240 NEW YORK STATE MUSEUM 



hall" a leiilury by iiuiiierous observers wliether amateurs or official 

 geologic surveyors has failed to bring to light postglacial marine 

 fossils above the level of the sea at the mouth of the Hudson. 



The comparison of the extreme ends of the gi^eat Hudson-Cham- 

 plain depression sjjeaks eloquently of marine submergence on the 

 north during a time when the region on the south about the mouth 

 of the Hudson at least was as it now is above the level of the sea. 



SUMMARY OF GEOLOGIC HISTORY BEGINNING WITH THE RETREAT OF THE 



WISCONSIN ICE SHEET 



Fi'oiii the foregoing iiiore or less detailed but as yet incomplete 

 account of the successive frontal moraines in the Hudson and 

 Champlain valleys, it follows that the ice front after receding 

 from the moraine at New York Narrows became more and more 

 irregular in outline, more and more reduced to a long loop pro- 

 jecting southward in the Hudson valley and receding northward 

 over the highlands which formed a wall on either side of it. When 

 the ice had so far dwindled away as not to be able to surmount 

 the Archean ridge of the Highlands, it still pushed southward 

 through the Hudson canyon in this elevated district a narrow 

 tongue of ice which has left its marginal deposits of stratified 

 gravel, sand and clay, at Croton Point, North Haverstraw, about 

 Peekskill, and in the vicinity of West Point. During this stage 

 of the waning Wisconsin epoch, the land from the Highlands 

 southward through the lower Hudson valley appears to have been 

 occupied by standing water about the margin of the receding ice. 

 The level of this body of water is now marked by proglacial deltas 

 which rise to the north at the rate of about 2.6 feet a mile;^ an 

 inclination very close to that found by Kiimmel for the shore lines 

 of Lake Passaic in New Jersey. 



When the ice disappeared from the Wallkill valley about the 

 northern slopes of the Highlands, it formed a long tongue from 

 Newburg northward covering the greater j)art of the width of the 

 floor of the Hudson valley. About its margins were accumulated 

 stratified gravels and sands now in the form of terraces, with ket- 

 tles and ice-block holes, extendi ni> on its eastern ninr":in north- 



'r> 



'This estimate Is obtained bj' taking the distance, 34.5 miles, from tlio 

 College Point delta with an elevation of 30 feet, to the teiTaee used for a 

 state militarj' oamp at Peelvslvill w itli mii elevation of 120 feet. 



