The Threetop series consists of well drained soils that are moderately deep to hard, argillaceous sandstone on dipslopes. They formed in slopewash alluvium and residuum. Slopes range from 3 to 12 percent. Depth to lithic contact is 20 to 40 inches. Depth to the base of the argillic horizon is 10 to 18 inches.
Rock fragments are mainly angular sandstone pebbles.
It is slightly acid through mildly alkaline. It is loam, sandy clay loam or clay loam with 25 to 35 percent clay and more than 35 percent fine sand or coarser. Rock fragments range from 0 to 15 percent. It is neutral through moderately alkaline. It is sandy clay loam, sandy loam, or loam.