do NEW YORK STATE MUSEUM 



CORRELATION OF THE ONTARIAN AND HUDSONIAN 

 ICE LOBES 



Maps of the waning ice sheet. Plates 9-17 

 The criteria or data for determination of the positions of the ice 

 fronts and the construction of the maps are found in the following 

 facts : 



1 Deltas or sand plains indicate approximate heights of lake sur- 

 faces and prove that open water existed at the locality when the 

 deposits were laid, the ice being removed. 



2 Stream channels indicate approximate heights of the outflowing 

 lake, with some allowances for depth of the rivers, and imply lower 

 levels for the receiving waters. Streams across cols must be lake 

 outlets. 



3 A series of stream-cut channels on a land slope imply a lower- 

 ing barrier, the receding ice front holding the streams to their work. 



4 A channel in the bottom of a col or valley with no stream work 

 on the wall of the valley indicates that standing or open water was 

 lowered on the valley until flow was established acrdss the divide. 



5 The border of the glacier must have had general uniformity of 

 height, resting like a lake or river against the land, but with some 

 rise toward the source of supply and declining downstream. 



6 In comparison of features relating to lake planes some deforma- 

 tion of the land must be recognized. 



Contemporaneous positions of the two ice fronts, far separated in 

 the Ontario and Hudson valleys, have been quite definitely deter- 

 mined in a few localities by means of criteria noted above. In the 

 Ontario basin near the head of the Mohawk valley we find an 

 abundance of glacio-lacustrine phenomena which must have altitude 

 relation to the eastward escape of the same lake waters; and it .is 

 found that such eastward outflow was controlled by the glacier in 

 the Hudson valley. When the outlet channels are along a steep 

 slope, like those on the face of the Helderberg scarp or on the hills 

 west of Schenectady, we may be sure of the precise position of the 

 ice front there at the time when the imprisoned lake had correspond- 

 ing altitude. At the Ontario end of the lake the positions of the ice 

 front can be determined approximately by means of inflow channels 

 and deltas, in their relation to the lake surface. For example, the 

 deltas at Trenton and Trenton Falls indicate subsiding lake waters, 



