Clay is mined in open pits by several companies in the Alberhill district, and the refractory clays of relatively high alumina sediment are used largely for fire brick. The Alberhill Coal and Clay Company is the largest operator and has produced a little over 2,000,000 tons of clay, of which nearly half was the refractory type.
The clay occurs at the contact of the lower Tertiary and the Mesozoic basement complex. The weathered surface of basement rocks includes much clay of high iron and low alumina content, and the better clay occurs in the basal Tertiary sediments. The clay deposits vary rather abruptly in thickness and quality, and only local lenses contain workable deposits. Structural deformation makes dips of 10 to 20 degrees common and the clay strata therefore pitch under excessive overburden in short distances. Extensive deposits of thick alluvial fan deposits cover the clay-bearing strata over most of the area, and add to the overburden problems.
The apparent lack of clay deposits of good quality that would total several million tons of ore, and the geological conditions that would make exploration and mining difficult and expensive make this district unpromising.