Walnut Creek is a branch of Middle Creek from the west. Mound Creek rises in the southwest corner of the county, and flows east, then southeast, into the Marais des Cygnes, Linn county. These grow mostly along the creeks, average width of belts being about one-half mile, and occupying about ten per cent of the surface, open prairie occupying ninety per cent. It is everywhere exceedingly fertile. The subsoil is usually clay.
Good limestone is found in almost every locality.
There is also considerable sandstone. This marble resembles the Junction City stone, is readily shaped with saw or plane, and is quite valuable for building purposes. Coal is believed to underlie about one-fourth of the county but the quality is generally too poor, and the layers are too thin to justify working. The existence of some of these springs had been known to the Indians from time immortal, and to the white men about thirty-five years. But prospecting for petroleum was not undertaken until 1882.