Appalachian Geosyncline. 239 



nor the Green Pond conglomerate. On the contrary, quartz- 

 ite formations resembling Cambrian quartzite where these are 

 extensively developed, had become uplifted and subjected to 

 erosion. Such a supply of material could not have existed 

 short of some region now concealed by Coastal Plain deposits 

 or the waters of the Atlantic ocean. 



Gravels as coarse and thick as those of the Skunnemunk 

 formation are commonly developed skirting the larger moun- 

 tain ranges, swept out by rivers and forming piedmont slopes. 

 From the Himalayas they are swept southward in abundance 20 

 to 30 miles. In the basins of Asia from ten to forty miles is a 

 common distance. In the valley of the Po, Bonney has noted 

 that gravels with pebbles commonly 3 to 4 inches in diameter 

 and more, reaching rarely 8 inches, are found after the river and 

 its tributaries have flowed over from 35 to 50 miles of plain.* 

 Gravels in some instances are carried much farther, even 

 several hundred miles, bat in such cases the pebbles are small 

 and constitute a lesser proportion of the entire deposit. The 

 well-worn and rounded character of the Skunnemunk gravel 

 indicates that it had suffered a large reduction in size of peb- 

 bles and had been moved a considerable distance. The mar- 

 gins of the gravel plain may therefore be estimated at perhaps 

 from 20 to 35 miles to the southeast of the Green Pond syn- 

 cline, making the original limits of the upper Devonian deposits 

 from 45 to 60 miles southeast of the present outcrops in Penn- 

 sylvania. Considering all the lines of evidence, especially the 

 distance to an available source for the quartzite and the short- 

 ening of this distance due to later folding, the larger figure 

 seems more probable than the smaller. Sixty miles during 

 the final period of folding may well have been shortened to 

 forty-five miles, so that this has been used as a minimum limit 

 in locating, the original margin in New Jersey and Pennsyl- 

 vania. The distance to the northwestern edge of the crystal- 

 line floor concealed beneath the Coastal Plain is now about 

 sixty-flve miles, and from beyond this line it would appear that 

 the greater portion of the quartzite must have been derived. 



Relations of Piedmont Gravels to Climatic and Crustal 

 Movements. 



Broad piedmont plains, such as those which skirt the east 

 side of the Pocky Mountains and whose dissected surfaces form 

 the High Plains, are in delicate adjustment to the grade of the 

 rivers which built them. A climatic movement away from a 

 state of semi-aridity toward a rainy and humid condition 

 increases the hold of vegetation on the soil, decreasing the 



* Bounding of Alpine Pebbles, Geol. Mag., decade III, vol. v, p. 58, 1888, 



