Clay pits at Butler, northern Bastrop County, are the type locality of the Butler Clay Member of F. B. Plummer (1933). This member is the basal Calvert Bluff Clay Member, which is actually the upper of three members described by Plummer and not the lowest underlying the Simsboro Sand Member as originally described by Plummer. The members proposed by Plummer, except the Butler, are raised to formation rank, and the Hooper Formation, proposed by H. B. Stenzel, has been adopted instead of the Butler Member. Clay beds in the Calvert Bluff and Hooper Formations weather to clay soil which supports prairie grasses and mesquite, whereas the sand beds of the Simsboro Formation weather to a pedalfer soil which supports dense growth of blackjack oak, post oak, and cedar. Thus, it is possible to map these formations in areas of poor outcroppings with some reliability.

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