Fig. 183. Joints utilized by a River in widening its Valley, Iowa
Joints are used to great advantage in quarrying, and we have seen how they are utilized by the weather in breaking up rock masses, by rivers in widening their valleys, by the sea in driving back its cliffs, by glaciers in plucking their beds, and how they are enlarged in soluble rocks to form natural passageways for underground waters. The ends of the parted strata match along both sides of joint planes; in. joints there has been little or no displacement of the broken rocks.
Fig. 184. A Normal Fault
Faults. In [Figure 184] the rocks have been both broken and dislocated along the plane ff´. One side must have been moved up or down past the other. Such a dislocation is called a fault. The amount of the displacement, as measured by the vertical distance between the ends of a parted layer, is the throw (cd). The angle (ff´v) which the fault plane makes with the vertical is the hade. In [Figure 184] the right side has gone down relatively to the left; the right is the side of the downthrow, while the left is the side of the upthrow. Where the fault plane is not vertical the surfaces on the two sides may be distinguished as the hanging wall (that on the right of [Figure 184]) and the foot wall (that on the left of the same figure). Faults differ in throw from a fraction of an inch to many thousands of feet.
Slickensides. If we examine the walls of a fault, we may find further evidence of movement in the fact that the surfaces are polished and grooved by the enormous friction which they have suffered as they have ground one upon the other. These appearances, called slickensides, have sometimes been mistaken for the results of glacial action.
Normal faults. Faults are of two kinds,—normal faults and thrust faults. Normal faults, of which [Figure 184] is an example, hade to the downthrow; the hanging wall has gone down. The total length of the strata has been increased by the displacement. It seems that the strata have been stretched and broken, and that the blocks have readjusted themselves under the action of gravity as they settled.