Terrace Deposits of the JIo)io)i(/ahel(i Hiver. — White. 375 
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 depos- 
its, 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 Alle- 
gheny, 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 
furtiier down the Ohio river comes at the same elevation, 
while the summit of the same deposit there, as w^ell as in Pitts- 
burg and Alleghen}^ 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-fllled 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 north- 
westward to the lake Erie basin has not yet been clearly de- 
lineated, but Mr, Leverett's studies leave little doubt that from 
Sharon the course was northwestward through the low drift- 
fllled divide at Warren, O., (now only 900 feet A. T., and its 
rock floor probabl}' 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 Lit- 
tle Kanawha gateways, cannot be estimated except by the de- 
posits 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 
