244 J. Barrell — Upper Devonian Delta of the 



An adequate geologic source for this volume of material 

 must be sought. In part the sand and clay may have come 

 from a distance of hundreds of miles, but the gravels and the 

 feldspar sand indicate that much of it was from a nearby 

 mountainous tract whose rocks were in large part of quartzites, 

 gneisses and granites. The questions, therefore, arise, — how 

 far away was the field of great erosion ? From what direction 

 were derived the Upper Devonian sediments? Did they come 

 from New England and enter the geosyncline by way of the 

 Catskill Mountain region, did the} 7 come from some other dis- 

 tant source, or were they poured in from a longitudinal land 

 mass lying along the southeastern side of the geosyncline % 



If attention were confined to those lands now above the sea 

 and to the portion of the Upper Devonian rocks still uneroded, 

 a derivation of the sediment from the northeast would be 

 favored. There is in New England a broad region of meta- 

 morphic rocks and the Upper Devonian sediments still uneroded 

 show further that subaerial conditions began to prevail first in 

 eastern New York, gradually advancing westward and south- 

 westward. A closer examination shows, however, that these 

 facts are not real evidences of a derivation from the northeast. 

 The strike of the New England rocks is southward under the 

 submerged portion of the Coastal Plain. The Coastal Plain 

 deposits rest on a surface which indicates profound erosion 

 previous to Comanche time. The fact that the Coastal Plain 

 in southern New England is diagonal to the structure and hides 

 the older basement to the south indicates the lack of relation 

 to the distribution of lands in Paleozoic time. The structure 

 of that basement is the more significant feature. 



The Catskill shore line advanced westward during the Upper 

 Devonian past the axis of deepest and most rapid subsidence 

 and encroached on the less rapidly subsiding sea bottom 

 beyond. This is true in New York, in Pennsylvania, and in 

 Maryland. If the sediment had been derived from the north- 

 east the advancing shore line would, on the contrary, have been 

 different from what is observed to be the case. The great 

 zone of subsidence in southeastern Pennsylvania would have 

 long remained as sea. The shore line would have trended east 

 and west, advancing south, and a terrestrial phase in Maryland 

 would have been deferred until the marine phase had entirely 

 disappeared to the north. On the contrary, the Pawpaw- 

 Hancock quadrangles show that the Catskill conditions in 

 Maryland appeared in full force on the east, where lay the 

 zone of greatest subsidence, before they encroached westward. 

 The Catskill beds are also much thicker to the east, being 

 given as 3800 feet in the eastern part of the Hancock quad- 

 rangle and thinning to 2000 feet within a distance of 15 to 20 



