56 NEW YORK STATE MUSEUM 



of glaciers, as is well known, has a certain amount of plasticity and 

 will flow under the pressure of its own weight, somewhat after the 

 manner of a mass of pitch. The flow of the great Laurentian glacier 

 was outward in all directions from the center of accumulation, local 

 topographic features exerting a deflecting influence only in the more 

 attenuated marginal portions. In its basal portions, the ice was well 

 supplied with rock debris, from the finest rock flour and clay to 

 boulders often of very great size. This material was derived from 

 the surface over which the ice flowed, and it measured in part the 

 amount of erosive work which the ice had accomplished. The rock 

 fragments frozen into the bottom of the moving ice mass, served as 

 efficient tools for grooving and scratching the bedrock over which 

 the ice flowed, while at the same time the finer material smoothed 

 and polished the rock surfaces. The direction of the grooves and 

 striae on the rock surfaces in general indicate the direction of the 

 movement of the ice which produced them, but this may not alwa3''s 

 represent the direction of general ice movement for the region, 

 since, at the time of making the striae^ the ice ma}^ have been thin 

 enough to be influenced by the local topographic features of the 

 region. In the Niagara district the striae have a direction extending 

 about 30° west of south (Gilbert) which direction, being inhar- 

 monious with the trend of the lowlands, indicates that these striae 

 were formed by the general movement of the ice, rather than by 

 local movements, controlled by topography.^ 



While the surface rocks of this region were everywhere scratched 

 and polished by the ice, these markings are only exhibited where 

 the protecting mantle of loose surface material or drift has been 

 recently removed. For where the polished rock surfaces are ex- 

 posed for any considerable period of time, weathering usually 

 obliterates these superficial markings. The best place in which the 

 striae of the region about Niagara river may be studied is near the 

 quarries on the edge of the escarpment, a mile or more west of 

 Brock's monument, where the ledges are progressively uncovered 

 previous to quarrying. 



^ For an account of the glacial sculpture in this region, see Gilbert. Bui. 

 geol. soc. Am. 1899. 10:121. 



