The upper layers are stream channel deposits while deeper they are marine. Upper Pleistocene deposits of the Lakewood Formation in the coastal area range in thickness from a few feet to 300 feet in the area shown in the cross section. These sediments are composed of interbedded clay, silt, sand and gravel. Outcrops of the Pleistocene San Pedro Formation are found in the western portion of Huntington Beach Mesa. These sediments are composed chiefly of marine sands, gravel, silts, and cloys.
Marine mollusks are locally abundant within the deposit.
Thicknesses vary from about 275 feet to about 500 feet in the study area. Near the base of the formation there is a sequence of coarse sand and gravel, interbedded with lenses of silt and cloy. This unit has been designated the Main aquifer. The San Pedro Formation, in general, and the Main aquifer, in particular, have been the most prolific water-bearing deposits in Orange County. The most recent elevation check of bench marks along the Pacific Coast Highway indicates an average settlement between Seal Beach and Bolsa Mesa of between 0.5 to 0.7 foot in a 20-year period.