64 NEW YORK STATE MUSEUM 



the clays are proof of this. When the margin of the ice sheet 

 receded, on the high ground north of the Adirondacks, to the inter- 

 national boundary, it uncovered a low notch, the Covey pass, which 

 permitted the outflow of the glacial waters of the Ontario-St 

 Lawrence basin. Lake Iroquois; and from this time the St Law- 

 rence waters were tributary to the Champlain valley, instead of, 

 as formerly, to the Mohawk-Hudson. At that time the Covey out- 

 let was 740 feet lower than today, and only 290 feet above the sea. 



With further recession of the ice barrier on the north slope of 

 Covey hill, Lake Iroquois was drained down to sea level. The 

 Hudson-Champlain estuary then passed westward around the 

 Covey salient and occupied the St Lawrence basin. The altitude 

 relations of Lake Iroquois and the sea-level waters are tabulated- 

 in plate 12. 



The postglacial rise of the land was by a wave uplift, progress- 

 ive from south to north, following the removal of the ice burden. 

 The rise was laggard or dilatory, and it appears that no appreciable 

 rise occurred at any point in the Hudson valley while the ice lay 

 on that point. But it seems more than possible that the uplift wave 

 overtook the receding ice front in the Champlain valley; and that 

 a relatively small rise took place at Covey hill while the ice held 

 Lake Iroquois to the Covey outlet. This would account for some 

 shore features in the districts of Crown Point, Port Henry and 

 Peru somewhat above the profile shown in plate 10. In such case 

 the bars at Covey hill, 740 feet, do not give the full amount of 

 glacial depression and postglacial uplift. The maps and profiles 

 do show the positive minimum ot recent diastrophism. 



It is recognized that there may be complications in the Pleistocene 

 history, and that the present altitude of the upraised water plane 

 is only the final or average result of any up and down land move- 

 ments, and any variations of the sea level ; the arithmetical sum of 

 all the plus and minus elements. But it appears that such com- 

 plicating factors would probably increase the record of land depres- 

 sion and subsequent rise, instead of reducing it. To the degree 

 that the ocean level was lowered by removal of water to build the 

 ice caps, the ancient water plane was so much beneath present 

 ocean level. The present height of the summit plane above the sea 

 must be only the excess of land uplift over any rise oi the ocean 

 surface. 



The gravitational pull on the sea level by the ice body at the 

 attenuated margin of the valley lobe is thought to have been a 

 negligible factor in the New York district ; and probably so at all 



