WATERFALL, a point in a watercourse where descent is perpendicular or nearly so. The beauty of a waterfall is de pendent on its height rather than on the volume of water. Small but immensely deep falls are common, for a small stream has insufficient power to erode a steady slope, and thus any consider able irregularity of level in its course is marked by a fall. In mountainous districts a stream may descend into the main valley as a "hanging tributary" by way of a fall, its own valley having suffered less erosion than the main valley. The chief cause of a waterfall is a sudden and marked change in geological structure. For example, if a stream crosses a harder stratum it will be able to grade its course through the upstream or downstream soft strata more rapidly than through the intermediate hard stratum, over a ledge of which it will subsequently fall. Such barriers may
be produced by the ordinary rock sequence or by an intrusive dyke of basalt, or by glacial deposits. Where a river falls over a shelf of hard rock overlying softer material there is a rapid erosion of the soft rock, with undercutting and consequent collapse of the edge of the hard shelf. In this way, the fall gradually moves up stream and a gorge occurs below it ; the Niagara Falls provide an excellent illustration of this process.