GEOLOGY OF THE BLUE MOUNTAIN QUADRANGLE 49 



with an average direction of about south 30° to 40° west. All but 

 three of these sets of striae were found along roads, a reason for 

 this being that tmweathered, glaciated rock surfaces are there fre- 

 quently stripped of drift artificially, thus exposing the glacial marks. 

 •This is quite the rule in the Adirondack region. 



No striae were noted in the southwestern portion of the quad- 

 rangle, nor on the great mountain mass between Mount Sabattis 

 and Dun Brook mountain. The eighteen observed striae are, how- 

 ever, so distributed over the quadrangle as to furnish positive evi- 

 dence that the general ice current was southwestward across this 

 portion of the Adirondacks. This accords essentially with, the 

 observations made by Gushing on the Long Lake quadrangle im- 

 mediately to the north, and by the writer on the Lake Pleasant quad- 

 rangle to the south. 



Certain striae, as, for example, those of the Long Lake basin, 

 closely follow the trend of, and may have been influenced by, the 

 topography, though it is quite possible that such parallelism may be 

 largely a coincidence. Other striae are so situated as to demon- 

 strate independence of direction of ice flow and trend of the topog- 

 raphy. Examples of such are in the depression between the 

 mountains southeast of Forest House; on the south side (several 

 hundred feet below the top) of the northwestern spur of Blue 

 mountain; and well up on the mountain (altitude 2550 feet) in the 

 northeastern part of the quadrangle. 



That the great ice sheet was thick enough to overtop the highest 

 mountains of the region is practically demonstrated by the per- 

 sistence of the southwesterly ice current in spite of the topography. 

 Actual striae 2550 feet above sea level near the top of the mountain 

 in the northeast and distinctly glaciated ledges (without striae) 

 above 3000 feet on the side of Dun Brook mountain conclusively 

 prove ice currents at such altitudes. Well-worn pebbles and small 

 boulders of Potsdam sandstone derived from the St Lawrence valley 

 were occasionally observed on mountain tops at altitudes of from 

 2500 to 3500 feet. Again, the great surfaces of comparatively hard 

 and fresh rock on such mountains as Owl's Head, Mount Sabattis, 

 Blue mountain, etc., were quite certainly stripped of rotten rock by 

 the ice and have since been only slightly modified by Postglacial 

 weathering. 



The great ice sheet, in its passage across the region, was a suffi- 

 ciently active agent of erosion to remove most of the Preglacial 

 soils and rotten rock from their original positions. For this reason, 



