18 NEW YORK STATE MUSEUM 



clumps of till or rudely assorted, coarse debris. I suspect also 

 that west of Irona, toward the western margin of the quadrangle, 

 what I have mapped as ice-laid drift may in reality be in some 

 part torrent-made debris; nevertheless, I was not able to come 

 to such a decision regarding it at the time of going over it. 



In the accompanying sketch map [pi. 5] I have attempted to 

 show the fullest extension of the flat rock areas from Altona to 

 those on the northwest. 



Altona Flat Rock. This is the largest of these spillways in the 

 district. It is at least 5 miles in length and varies from 1 to 2 

 miles in width. A number of its features are worthy of note. 



Cold brook, one of the head streams of the Little Chazy river, 

 flows southeastward along the strike of the Potsdam (Sara- 

 togan) sandstone in a depression about 100 feet in depth. 



The southern and higher margin of the stripped area is 

 roughly determined by the 900 foot contour line. Its lower 

 margin on the north and east is bounded by a beachlike deposit 

 of waterworn cobbles at about 680 feet. For about a mile along 

 this border the bare rock descends to a lower level (630 feet). 

 On the southeastern extremity of this margin the bare ledges of 

 "Pine ridge" are extended southward by a bouldery moraine which 

 I have- called Cobblestone hill. The wave-washed slopes of a por- 

 tion of this moraine are evidently later than the moraine, which 

 must have been constructed when the ice sheet lay against the 

 northern and lower margin of the stripped belt. 



The surface of the flat rock bears a few isolated patches of 

 what appears to be till, remnants of the deposit which was laid 

 on by the ice sheet during its occupation of the belt. Two of 

 these patches of conspicuous extent are shown on the map. 

 The boulders in these deposits are largely of the local rock, and 

 many are well rounded, and it is possible that the deposits, as 

 Dr Gilbert lias suggested to me, are of torrent origin even where 

 the rocks are decidedly angular. Loose, frost-riven blocks of the 

 sandstone and the pebbles and sands derived from the secular 

 weathering of the Potsdam are the sole surficial rock debris over 

 inosi of the flat rock area [pi. 6]. 



There are several noticeable rock cuts which indicate the 

 action of powerful streams capable of removing blocks of the 



