90 J. Barrell — Upper Devonian Delta of the 



In southern New York the strata dip gently southeast and 

 outcrop in a broad hilly upland from 1500 to 2000 feet in ele- 

 vation ; the general surface however slopes north, contrary to 

 the slope of the strata, and overlooks the central lowland of 

 .New York, lying at a general level of 400 to 500 feet, devel- 

 oped on the soft Ordovician and Silurian strata. The old 

 upland surface is now trenched by deep transverse valleys 

 leading to the north or south and giving a sinuous outline to 

 the limiting outcrop. In eastern New York the strata are 

 more resistant, dipping gently southwestward and extending 

 northeastward in the Catskill Mountains until the higher peaks 

 reach elevations in the neighborhood of 4000 feet. Close in 

 front lie the Mohawk and Hudson Rivers near the level of 

 the sea. The thickness of the Upper Devonian at its margin 

 is about 2000 feet in western, about 4000 feet in eastern New 

 York. The strata must therefore have extended at one time 

 ranch farther to the north. 



Facing the Appalachians the Upper Devonian as far south 

 as Lat. 41° forms a mountainous tableland of gently flexed 

 strata, the elevations descending from, the heights of the Cats- 

 kills to about 2000 feet. Immediately to the east lie the 

 folded structures of the Appalachian system over which the 

 Upper Devonian strata, here from 4000 to 6000 feet thick, 

 must once have extended, participating in the folded zone. 

 South of Lat. 41° to 41° 15' the folded structures advance 

 westward and involve a greater breadth of the Appalachian 

 geosyncline. The Upper Devonian strata plunge steeply 

 beneath the synclinorium of the anthracite coal basins, the 

 eastern outcrop showing here maximum thicknesses of between 

 7000 and 9000 feet. West of this synclinorium is a broad 

 cross anticlinorinm running north and south between Long. 

 77° and 78°. This upwarp crosses a series of great folds, pro- 

 ducing very zigzag outcrops in the strata pitching away from 

 it. The Upper Devonian is completely removed across this 

 transverse anticlinorium and exposes in its flanking outcrops 

 a thickness decreasing northwestward to 6000 feet. On the 

 west the isolated Broad Top Coal basin shows the Upper 

 Devonian on all sides and offers good opportunities for meas- 

 urement. The last exposure of the full thickness is on the 

 Allegheny front, at the limit of the strongly folded structures, 

 but the upper part outcrops in three gentle anticlines to the 

 west and bore holes show the continuation of the strata. 



In the region of folding the measurements of thickness can 

 be made in a limited distance and with small error but transi- 

 tions at base and top prevent a high degree of accuracy. 

 Some changes of thickness due to folding are also probable. 

 In the other regions the thicknesses are not so easy to obtain. 



