National Geographic : 1893 Jul 10
The Rocks of Blue Mountain. valley. On the western or Blue Ridge side it is built up of sedi mentary rocks originally deposited in the sea on the bottom and, it may be, the side of the Appalachian trough. In the interven ing valley it consists to a considerable extent of eruptive rocks, which poured out as flows the ancient land surface prior to the existence of the Appalachian trough and before the deposition of the stratified rocks which so largely form the North American continent within the limits of the United States. The elevated eastern side forms the Catoctin ridge, which is capped by a com pressed fold of the old shales and quartzites. Both ridges con tinue south of the Maryland line toward Harpers Ferry and far into Virginia as compressed synclinal folds of the Cambrian rocks, resting on the rocks of the ancient Appalachian trough, the older rocks and the more recent rocks having been involved in the same series of folding. In addition to this folding, numer ous thrusts of one mass of rocks upon another are to be found all along the Blue ridge, especially north of the Pennsylvania Maryland line, in the northern extension of Blue mountain, or the South mountain of Pennsylvania. In some instances the ancient eruptive rocks have been thrust westward, so as to rest upon and above the more recent sandstones and shales which were originally deposited upon them in the bottom and along the shore of the Appalachian trough. Often the pressure has cleaved the massive lavas and formed slates and shales that appear like those deposited in quiet waters. The result of this has been to complicate the geologic structure and topography of South mountain and the Blue ridge, and to make the region one of great interest to both professional and amateur geolo gists. Erosion has aided their study by cutting away thousands of feet of strata from above the present mountain area and adja cent valleys, and thus laying bare a portion of the ancient shore line of the Atlantic coast area of Cambrian time and of the foundation upon which much of the present continent is built. The history of the Blue ridge and its rocks as now interpreted is essentially as follows:* It began long after the first known primitive rocks of the earth were raised into plateaus and ridges to form the platforms of the present continents. At the close of the periods in which the earlier crystalline rocks of the conti nent were formed, and also the great masses of bedded rocks beneath those containing the Cambrian or oldest known fauna, *See Am. Journ. Sci., vol. xliv, 1892, pp. -.
1894 Jan 31
1893 May 05