GEococr. The Appalachian Mountains are folded mountains; that is, they have been formed by plications or. folds of the rock layers that make up the crust of the earth in this region, and the particular type of plication is so well developed in this region that it has received the name of the "Appalachian type" of folding. The Blue Ridge, along the eastern side, con sists of layers of crystalline rocks, the oldest known in the Appalachians, that have suffered so great an amount of metamorphism as to render the determination of their exact age a matter of considerable difficulty. They are grouped under the term "fundamental complex," and it is certain that they are in large part pre Cambrian; and sonic are even Archaean on the eastern edge of the Blue Ridge. On the western edge isolated masses of Cambrian rocks are found. All these rocks of the Blue Ridge have been much folded and compressed, so that the layers now stand almost on c11(1 and are even overturned. Great faults and overthrusts are common, and add to the difficulty of unraveling the structure of the district. In the Appala chian Valley the geological structure is also quite complex, though the strata are not so intensely metamorphosed. The rocks are limestones, shales, and sandstones, and they lie in closed folds that become more open toward the western side of the valley. These folds are peculiar in
that their eastward slopes are always steeper than the westward. \Vhen the folds are over turned the inversion is toward the east; and overthrusts are also toward the east, and often of considerable extent. This valley is largely the result of the erosion of a great limestone for mation, of Cambro-Silurian age, that extends its entire length. The Alleghany \lountains consist of rocks of Paleozoic age, Cambrian to Carbon iferous, inelusive, that have been elevated into folded ridges and then eroded to their present topography. The softer beds have been worn into valleys, and the harder beds, having resisted erosion, have been left to form the ridges and benches. In this limestone also have been erod• ed the wonderful series of caves of the Shenan doah Valley and elsewhere, of which that at Luray, Va., is a striking example (see CnvEs). Anticlinal and synclinal folds alternate in di minishing intensity toward the west, where they disappear in the nearly horizontal beds of the Cumberland Plateau, which is made up of carboniferous rocks.