The Ardath Shale (part of the Rose Canyon Shale Member of Hanna, 1926) crops out along the sea cliffs from Bathtub Rock in Torrey Pines State Park, south to the pier at Scripps Institution of Oceanography, where it is overlain by Pleistocene deposits. From Rose Canyon it can be traced south to the northeast corner of Mission Bay, where it is overlain by younger rocks. In the northwest corner of the area, it thins below the Scripps Formation as it grades into the Torrey Sandstone below. The Ardath Shale is predominantly weakly fissile, olive-gray shale. Concretionary beds containing molluscan fossils are common. Expansible claystone locally comprises as much as 25 percent of the unit and landslides are commonly associated with those areas. Sieve analyses indicate that the particle size distribution is 81 percent silt, 16 percent clay, and 3 percent sand. The clay is mostly kaolinite but montmorillonite is also present. The sand consists of quartz (70-75 percent), potassium feldspar (15-20 percent), biotite (5-10 percent), plagioclase (less than 1 percent), and a trace of zircon, tourmaline, pyroxene, and amphibole. The base of the Ardath Shale is not exposed at its type section in Rose Canyon (Kennedy and Moore, 1971a), but underlying outcrops at the type locality of the Mount Soledad Formation, 1 km to the west, reveal the contact on the Mount Soledad Formation to be conformable. The Ardath Shale is estimated to be 70 m thick at its type locality. It grades alternatingly and conformably into the overlying Scripps Formation. Abundant fossils, including mollusks and calcareous nannoplan kton, permit an assignment of the Ardath Shale to the lower middle Eocene (Bukry and Kennedy, 1969). The unit correlates with the basal part of the Torrey Sandstone and with the middle part of the Santiago Formation.

Source - Geology of the eastern San Diego metropolitan area, California (Del Mar, La Jolla, Point Loma, La Mesa, Poway, and SW 1/4 Escondido quadrangles), 1975, California Div. Mines and Geol. Bull. 200, Section A, by Michael P. Kennedy

References

Bukry, David, and Kennedy, M.P., 1969, Cretaceous and Eocene coccoliths at San Diego, California, in Short contributions to California geology: California Division of Mines and Geology Special Report 100, p. 33-43.
Hanna, M.A., 1926, Geology of the La Jolla quadrangle, California: University of California, Dept. Geol. Sci. Bull., v. 16, p. 1 87-246.
Kennedy, M.P., and Moore, G.W., 1971a, Stratigraphic relations of upper Cretaceous and Eocene formations, San Diego coastal area, California: American Assoc. Petroleum Geologists Bull., v. 55, p. 709-722.