RESUME OF THE GEOLOGY 305 



ous). They include shales, sandstones, and limestones, which show con- 

 siderable variation in composition and appearance. They have suffered 

 more or less metamorphism and in consequence are frequently much 

 changed, both lithologically and structurally. They represent the entire 

 sedimentary column of the Shenandoah Valley and the Alleghany Eidges 

 physiographic provinces in which the dikes are found. The geology of 

 the region is shown on the accompanying map, figure 1. 



The formations recognized in central western Virginia are given in the 

 subjoined table. 7 



Paleozoic : Feet. 



Carboniferous (Mississippian) Pocono sandstone 700 



Devonian Hampshire sandstone 1,800 



Jennings shale and sandstone 3,800 



Romney black shale 1,300 



Monterey sandstone 300 



Lewistown limestone ") 



\ 1,050 



Silurian -. Lewistown shaly limestone J 



Rockvvood sandstone, etc 900 



Ordovician Massanutten sandstone 600 



Martinsburg shale 1 ,400 



Cambro-Ordovician Liberty Hall limestone 1,000 db 



(Shenandoah group. ) Murat limestone 150 



Natural Bridge limestone 3,500 



"Buena Vista" (AVatauga) shale. ... 900 



Sherwood (Shady) limestone 1,800 



Cambrian Lower Cambrian quartzite 800 



Lower Cambrian shale and quartzite. 3,000 



The Appalachian revolution closing the Paleozoic era of sedimentation 

 in western Virginia resulted in widespread folding, accompanied in many 

 places by faulting, chiefly of the thrust type, of the sedimentary forma- 

 tions listed above. This period of deformation, as is shown later in this 

 paper, occurred prior to that of the intrusion of igneous material as dikes. 

 The axes of the folds, as well as the trend of the major faults, extend in 

 a general northeast-southwest direction. 



Folding and faulting of the Paleozoic rocks of this region, together 

 with the long period of subsequent erosion, have resulted in the produc- 

 tion of nearly parallel ridges, which owe their existence to the resistanl 

 character of the rocks. The drainage of the region is well adjusted to 

 the rock structure, as shown by the streams haying established their 



7 Thomas L. Watson: A geologic map of Virginia. Virginia Geological Survey, 10] 



