Contra Loma Dam and Reservoir is located in a small off-stream valley on the northeast flank of the Los Medanos Hills, about 2.5 km south of Antioch, California. The Los Medanos Hills consist of a generally north dipping sequence of Tertiary strata on the north margin of the Mt. Diablo uplift, and on the southwestern margin of the Sacramento-San Joaquin Delta. In the vicinity of the dam and reservoir the relief of the rolling hills ranges up to about 200 m. The relief increases rapidly to the west and south and decreases to the north and east in the delta area.
In the immediate vicinity of the dam and reservoir the hills are mantled by clayey residual soil, colluvium, and alluvium, and are underlain by a sequence of moderately lithified Tertiary nonmarine and marine sedimentary rocks assigned, in ascending order, to the Eocene Kreyenhagen Formation, the Oligocene Kirker Formation, the Miocene Cierbo Formation, the Mio-Pliocene Neroly Formation, and the Pliocene Wolfskill Formation. The formation contacts generally strike north-northwest east-southeast, and dip to the north at angles up to about 25 degrees. Bedding within these formations is cut by several sets of joints, and locally by minor faults with up to a few feet of offset
The reservoir is in a small valley enclosed between low ridges and rather steep-sided rolling hills which support tall grasses and a sparse growth of trees. Two gentle saddles in the ridge along the east edge of the reservoir required low dikes. Most of the reservoir floor is blanketed by residual clayey soil , accumulations of fine-grained slopewash, and alluvium, except in the central portion where most of the surficial material was removed for use as impervious embankment. The northern (downstream) half of the reservoir is underlain by the Pliocene Wolfskill Formation of poorly lithified claystone, siltstone, and interbedded sandstone. Elsewhere, the marine Mio-Pliocene Neroly Formation of lithified Sandstone and interbedded, moderately lithified, claystone underlies the reservoir.
The residual clays (CL-CH) vary in depth from a few inches to nearly 6 feet. The bedrock beneath the soil cover consists of layers of shale, siltstone, and sandstone which trend east-west (nearly parallel to the darn axis) and dip 10 to 25 degrees northward (downstream). Some of the layers air slake when exposed, and some water slake.
Other Dams or Dikes which enclose Contra Loma Reservoir: Contra Loma Dike No. 2, Contra Loma Dike No. 3.
