378 The American Geologist. December, i896 
of the Monongahela, so that every feature of the topography 
tends to confirm the conclusion that the age of the river be- 
low the old base-leveled plain shown in the table, has been of 
comparative short duration, and must be measured by only a 
few thousand years at most. 
The writer had intended to present a map with this paper, 
showing in a crude way the pre-glacial drainage of the areas 
herein described, but so much detailed work must yet be done 
before any approach to accuracy can be hoped for that the map- 
ping is left for other and more skillful workers in this prom- 
ising field. In this connection I would call the attention of 
students of the subject to the former northward course of the 
Little Beaver and the Slippery Rock. 
The former once went northward along with the Mononga- 
hela drainage, but being dammed up by the northern ice it 
cut a new channel about fifteen miles long, southward into 
the Ohio river drainage, and this fully accounts for the won- 
derful change in topography along the lower portion of its 
course, as well as for the southward transportation of large 
granite boulders several miles beyond the limits of true gla- 
cial movement, thus giving rise to the phenomena called the 
"fringe"' by Lewis and Wright; for wherever a stream was 
thus impounded the blocks of ice floating southward across 
these temporary lakes from the terminus of the glacier would 
of course bear away and, melting, scatter over the surface 
many masses of imbedded rock. 
The Slippery Rock now joins the Connoqiienessing in a cu- 
rious manner, by meeting it direct, and the combined stream 
turns otr at right angles to enter the Beaver at Rock Point. 
In pre-glacial time the Slippery Rock left its present channel 
a short distance above Kennedy's upper mill, and following 
the present valley of Big run, turned northwestward near New 
Castle, and evitting squarely across the present gorge of the 
Neshannock (which is also of post-glacial origin) passed two 
and one-half miles north of New Castle and entered the an- 
cient Monongahela near Harbor Bridge. All this is fully at- 
tested by the wide drift buried valley, which can be followed 
from Slippery Rock clear through to the Shenango, and the 
course of this ancient stream also confirms the conclusion of 
Leverett that the pre-glacial drainage took the Shenango 
