642 CHARLES EMERSON FEET 



(the lake hypothesis) the land was not only relatively lower at the 

 north than now, but at the south, from the beginning of the ice- 

 retreat or soon after, it was higher than now. Its final altitude, 

 however, before the recent submergence may not have been its alti- 

 tude when the ice began its retreat. If the land at the south had its 

 full altitude when the retreat of the ice began, then depression only 

 is necessary here since then. If the full altitude was attained only 

 after the ice had retreated some distance, the movement at the south 

 was first one of uplift and finally one of depression. During the 

 higher altitude, the channels, which are now under the waters of the 

 Hudson estuary, were eroded and subsequent depression of the land 

 has submerged them. Under the estuary or salt-water hypothesis the 

 land was lower than now both north and south, when the gravel 

 plateaus alid other standing-water features were produced, and was 

 subsequently uplifted; erosion produced the channels, and subse- 

 quent depression submerged them. Under either hypothesis, then, 

 the retreat of the ice was followed by a time of higher altitude of land 

 than now, and was succeeded by one of depression. The chief differ- 

 ence in the hypotheses is in the original altitude of the land and the 

 time of uplift. According to the salt-water or cstuarine hypothesis, 

 the time of uphft was on the inauguration of Higher Glacial Lake 

 Champlain, although the uphft may have been in progress in the 

 southern Hudson before the emergence of the barrier in the northern 

 Hudson that produced Higher Glacial Lake Champlain. The 

 amount of this uphft before the disappearance of the Hudson water 

 body is limited, however, by the levels which would give the sea 

 access to the northern Hudson, if the water body was an arm of the 

 sea. According to the lake hypothesis, the land was higher than now 

 when the ice began its retreat, or soon after, and had either attained 

 its full height then or did so during the retreat of the ice. According 

 to either hypothesis, the full altitude of the southern Hudson had been 

 attained before the close of Higher Glacial Lake Champlain history, 

 and probably southern depression had begun. 



3. The gaps in the moraine. — There are two gaps in the moraine 

 between Brooklyn and Perth Amboy, the Narrows gap and the Arthur 

 Kill gap (p. 426). The gap occupied by Arthur Kill has slopes which 

 indicate that it may have been cut down from an elevation of from 25 



