Tarkio Limestone

The Tarkio Limestone, is a Late-Carboniferous member of the Zeandale Formation in Kansas, extending into Nebraska, Iowa, and Missouri. The unit was named for outcrops in the Tarkio Valley in northwestern Missouri, however, R.C. Moore designated a typical exposure of this unit on Mill Creek, southwest of Maple Hill, Kansas. Owing to its unusual coloration, an early name for this rock was chocolate limestone. This rock can be recognized by its coloration and abundant fusulinids, Triticites ventricosus, protruding from the surfaces, giving a "raspy" texture.

The Tarkio Limestone forms bold bluffs along Deep Creek in southeastern Riley County, Kansas and can be easily accessed there at Pillsbury Crossing, about 30 miles west of Maple Hill.