Page:Popular Science Monthly Volume 68.djvu/399

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sandstone layers retard erosion and form falls, sometimes single leaps, sometimes step falls (Fig. 9) where the water cascades from ledge to ledge. With the variable spacing of the sandstone and shale layers, there is almost infinite variety in waterfall form.

The variation in gorge and waterfall form is still further increased by the joint planes which cleave the strata nearly vertically, and thus introduce a cause for vertical variation in erosion in addition to the horizontal. Some of the most beautiful of the cascades are those where the stream erosion has etched out combinations of horizontal and vertical irregularity in the rock bed over which the water falls (Fig. 8). The joint planes often guide the stream course between the falls also, sometimes confining it between narrow,