Pg. 16 (fig. 2a), 18-20, 61, 63. Atchinson formation. In most places, basal unit of this formation is black breccia composed of angular blocks of augite andesite lava; about 500 feet thick and grades irregularly into unbroken flow-rock in Atchinson Mountain; locally at base are found a little white limestone and red sandstone gravel. Upper unit is a red to red-brown monolithic breccia. Thickness ranges from a fraction of a foot to about 500 feet. Overlies Rencher and Grass Valley formations (both new). Age is Tertiary.
Named from exposures on Atchinson Mountain, [Washington Co.], southwestern UT. Mapped in northern half of Pine Valley Mountains, Washington and Iron Cos., southwestern UT.

Source: US geologic names lexicon (USGS Bull. 1200, p. 160).