Where a swift torrent, descending a steep slope, debouches on a plain or wide valley, its velocity is greatly diminished, and a large part of the material which it carries is thrown down and spread in a fan shape from the opening of the ravine in which the torrent flows. The thickness of the cone is greatest at the mouth of the ravine, while its breadth increases outward from that point. Where several such torrents open on the plain near together, their fans may coalesce and form a continuous fringe along the base of the mountain. The slope of the cone's surface diminishes with the size of the stream; in small streams it may be as steep as 10°. These cones are formed on much the same principle as deltas, and might, with propriety, be called terrestrial deltas. Very large alluvial cones are found in the Rocky Mountain And Great Basin Regions, generally in the forelands which front high mountains. In the western part of the Argentine Republic, along the front of the Andes, the temporary rivers formed by the melting snow bring down enormous quantities of mud and fine sand and spread it out over the plain.
Where the rivers discharge into the sea, this process of upbuilding is limited, except where there is slow downwarping of the foreland, but in interior arid basins, without outlet, it may accumulate very great thicknesses of river-made sediments.
Fig. 93. - Sand deposits, North Platte River, Nebraska. (U. S. G. S).
Fig. 94. - Alluvial cone, trenched by stream, with secondary cofte below.
(U..S. G. S).