﻿L$2 NEW YORK STATE MUSEUM 



tinguish the ice-heaped blocks from the frost fracture piles; the 

 frost work having, of course, also affected the ice deposits. 



In some places the morainal character of the drift is clearly ex- 

 pressed by the well known features of irregular surface, mound 

 and basin topography, but over most of the area the morainal ele- 

 ment has been discriminated by one or the other of two features ; 

 unusually stony patches or kettles. Very stony fields with heaps of 

 boulders and stone fences, specially if containing considerable per- 

 centage of nonlocal rock, have been diagnosed as moraine. Dis- 

 crimination is needed, for in a district of ledges, scarps or cliffs, 

 specially of the quartzitic Potsdam ? the ground may be strewn with 

 rubbish from the native rocks which is not strictly morainal, or 

 peripheral to the ice sheet, even if glacial. This criterion of stoni- 

 ness is often equivocal and in such cases is usually disregarded. 



In districts of clayey till the occurrence of kettles or inclosed 

 basins in the drift is interpreted as indicating ice margin deposits, 

 and sometimes they may correlate with stony tracts. Over lime- 

 stone floors small sinks may simulate kettles, but over the sand- 

 stone and crystallines this deception can not occur. 



The above description will suggest how difficult if not quite 

 impossible it would be to accurately map the morainal deposits over 

 the entire area, and this is not attempted. The heavier morainic 

 masses are shown in plates 44-47. 



Boulder moraines. Plate 47 shows the larger portion of the 

 Black river moraine, which continues southwest to and beyond 

 Watertown. On this map conventional signs indicate lines and 

 ridges of block moraine. Some of these have high relief and are 

 striking features in the landscape. One photograph is given in plate 

 56. The character of the ridges as bare limestone blocks is partly 

 the result of wave work of the falling waters of Lake Iroquois. 

 The Black river delta built in the lake was banked against the 

 moraine and partly buried its southeastern border. From the trend 

 of these ridges it is apparent that the ice flow constructing them 

 was from the northwest, and that the ice margin was spreading 

 or deploying on the plain. 



The great massing of limestone blocks with very few crystallines 

 could hardly have been effected by the earlier ice movement from 

 the northeast or north, as the limestone formations do not extend 

 far in that direction. The change in direction of flow enabled the 

 ice to sweep up the rubbish left on the limestone tract on the north- 

 west, and perhaps the new direction of impact,_ changing from south- 

 westward to southeastward, gave the ice a more effective grip for 



