Page:Origin of the High Terrace Deposits of the Monongahela River.pdf/8

Rh Brownsville, Belle Vernon, Charleroi, Monongahela, Elizabeth, McKeesport, Braddock, Homestead, all have their upper and more level portions situated on the ancient floor of the pre-glacial valley. It is this wide and almost level area of deposits, stretching from Braddock to the Monongahela, across to the Allegheny river via Homestead, and East Liberty, which forms the principal site of Pittsburg itself, as well as Allegheny, beyond. Here a remnant of this ancient valley floor is appropriately named Monument hill, rising, as in does, like an island between the present mouth of the Allegheny river and its filled-up and more ancient channel just north of the island. The bed rock under the terrace deposits of Monument hill is now 190 feet above low water or 890 feet above tide, and the rock floor under the vast terrace deposits at Bellevue, still further down the Ohio river comes at the same elevation, while the summit of the same deposit there, as well as in Pittsburg and Allegheny, has nowhere been observed above 990 feet A. T., or 290 feet above low water in the present rivers.

This old elevated valley floor can be followed down the Ohio river to Rochester, and up the Beaver river past New Brighton, Beaver Falls, Rock Point, Wampum, Mahoningtown, and New Castle, beyond which, up the Shenango, it sinks out of sight under the drift-filled valleys, and at Sharon is submerged by 60 feet of cover, 40 feet below the present water level, or 780 feet A. T.

The exact course of the pre-glacial river from Sharon northwestward to the lake Erie basin has not yet been clearly delineated, but Mr. Leverett's studies leave little doubt that from Sharon the course was northwestward through the low drift- filled divide at Warren, O., (now only 900 feet A. T., and its rock floor probably 200 feet lower), and thence northward along the general course of Grand river.

Just how long lake Monongahela existed, and drained its surplus waters southward through the Middle Island and Little Kanawha gateways, cannot be estimated except by the deposits just described; but finally the barrier along the upper Ohio, (probably at the "narrows" below Moundsville, W. Va., as believed by Profs. Chamberlin and Leverett) gave way, and the level of lake Monongahela was speedily lowered by the rapid cutting away of the soft rocks along the present Ohio