An informal name applied to a marker bed in the lower part of the Upper Cretaceous Williams Fork Formation of Mesaverde Group. Named for exposures along the Yampa River Valley near Craig, Moffat Co., CO in the Greater Green River basin. Is a claystone or diagenetically a regionally persistent altered ash bed 0.5 to 5 ft thick. Is poorly exposed. Weathers white to cream or light gray to light brown. Lacks stratification, is cohesive when moist, and displays blocky fracture. Occurs in subsurface where it is easily recognized on coal and gas exploration logs. The Buckpeak State 11-30 in sec 30, T6N, R98W (fig. 4) is an example of the geophysical response of the bed. Upper and lower contacts are usually sharp when associated with coal, and suggest minor transport where associated with lagoonal or fluvial rocks. Occurs 0-250 ft above base of formation.

Source: GNU records (USGS DDS-6; Denver GNULEX).