Proceedings of the American Philosophical Society Held at Philadelphia for Promoting Useful Knowledge Volume 36 (Paperback)

This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1897 edition. Excerpt: ...the Paleozoic region of southwest Arkansas. Let Fig. 2 be a very generalized columnar section, in which (a) represents the Silurian novaculites, b) the sandstone immediately overlying the novaculites, (c) the variable thickness of sandstones and shales, d) massive sandstones, and () the great thickness of soft and hard sandstones forming most of the section. The topography developed upon these beds will depend to a great extent upon the character of the folds. In the case of a simple fold, such as are shown on Figs. 3 and 4, where erosion has eaten down until the hard sandstones are exposed, an anticlinal ridge will follow the axis with a valley on either side. Figs. 3 and 4 show the effect of the dip upon the width and character of the valleys. As a rule it is found that, of the ridges formed by the sandstones overlying the shales, the one on the side from which the principal drainage comes is cut down so as not to present an abrupt face. Fig. 5, shows the effect of two such anticlines close together. Generally one side of the anticline is steeper than the other, as is suggested in Fig. 8, when this becomes overturned the effect is modified slightly, as is shown in Fig. 6. Where shale underlies the sandstone of the anticlinal axis, in time this anticline becomes breached (Fig. 7), and the topography resembles that of which the Wall mountain (Figs. 27 and 28) is an excellent example. If erosion has not cut down to the underlying sandstone, the result will be the same, except that the anticlinal ridge is omitted and the valley will be narrower, possibly very much narrower. Such an anticline (Fig. 8), is difficult to locate accurately. This is a common type; it is illustrated by the Prairie Bayou anticline (Fig. 16). Overturns of the kind...