GEOLOGY OF MOUNT MARCY 65 



The major fault-line valleys furnished passes for ice tongues to 

 push through. The slopes were smoothed and carved into U-shaped 

 defiles. Such phenomena are observed in the Cascade fault-line 

 valley, and the valley holding the Lower Ausable lake, which were 

 blocked by crescent-shaped moraines deposited upon the retreat of 

 the ice lobes. 



The occurrence of glacial boulders is quite common, some of which 

 appear to have been transported from great distances, while others 

 can be traced to parent ledges in the neighborhood. Rounded 

 boulders of " Potsdam " quartzite have been noted all over the quad- 

 rangle. Large irregular slabs of Potsdam sandstone and quartzite 

 are encountered in some of the brook valleys where the drift is 

 abnormally thick. In the brook valley where the Weston mines were 

 located irregular nonglaciated flagstones were found in such numbers 

 as to strongly suggest that a ledge of the Potsdam existed there 

 before the ice invasion broke it up. Similar occurrences in the Eliza- 

 bethtown ^ and Lake Placid " quadrangles together with outliers ^ 

 point to the conclusion that the Adirondacks were more or less com- 

 pletely mantled by the Potsdam. Dr D. W. Johnson suggested to 

 the writer that the sawtooth shape of the Niagara mountain block 

 fault in the southeast corner of the quadrangle may have been pre- 

 served by the deposit in it of a ledge of Potsdam sandstone that was 

 subsequently eroded and destroyed by the ice. Doctor Kemp reports 

 that no remnant was found. 



Constructional Work 



There is little true morainal material,* for most of it has been 

 modified by water ; ^ the movement of the ice during the maximum 

 advance having, evidently, been too vigorous for deposition and the 

 material that was deposited as the ice retreated having been sorted 

 by the waters of the glacial lakes. 



The recessional moraines appear to be largely confined to the fault- 

 line valleys, being formed by the ice tongues as they withdrew from 

 the narrow defiles. At the southwestern ends, in the broad val- 

 leys, the rate of retreat was slow and moraines were formed; but 



1 Ruedemann, R., N. Y. State Mus. Bui. 138, p. 62. 



2 Ailing, H. L., N. Y. State Mus. Bui. in press; Bui. Geol. Soc. Am., 

 27 :650. 1916. 



3 Miller, W. J., N. Y. State Mus. Bui. 182, p. 44. 



4 Gushing, H. P., N. Y. State Mus. Bui. 115, p. 495. 

 ^ Ogilvie, I. H., Jour. Geo., 10 :4c6. 



