Journal of Science, for June, 1891. 



