erosion of deep rock , valleys 513 



Detailed Descriptions 

 erosion of deep rock valleys 



Description. — Borings made near the coast in all the large valleys en- 

 tering the Atlantic between New York and central Maine have shown 

 that the bed-rock floor underlying the drift is very irregular, and that 

 buried valleys extend inland often for many miles, and some of them bear 

 little relation to the direction and distribution of the present streams. 



The buried gorge of Hudson river at New York is known to be at least 

 300 feet deep below the present sealevel,* and its course has been traced 

 over 100 miles out to sea in the form of a deep canyon-like gorge several 

 thousand feet deep.f Eecent borings by the New York commission on 

 additional water supply in the Hudson river near West Point are reported 

 by the engineers to show that the valley floor at that place is 450 feet or 

 more deep below tide. 



Numerous wells and borings sunk at Boston have shown that the bed- 

 rock floor of the ancient rivers at that city lies 200 to 400 feet in maxi- 

 mum depth below the present sealevel. J Wells of the Amesbury water 

 works, situated on the banks of Merrimac river 6 miles from its mouth, 

 strike rock at 100 feet IdcIow sealevel, and the wells of the Salisbury Beach 

 water works go nearly to that depth without striking rock. Across Fore 

 river, between Portland and South Portland, Maine, a line of borings has 

 been made which shows a bed-rock valley 110 feet in maximum depth 

 below tide. Test borings made for the Brunswick-Topsham water district 

 on the banks of Androscoggin river at Brunswick struck rock at 155 feet. 



Age of erosion. — It is commonly supposed that these old gorges were 

 eroded during the continental elevation immediately preceding the first 

 recorded ice invasion, but in most of them there is no known reason why 

 the erosion may not date from the Aftonian or even the Yarmouth inter- 

 glacial stage. According to Fuller, the evidence on Long island indicates 

 that the main erosion of the Hudson Eiver channel was subsequent 

 to the deposition of the Manetto gravels (page 516), and in all proba- 

 bility antedated, in part at least, the deposition of the Kansan materials 

 there ; hence the principal work of erosion was presumably Aftonian. In 

 all the valleys the erosion far antedates the Montauk or principal stage of 



* W. H. Hobbs : The configuration of the rock floor of Greater New York. BuU. no. 

 270, U. S. Geological Survey, 1905, p. 41. 



t J. W. Spencer : Submarine great canyon of the Hudson river. American Geologist, 

 vol. xxxiv, 1904, pp. 292-29,3. 



t P. G. Clapp : Geological history of the Charles river. Technology Quarterly, vol. 

 xlv, 1901, pp. 171-201 ; American Geologist, vol. xxix, 1902, pp. 218-233. 



W. O. Crosby: Report of the committee on the Charles River dam, Boston, 1903, pp. 

 345-369, and earlier papers. 



