Page:Popular Science Monthly Volume 3.djvu/217

Rh No 3—The great deposit that follows No. 2, whether it be of clay or of gravel and sand, is that designated in common usage "hard-pan." It constitutes the chief member of the drift throughout the Northwest. It is rarely found entirely wanting, whereas the foregoing are very often wanting. It seems to be the parent member of which the former two are offshoots, or modifications. It sometimes has a thickness of more than two hundred feet, and rises to the surface forming the basis of the soil. It consists of a heterogeneous mixture of clay and gravel-stones, with bowlders of northern origin. It is nearly impervious to water, and occasionally, but rarely, shows a rude arrangement in alternating bands, as if, in a plastic state, it had been folded upon itself. Such arrangement discloses no assortment of the



materials that can be likened to the assortment seen in No. 2. The bowlders embraced in this member of the drift almost invariably show glaciated surfaces. Although apt to be more abundant near the bottom of the deposit, they are not confined to it, as is sometimes stated, but occur throughout the whole. This deposit of hard-pan sometimes encloses lenticular masses of assorted materials. It has even been seen to overlie a considerable thickness of fine stratified sand the extent of which could not be ascertained. In general, however, it is one compact, uniform mass, varying slightly in the proportions of its different parts, from State to State, according to the readiness of supply of any