of the Monongahela, so that every feature of the topography tends to confirm the conclusion that the age of the river below 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 mapping is left for other and more skillful workers in this promising 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 Monongahela 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 wonderful 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 glacial 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 Connoquenessing in a curious manner, by meeting it direct, and the combined stream turns off 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 cutting 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 ancient Monongahela near Harbor Bridge. All this is fully attested bv 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
