191^.] STEVENSON— THE FORMATION OF COAL BEDS. 513 



The long serpentine course of the Brush Creek limestone from Mary- 

 land to central Ohio shows that, while there had been no great 

 topographical change and the whole area was still lowland, there 

 had been enough change during deposit of the underlying sandstone 

 to make possible, by slight submergence, a broad, continuous drowned 

 valley across nearly the whole bituminous region, — from very near 

 the eastern outcrop westward to the line of the pre-Beaver valley. 

 This depression did not affect the southwestern part of the region 

 as there is nothing there answering to the Brush Creek ; the invasion 

 was from the east. But during the next interval, the earlier con- 

 ditions were restored and one finds the Cambridge limestone follow- 

 ing approximately the course of the Vanport in Pennsylvania, Ohio 

 and Kentucky, though reaching farther eastward in the latter states. 

 But at about the same time, there was a new invasion on the eastern 

 side along the line of Brush Creek estuary, for a marine limestone 

 is present in western Maryland while farther west near the Monon- 

 gahela River in West Virginia, there is a non-fossiliferous limestone, 

 which may represent the shore phase of the same deposit. 



The Cambridge limestone marks the temporary culmination of 

 a long continued subsidence which brought a constantly widening 

 area on the west side to sea-level and eventually below it. That 

 area continued to widen eastward and reached its maximum when 

 the Ames limestone was deposited. Peneplanation of the bituminous 

 region had become far advanced ; for a long period only fine material 

 had been brought down by the streams and a very great part of 

 the region had become converted into mud flats through which the 

 streams meandered in shifting channels, sorting the fine clays and 

 sands. The rivers must have emptied into estuaries around the 

 border for marine conditions existed as far north as the southern 

 line of Pennsylvania just prior to the Ames. Limestone deposition 

 began first at the west, the thickness and purity of the rock being 

 best marked in Ohio, and advanced east and northeast until the bed 

 covered a great part of the bituminous region. The efflux of the 

 cleaner water was abrupt, but the limestone is followed by fine 

 deposits at most localities and it was not until 70 feet of shales had 

 been laid down in much of the region that the normal conditions 

 were restored. It is possible that the 70 feet of muds, in which 



