Outcrop of Strata.—It will be observed that some of these ridges, A, B, are unbroken on the summit, whereas one of them, C, has been fractured along the line of strike, and a portion of it carried away by denudation, so that the ridges of the beds in the formations a, b, c come out to the day, or, as the miners say, crop out, on the sides of a valley. The ground-plan of such a denuded ridge as C, as given in a geological map, may be expressed by the diagram, Fig. 64, and the cross-section of the same by Fig. 65. The line D E, Fig. 64, is the anticlinal line, on each side of which the dip is in opposite directions, as expressed by the arrows. The emergence of strata at the surface is called by miners their outcrop, or basset.
If, instead of being folded into parallel ridges, the beds form a boss or dome-shaped protuberance, and if we suppose the summit of the dome carried off, the ground-plan would exhibit the edges of the strata forming a succession of circles, or ellipses, round a common centre. These circles are the lines of strike, and the dip being always at right angles is inclined in the course of the circuit to every point of the compass, constituting what is termed a quâ-quâversal dip—that is, turning every way.
There are endless variations in the figures described by the basset-edges of the strata, according to the different inclination of the beds, and the mode in which they happen to have been denuded. One of the simplest rules, with which every geologist should be acquainted, relates to the V-like form of the beds as they crop out in an ordinary valley. First, if the strata be horizontal, the V-like form will be also on a level, and the newest strata will appear at the greatest heights.
Secondly, if the beds be inclined and intersected by a valley sloping in the same direction, and the dip of the beds be less steep than the slope of the valley, then the V’s, as they are often termed by miners, will point upward (see Fig. 66), those formed by the newer beds appearing in a superior position, and extending highest up the valley, as A is seen above B.
Thirdly, if the dip of the beds be steeper than the slope of the valley, then the V’s will point downward (see Fig. 67), and those formed of the older beds will now appear uppermost, as B appears above A.
Fourthly, in every case where the strata dip in a contrary direction to the slope of the valley, whatever be the angle of inclination, the newer beds will appear the highest, as in the first and second cases. This is shown by the drawing (Fig. 68), which exhibits strata rising at an angle of 20°, and crossed by a valley, which declines in an opposite direction at 20°.
These rules may often be of great practical utility; for the different degrees of dip occurring in the two cases represented in Figs. 66 and 67 may occasionally be encountered in following the same line of flexure at points a few miles distant from each other. A miner unacquainted with the rule, who had first explored the valley Fig. 66, may have sunk a vertical shaft below the coal-seam A, until he reached the inferior bed, B. He might then pass to the valley, Fig. 67, and discovering there also the outcrop of two coal-seams, might begin his workings in the uppermost in the expectation of coming down to the other bed A, which would be observed cropping out lower down the valley. But a glance at the section will demonstrate the futility of such hopes.[[5]]
Section of carboniferous rocks of Lancashire. (E. Hull.[[6]])