The effect of joints on erosion may often be seen along a stream which flows in a rock gorge. In such situations, the outlines of the banks are sometimes angular, and sometimes crenate ([Fig. 138]), the reëntrants being located at the joints. By working into and widening joints, running water sometimes isolates masses of rock as islands ([Fig. 139]). In a region free from mantle rock, or where the mantle rock is meagre, joints often determine the courses of valleys by directing the course of surface drainage. This is shown in many parts of the arid west. In regions where the rocks are notably faulted, the courses of the streams are sometimes controlled by the courses of the fault planes. This is the case, for example, in central Washington.[55]
Fig. 137.—Shows the sagging of beds along joints. The disturbance does not extend far below the surface. Cook’s quarry (Niagara limestone) near La Salle, Niagara Co., N. Y. (Gilbert, U. S. Geol. Surv.)
The jointing of rocks often shows itself distinctly in the weathered faces of cliffs (Figs. [140] and [141]), especially in arid and semi-arid regions, or where the slope is too steep for the accumulation of soil and rock-waste on its surface.
If a stream flowing over jointed rock has falls, the conditions are sometimes afforded for the development of an exceptional and striking scenic feature. If above Niagara Falls, for example, there were an open joint in the bed of the stream (as at b, [Fig. 142]), some portion of the water would descend through it. After reaching a lower level it might find or make a passage through the rock to the river below the falls. If even a little water took such a course, the flow would enlarge its channel, making a passageway between the joint through which the water descended and the valley below the falls (bcde, [Fig. 142]). This passageway might become large enough to accommodate all the water of the river. In this case, the entire fall would be transferred from the position which it previously occupied (f) to the position of the enlarged joint (b). The fall would then recede. The underground channel between the old falls and the new would be bridged by rock (bf″ and f‴, [Fig. 143]), making a natural bridge. The natural bridge near Lexington, Va. ([Fig. 144]), almost 200 feet above the stream which flows beneath it, is believed to have been developed in this way. A similar bridge is now in process of development in Two Medicine River in northwestern Montana ([Fig. 145]). Once in existence, a natural bridge will slowly weather away.
Fig. 138.—Figure showing crenate river bank, the reëntrants being determined by joints. Dells of the Wisconsin River, near Kilbourn, Wis. (Atwood.)
Fig. 139.—Lone Rock. An island isolated by the notable widening of a series of joints. The joints in the rock of the island have themselves been so widened that a rowboat may be taken through it in two directions. Lower Dells of the Wisconsin. (Meyers.)
