Geological investigation of the clays of Riverside and Orange counties, southern California

One of the three major clay-producing districts of California lies in Riverside and Orange Counties. The more commercially important clay pits and mines are distributed in the form of a horseshoe, whose axis, trending northwest-southeast, roughly coincides with the crest of the Santa Ana Range. The eastern limb lies in the Elsinore-Temescal Valley. The apex or closed end corresponds with the northern tip of the range, while the western end lies in the foothills of the Santa Ana Mountains. According to ceramic use, these clays comprise many types but all can be classified into three broad divisions, namely, fire-clays, refractory bond-clays, and red-burning clays. Although more than thirty different clays are mined within this area, no one clay is used alone, all clay wares produced being a mixture of two or more separate types. According to origin, both residual and transported clays are present. All of the transported and a portion of the residual types apparently belong, geologically, to one Tertiary formation, known as the Martinez, of Eocene, age. The clays are thus a part of a sedimentary series, considered to have been deposited under estuarine conditions at the edge of a gradually encroaching sea. Coal is found in close association with all of the transported clays. Its presence indicates that a humid-temperate climate existed when these clays were deposited. Although the presence of organic acids undoubtedly played a part in the formation of some of the clays derived from the original materials, certain other types of the residual class can not be explained in this manner, since they are not associated with coal. Volcanic material of a rhyolitic type has been found to be present both in situ and in the sediments and to have been altered in place, thus giving rise to the formation of residual and transported clays, which are bentonitic when derived from tuffaceous material