60 NEW YORK STATE MUSEUM 



Then the great peneplain was upraised and a new period of 

 active erosion was inaugurated to continue to the present day. 

 There is some reason to think that this erosion proceeded far 

 enough to permit the larger rivers to reach an almost graded con- 

 dition after which there was moderate renewed uplift. 



Much of the faulting of the eastern and southern Adirondacks 

 ■ dates from the time of this peneplain uplift or even later, through 

 it is likely that some took place much earlier. 



The major existing relief features of the quadrangle are the 

 direct results of dissection by erosion and faulting of the upraised 

 peneplain. Many mountains of the quadrangle have become more 

 or less isolated as domelike masses as a result of the erosion, in 

 many cases aided by the faulting. Among numerous fine examples 

 are Potash mountain, the Three Sisters (south of Thurman sta- 

 tion), and Jefl;ers mountain. These mountains are literally peeling 

 off by the removal of exfoliation sheets of great size, some having 

 been noted as much as 50 to 75 feet across and from i to 3 feet 

 thick. Plate iiB gives a good idea of this phenomenon as observed 

 on the face of the mountain north of Thurman station. Some- 

 times, especially during the fall and spring months, slabs loosen up 

 and go thundering down the mountain sides. Though the igneous 

 rocks of the mountains are all gneissoid, the exfoliation appears 

 entirely to disregard the direction of that structure and often great 

 sheets come off at right angles to the foliation. 



Immediately preceding and probably during much of the great 

 Ice Age of the Quaternary period, this region, like all of north- 

 eastern North America, was considerably higher than now. 



During the Ice Age the area of the quadrangle, in common with 

 nearly all of New York State, was buried under the vast ice sheet 

 which has left many records, such as striae on the ledges, glacial 

 boulders, moraines and glacial deposits in general. The preglacial 

 topography was not notably affected by ice erosion and deposition. 

 The extinct and existing lakes of the quadrangle were formed 

 either by the actual presence of ice dams or by irregular accumula- 

 tions of glacial deposits. During the retreat of the great glacier 

 from the Adirondacks, long narrow bodies of water occupied the 

 Hudson and Sacandaga valleys of the quadrangle. 



A subsidence of the land several hundred feet below the present 

 level took place during the closing stages of the Ice Age or very 

 soon after when arms of the sea extended through the Champlain 

 and St Lawrence valleys. The area of the Luzerne quadrangle 

 was then of course lower than now. 



