The Colthorp series consists of shallow to a duripan, well drained soils on basalt plains and terraces. They formed in silty alluvium from loess and weathered volcanic ash.
Permeability is moderately slow.
Slopes are 0 to 20 percent. These are the Argalt , Bartome , Bigfrog , Bruncan , Chuckridge , Chuska , Dewar , Grassval , Handpah , Packwood , Pipeline , Roseworth , Thoms , and Wellington series. Bartome, Bigfrog, Chuska, Dewar, Grassval, Pipeline, Roseworth, and Thoms soils lack bedrock above depth of 40 inches. Argalt soils have bedrock at less than 20 inches. Packwood soils typically have bedrock at depths of less than 20 inches and they have a hue of 7.5YR or 5YR in the B2t horizon.