50 NEW YORK STATE MUSEUM 



it is quite the rule to find ledges of relatively fresh rock, such ledges 

 frequently being in sharp contact with overlying loose glacial debris. 

 Deeply weathered to even rotten rock is, however, not of veiy- rare 

 occurrence, and the writer beheves that such decomposed rock is 

 appreciably more common in this central Adirondack region than, 

 for example, in the southeastern portion. This is quite in harmony 

 with the generally accepted beUef that the central Adirondacks 

 were neither so long ice occupied nor subjected to such vigorous ice 

 currents as were the border portions of the mountains. 



Rock basins due to ice erosion are not known within the map 

 limits though certain valleys, like those of Long Lake and Blue 

 ^fountain lake, may have been deepened by ice erosion, the available 

 data not being sufficient to make any certain decision. 



Glacial Deposits 



Erratics. Erratics (glacial boulders) are numerous and widelj- 

 scattered over the quadrangle. As usual they are mostly of ver}- 

 local origin so that, in the absence of sufficient outcrops from cer- 

 tain areas, some idea of the imderhing rock formations may be 

 gained by noting the relative numbers of glacial boulders. 



Among the erratics which have been derived from ledges wholly 

 without the area under consideration are those of anorthosite and 

 of Potsdam sandstone. Anorthosite boulders, w4th maximum 

 diameters of 8 or lo feet, are not very common, they ha\-ing been 

 most frequently noted in the northern portion of the quadrangle. 

 In ^-iew of the fact that the extensive anorthosite body is exposed in 

 great force on the quadrangles inmiediatek' to the north and north- 

 east, with the nearest exposures only 5 to 7 miles distant, it is some- 

 what surprising that fragments of this formation are not more 

 common over the Blue ^lountain quadrangle. The combination of 

 shorter time of ice occupancy and less ^-igorous ice currents over 

 the- central Adirondacks than around its borders probably accounts 

 for the relative scarcity- of anorthosite boulders Avithin the Blue 

 Mountain quadrangle. 



Pebbles and small boulders of Potsdam sandstone occasionally 

 noted, even on moimtain tops at altitudes of from 2500 to 3000 feet, 

 are of special interest because of the long distance they have been 

 transported, the nearest known outcrops being some 50 to 55 miles 

 away in the St Lawrence and northern Champlain valleys. These 

 erratics are alwavs well worn and hard. 



