form good-sized tanks in hard rock. A scour depression in relatively soft hardpan appears to be the cause of one of the water holes in the Vekol Valley, Maricopa County (fig. 22). This consists of a hollow at the point of an elbow in the sandy channel of the stream. A cavity in the underlying hardpan holds water after a flood. The pool of water is 6 by 4 feet, 2 feet deep, and lies under the bank. Seepage from the sand of the stream channel and from the sandy clay of the banks maintains the water for a considerable time after adjacent charcos have dried up. The water does not last very long, for the level fell 112 feet between September 20 and September 22, 1917. Riffle hollows occur where the bed of a stream is composed of alternate layers of hard and soft rock, as illustrated in Figure 21, C. Erosion of the softer rock is carried below the grade established by the harder rocks, which project in the stream bed and constitute obstacles to the stream flow. Such hollows are commonly from 3 to 12 inches deep and vary in size according to the spacing of the FIGURE 22.-Cross section of the stream channel and water hole in the Vekol Valley, Ariz. harder portions of the rock in the stream bed. Riffle hollows make very shallow pools unless they are deepened by pothole action or unless they grade into plunge pools. A pothole is formed by the grinding or drilling of an original hollow in a stream bed by sand, pebbles, or boulders rotated by the current. The top of a pothole is nearly circular, and the diameter generally increases below, as shown in Figure 21, D. The diameter may range from 3 inches to 10 feet or more and the depth from 6 inches to 8 feet or more.18 Potholes are developed in all streams that are actively eroding their channels in consolidated rocks, but they are more likely to be found in gorges and below waterfalls. They are thus associated with and grade into plunge pools. A plunge pool is formed by the impact of water and the sand and gravel which it carries at the foot of a waterfall (fig. 21, E). A fall differs from the changes in grade of a stream bed described in connection with riffle hollows in that it is usually high enough to cause a flexure in the flood surface of the stream. Consequently, a very 18 Elston, E. D., Potholes, their variety, origin, and significance; Sci. Monthly, vol. 5, pp. 554-567. 1917; vol. 6, pp. 37-51, 1918. |