The Delmar Formation is believed to hav a lagoonal deposiional environment, because of facies relationships and paleoecological studies. This type of environment is protected from normal 1 ittoral processes by the barrier islands (Torrey Sandstone). The areas in and around tidal)nlets, however, have relatively higher energy conditions. The 1 ithology is predominately dusky-yellowish-green sandy mudstone interbedded withyellowish gray, medium to coarse-grained sandstone. Coal and fossil wood fragments are common. Fifty samples were taken from vertical channels dug at six locations - three along the coast and three inland. The claysized fraction (less than 2 microns) was extracted, and then analyzed using X-ray diffraction techniques. The diffractograms show kaoi inite, smectite, vermiculite, ill ite, chlorite, and mixed-layer minerals. The expandable clays, smectite and vermiculite, dominate each sample. Kao-, linite is present in every sample. lllite varies laterally; it was not detected at the sample site in San Dieguito Valley, and at the sample site along the coast just south of Del Mar. The thin claystone beds contain 57% expandable clay minerals. Vertical variations in clay mineralogy are grossly similar in the exposures that were sampled. The variations approach the accuracy of the data. Variations that do occur could be attributed to current patterns in the lagoon and/or source area.