Fig. 75.
Slope of valley 20°, dip of strata 50°.
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. 76.]), which exhibits strata rising at an angle of 20°, and crossed by a valley, which declines in an opposite direction at 20°.[57-A]
Fig. 76.
Slope of valley 20°, dip of strata 20°, in opposite directions.
These rules may often be of great practical utility; for the different degrees of dip occurring in the two cases represented in [figures 74] and [75.] 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. 74.]), 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. 75.], 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.
In the majority of cases, an anticlinal axis forms a ridge, and a synclinal axis a valley, as in A, B, [fig. 62.] [p. 48.]; but there are exceptions to this rule, the beds sometimes sloping inwards from either side of a mountain, as in [fig. 77.]