Another common, though seldom noticed, post-glacial feature of the park is the alluvial fan. These are fan-shaped accumulations of gravel deposited by swift, tributary streams where they enter a main valley. Some of them have grown so large as to dam the stream in the major valley and cause a lake (Figure 2). St. Mary, Lower St. Mary, Lower Two Medicine, and Waterton Lakes are held in by such dams. The alluvial fan of Divide Creek which holds in St. Mary Lake can easily be distinguished from Going-to-the-Sun Highway on the north side of the lake near its outlet. The St. Mary Entrance Station is located on this fan. The lower lake is dammed by a large fan built into the St. Mary Valley by Swiftcurrent Creek. The straight section of highway between the town of Babb and the St. Mary River bridge lies on the lower part of this fan. Inasmuch as the Pleistocene glaciers undoubtedly removed any such fans made previously, those which are present today must have been constructed since disappearance of the ice, and are then not more than 12,000 years old. Most of them are somewhat older, possibly by as much as two or three thousand years, than the gorges mentioned above, because the latter are located nearer the source of the glaciers, and their sites were thus still covered by ice after the fans had already begun to form. After the Pleistocene glaciers began their final retreat several thousand years elapsed before they disappeared from the mountains. [Illustration: FRONT OF LEWIS RANGE, NORTH SIDE OF SWIFTCURRENT VALLEY. THE LEWIS OVERTHRUST LIES AT THE BASE OF THE CLIFF. THREE LARGE TALUS CONES ARE VISIBLE BELOW MT. ALTYN ON THE LEFT. (DYSON PHOTO)] One of the most conspicuous of all post-glacial features is the talus cone, an accumulation of angular rock fragments which fall from cliffs. It is only at the base of a crevice or chimney that this material takes the apparent form of a distinct cone. Elsewhere it is referred to as a talus slope or simply as talus, or, in the parlance of some mountaineers, as scree. Although several thousand years have been required for their formation most talus accumulations in the park are still actively growing, especially in spring and early summer when rocks are pried loose by the alternate freezing and thawing of moisture within fractures. The artillery-like crack made when a falling rock crashes to the base of a high cliff is a familiar sound to anyone who has spent much time in the mountains. The Future