The Potomac Formation in Virginia (Volume 145)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1896. Excerpt: ... adjoining pre-Cambrian rocks the waterworn stones are not associated with clay. Westward of the area, in the vicinity of Hanover Junction, and westward of the Petersburg area, deep-red clay is commonly found with the stones. The stones over all the Potomac areas are well rounded. As we pass from the Potomac formation westward into the pre-Cambrian terrane, they gradually give way to the more or less angular drift that comes from the degradation of the pre-Cambrian. These rounded stones evidently came originally from the Potomac beds, for they abound in the vicinity of outcrops of that formation and diminish as we depart from its exposures. It is a remarkable fact that cobblestones of Cambrian quartzite are common in the gravel that lies upon the pre-Cambrian along the western margin of the Potomac on the North Anna River. We might explain the existence of such material in the vicinity of the James and the Potomac by assuming that it was brought down by these streams, for west of the Blue Ridge they cross the Cambrian formation; but this is not the case with the North Anna. The above account gives approximately the location and limits of the several detached Potomac areas. THE GEOLOGIC CHARACTER OF THE POTOMAC FORMATION AT DIFFERENT LOCALITIES. We may now turn our attention to the geologic character of the formation. It has been found to be impossible to give a general account of the geology of this group of beds. The materials that form the Potomac change so rapidly that representative sections do not exist. In any given exposure, if vertical sections should be made at intervals of, say, 50 feet, they would be as a rule widely different. The changes, it is true, do not have large range, but they occur suddenly and without any evident law. It is therefore ...