252 The American Geologist. October, 1902. 
The striae so often referred to and so frequently found 
differ from grooves mainly in magnitude, though the abrading 
agents in this case are in general of finer character. The hair- 
like striae which are often sharply defined upon rocks, were 
probably produced by sand and silt acting not immediately be- 
neath the glacier ice, but under the larger boulders moved along 
by it. Striae often present rough outlines, and vary in direc- 
tion on the same surface, the latter being due either to a com- 
bination of movements of the abrasive materials during a given 
period, or to successive periods of glaciation. On the Avhole. 
however, taken in connection with the grooves they are a deter- 
minative factor in showing the direction of glacier flow, the 
markings generally being less prominent on the lee side of 
rock prominences than on the stoss side. This latter feature is 
amplified to a greater extent, however, in areas of less severe 
glaciation than in those where the erosive action has been pov/- 
erful, for in the latter case the striating material has been 
crowded about obstacles, to a greater extent with consequent 
multidirectional scoring. 
That the above is true is obvious on the bacis of the theory 
of motion for Alpine glaciers previously considered. 
Where the glacier was of only average depth or less 
its relative rigidity was greatly increased because of 
the ice being largely in the zone of fracture, and con- 
sequently the plasticity of the mass relatively small, ad- 
justment to underlying topographic conditions was but iiuper- 
fectly made, with the result that scoring took place almost en- 
tirelv in the general direction of the ice flow, and at the same 
time scarcely aiTected the lee side of rock prominences. On the 
other hand where the ice was of great depth the enormous 
weight caused the ice in the zone of plasticity to mold itself to 
everv inequality of the ice floor, to fill all cracks and fissures, 
and to adjust itself to all topographic irregularities, and in so 
doing to carry with it the abrasive material in any or all direc- 
tions irrespective of the general trend of the main ice movement. 
In a paper published in 1888 Chamberlin states that there 
had been recorded about 2500 observations on drift striation in 
America, the same being made by 104 observers. * Of this 
*For a very comprehensive treatment of this matter, see "The Rock- 
Scorings of the Great Ice Invasions,'' bv T. C Chamberlin, 7th Ann. 
Report of the Director of U. S. Geol. Survey, 1888. 
