The Eocene Friars Formation in southwestern San Diego County is an immature to submature, calcite-cemented, micaceous feldspathic plutonic-volcanic-metamorphic litharenite. The formation can be divided into two units, a lower clayey unit and an upper sandy unit. Clays, dominantly expandables, are concentrated at the base and decrease to less than 1 percent in the upper sandy unit. Basement rocks east of the San Diego area are the source of the Friars Formation. Deposition by intermittent streams occurred in stream valleys on the basement rocks. The climate was arid to semi-arid. Clays of the Friars Formation are of low to medium plasticity and present an engineering problem to land developers because of the expandable nature of the clay minerals.
