78 NEW YORK STATE MUSEUM 



mantle of drift and show very infrequent rock outcrops. As the 

 ice movement was to the southwest, the back slopes were, how- 

 ever, largely protected from wear. The cliffs are sometimes 

 around the sides of coves produced by the action of local glaciers 

 high up on the mountain sides and called cirques. At other times; 

 they are independent of such features, but in either case their 

 origin seems referable to the same cause, namely that the glacier 

 did not closely hug the mountain side, but that a gap (bergschrund} 

 existed between the two, in which considerable range of daily 

 temperature with alternate freezing and thawing would occur and 

 produce rapid scaling off of the surface rock, the process being- 

 made much more efficacious by the almost universal jointing of 

 the rocks. These joints have commonly a close approach to» 

 verticality, and the scaling off of the rocks along them gives rise 

 to perpendicular cliffs. 



By no means all the cliffs of the region have this origin however. 

 Many steep-sided hills are found, where it seems strongly probable 

 that the steep face is a fault scarp, though usually it is not pos- 

 sible to demonstrate the truth of this view. 



The outer Adirondack hills on the north are mainly composed 

 of red, orthoclase gneisses of unknown age and origin. These 

 hills usually show typically the low, even north slope and some- 

 what steeper back slope (see pi. 2). These rocks are folded, and 

 the folds pitch to the northward, sometimes only slightly, at others- 

 considerably. With this variation seems to come a variation in 

 the steepness of the north slope of the hills, though ordinarily 

 this is difficult of demonstration (pi. 3). 



The anorthosite hills have usually steeper and somewhat more 

 rugged outlines than have those of gneiss, though this is not ne~ 

 cessarily the case. Undoubtedly the lack of foliation in these rocks- 

 has some influence in the matter. 



Old base levels of erosion. The Adirondack region has been con- 

 tinuously above sea level since Lower Silurian times, an interval; 

 which represents a very considerable part of geologic time. Dur- 

 ing all these ages it has been subjected to unceasing wear, result- 

 ing in slow decrease in hight. The elapsed time has been suffi- 



