Glacial Erosion. — Tarr. 117 
the first sandstone at Fredericksburg. It is extremely import- 
ant that this should be definitely recognized. In reality he not 
only fails to do this, but he seizes upon an extremely doubtful 
case and affirms that what may be, and probably is, only a 
lenticular bed of sandstone of uncertain dimensions, is the 
disappearing second sandstone. 
These are, as the writer understands them, the three princi- 
pal points taken up by Prof. Broadhead in the paper men- 
tioned : First, the insufficient reasons given by the older 
geologists for separating the sandstones ; second, the appar- 
ent discrepancy of the writer's statements in regard to the 
occurrence of fossils: and third, the occurrence of sandstone 
at the river level at Mt. Sterling. This fact still remains clear 
to the writer's mind, that neither Prof. Broadhead nor the 
older geologists have given sufficient data upon which to base 
their divisions into the First, Second, and Third sandstones, 
and the First, Second, and Third limestones. 
GLACIAL EROSION. 
By Ralph S. Tare, Ithaca, X. Y. 
Mr. Warren Upham, in a valuable paper,* has arrived at 
the conclusion that *' from the volume of the drift and the to- 
pographic features of the country a geologically brief period, 
at the longest perhaps 10,000 or 20,000years, would suffice for 
the observed volume of the Pleistocene glacial erosion and re- 
sulting drift." It has for some time seemed to me that upon 
the basis of the character of glacial erosion and drift accumu- 
lations, one must conclude that some such brief period of time 
ms the above must be assigned to the last glacial period in 
place of the commonly stated great lapse of time. Without 
entering into the consideration of the disputed question of 
the complexity of the glacial period. I shall state the line of 
argument upon which this conclusion i- based. 
Ice cannot destroy the rocks over which it passes without 
the aid of cutting tools. Leaving out of consideration valley 
glaciers, where a supply of debris comes from the bordering 
cliff's, and the rare peaks which may project above the surface 
of a continental ice-sheet, there are but three ways in which a 
*Bull. Geol. Soc. Am., vol. iv, pp. 191-204. 
