248 «oS. J. Stevenson—Surface Geology of Pennsylvania. 
them by the draining away of a great lake or by the action of a 
great flood sweeping over the whole region. They can be no 
other than sea-beaches, marking stages in the withdrawal of the 
ocean. This supposition involves a submergence of the land to 
a depth of fully 2,600 feet, if we regard the higher benches as due 
the same cause with the lower ones, and the submergence 
would have to be somewhat greater to account for the even 
crests of the Alleghanies and other ridges of the Appalachian 
region west from the Blue Ridge 
The River Terraces. 
The persistent terraces are five in number and their relations 
are shown at the junction of Cheat and Monongahela Rivers. 
Three or four miles north from the West Virginia line, they are 
as follows :— 
Above river. Above river. 
1. 280 feet. 4, 80 feet. 
210" = SL 
a 180. 
The absolute elevation of the highest terrace at this locality is 
1,050 feet above tide. 
ese fall down stream and are covered by detritus, consist- 
ing of irregularly bedded sand, clay or gravel, in which are 
po 
streams, being divided by the channel-way just as the present 
“bottom” is divided. In some instances a terrace is wanting 
on one side; but there it is clear enough that that corrasion 
was confined to one side, for the terrace is unusually wide on 
the other. The same condition is often seen in the Hood plain 
of the river now. 
in my report for 1875, these terraces are simply 
n the rock on which rests a thin coat of detritus. Mr. 
Gilbert, in his memoir on the geology of the Henry 
Mountains (not yet published), describes similar terraces as 0C- 
curring there, though it does not appear that they are found at 
the same height on both sides of the streams. 
The terraces on the Ohio, below Pittsburg, consist largely of 
northern drift brought down by the Allegheny and Beaver 
Rivers, so that they certainly date from a time later than that 
shelves i 
G. 
