The Del Mar formation was studied in three outcrops along the coast of San Diego County between Torrey Pines State Park and the mouth of the San Dieguito Valley. The rock consist of sandstones, siltstones, and silty shales with a great deal of lateral variation. The sandstones and siltstones consist mostly of quartz and rock fragments. The shales commonly contain pyrite, marcasite, and sulfur-rich coal indicating a reducing environment, which is usually related to a humid climate. The fauna in the rocks contains twenty-one species of molluscs, none of which are alive today. Six of these species are considered brackish-water indicators. The environmental information for the other fossils is based on characteristics of the genera. They indicate a sandy to muddy environment at a depth of about sixty feet or less.