The Rillito series consists of very deep, somewhat excessively drained soils that formed in mixed alluvium.
Rillito soils are on fan terraces or stream terraces.
Slopes are dominantly 0 to 5 percent, but range to 40 percent. Rillito gravelly sandy loam - rangeland. Driest during May and June. Typic aridic soil moisture regime. The surface layer contains 5 to 35 percent gravel.