The Gerty sand consists of unconsolidated deposits, as much as 50 feet thick, of sand, gravel, and clay. The sand and gravel are almost entirely siliceous, the main constituents being quartz, quartzite, chert, flint, jasper, and silicified wood. H. D. Miser has found one pebble of schist. The silicified wood has been examined by C. B. Read, of the United States Geological Survey, who believes much or all of it to be of Cretaceous age. The nearest source for these materials is in the Cretaceous and older rocks of the Rocky Mountains or the Tertiary deposits of the high plains, which contain pebbles and cobbles derived from rocks that are exposed in and near the Rocky Mountains. A single elephant tusk, found in the Gerty sand by A. E. Brainerd,1 suggests that the deposits are of Pleistocene age.
These unconsolidated deposits form the surface . . .