On slopes where water flows only during rain or the melting of snow, the same erosive effects may be seen as between the heads of ravines and their outlets. (See figure 2,) It is there too that the surprisingly rapid cutting-out of channels by the aid of water charged with rock fragments or gravel, can readily be observed, and the enormous power of water erosion convincingly shown. In the United States the stupendous gorges of the Columbia and Colorado rivers, the former cut to a depth of over 2000 feet into hard basalt rock, the latter to over 5,000 feet, partly into softer materials, partly into granite, are perhaps the most striking examples of this power; the manifestations of which can, however, be as convincingly seen in thousands of minor rivers and streams. All the materials so carried off from the higher slopes are finally deposited on a lower level; whether only a short distance away on a lower slope (colluvial soils), or farther away in the flood plain of streams, rivers, or lakes (alluvial soils). Other things being equal, the finest materials are of course, carried farthest, and often into the sea; in which, however, they cannot long remain suspended, but are quickly thrown down, forming river bars, flood plains, and deltas. The fineness of the material of delta soils, like that of those made from glacier flour, insures them the same advantage, viz. great fertility and durability. It is calculated that the Mississippi River carries into the Gulf of Mexico annually some 7469 millions of cubic feet of earthy deposits, which would fill one square mile of surface to the height of 268 feet, or would cover that number of square miles to the depth of one foot. [Illustration: FIG. 3.—Cliffs and caves on sea-beach at La Jolla, Calif. showing effects of Wave action.] _Wave-Action._—The powerful effects of the beating of waves upon abrupt shores of seas or lakes are in evidence all over the world, and these effects are so characteristic that they can be recognized even where no sea or lake exists at present. Gravel and sand are carried in the surf and serve as grinding materials, wearing even the hardest rocks into grooves, rills, channels and caves, defining sharply the varying degrees of hardness or tough resistance in different parts of rocky cliffs; frequently undermining them and causing extensive rock falls. The latter then serve for a time to break the violence of the waves’ onset, and may even cause permanent shore deposits to be formed under their lee. Such deposits are very generally formed on gently sloping beaches, and as the water gradually recedes, sometimes by elevation of the ground, beach lines or beach-terraces are left, which indicate the successive levels of the lake or sea. Such old beach lines or terraces and level-surfaced “buttes” in the Great Basin country, and “bench lands” elsewhere, show in their structure the characteristic lines of wave-deposition.