The basal formation of the Rosario Group, the Lusardi Formation, was named by Nordstrom (1970) for exposures of boulder conglomerate near the confluence of Lusardi Creek and the San Dieguito River, 2 km north of the area in the Rancho Santa Fe quadrangle. These rocks consist of cobble and boulder conglomerate, with occasional thin lenses of medium-grained sandstone. Some of the clasts are 10 m in diameter. The Lusardi Formation at the exposures within the northeast quarter of the Del Mar quadrangle, and within the type area to the north, has a maximum thickness of 125 meters. At the Holderness No. 1 well, 17 km southeast of the tip of the Point Loma Peninsula, rocks considered to belong to the Lusardi Formation are 82 m thick, whereas at the Point Loma No. 1 well, 10 km north of Point Loma, these rocks are 295-376 m thick (Hertlein and Grant, 1944, p. 38). The Lusardi Formation at its type area is unconformably overlain by Eocene rocks, but 16 km to the north near Carlsbad, it is overlain conformably by siltstone and sandstone of the Point Loma Formation (Kennedy and Moore, 1971b). The Lusardi Formation is considered to be Late Cretaceous in age because it contains quartz diorite boulders eroded from the mid-Cretaceous southern California batholith, which has a minimum age of 105 & 10 million years (Bushee et al., 1963), and it is overlain by the Point Loma Formation which contains Upper Cretaceous (Campanian) Foraminifera (Sliter, 1968). The Lusardi Formation is lithologically equivalent to the Trabuco Formation of the Santa Ana Mountains on the north (Nordstrom, 1970), to an unnamed fanglomerate near the base of the Williams Formation, also in the Santa Ana Mountains (Morton, 1972, p. 39), and to the Redondo Formation of Flynn (1970) in northern Baja California.

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

Bushee, J., Holden, J., Geyer, B., and Gastil, G., 1963, Leadalpha dates for some basement rocks of southwestern California: Geol. Soc. America Bull., v. 74, p. 803806.
Flynn, C.J., 1970, Post-batholithic geology of the La Gloria- Press Rodriquez area, Baja California, Mexico: Geol. Soc. America Bull., v. 81, p. 1789-1806.
Hertlein, L.G., and Grant, U.S., IV, 1944, The geology and paleontology of the marine Pliocene of San Diego, California, pt. 1, Geo.logy: San Diego Soc. Nat. History Mem., v. 2, p. 1-72.
Kennedy, M.P., and Moore, G.W., 1971b, Stratigraphy and structure of the area between Oceanside and San Diego, California: geologic road llog, in Elders W.A., ed., 1971, Geological excursions in southern California: Geol. Soc. America Cordilleran Section Meeting, Riverside, California, field trip guidebook.
Morton, P.K., 1972, Geologic guidebook to the northern Peninsular Ranges, Orange and Riverside Counties, California: Prepared jointly by National Assoc., of Geol. Teachers and South Coast Geological Soc., for N.A.G.T. far western section meeting, Chapman College, Orange, California.
Nordstrom, C.W., 1970, Lusardi Formation a post-batholithic Cretaceous conglomerate north of San Diego, California: Geol. Soc. America Bull., v. 81, p. 601-605.
Slitter, W.V., 1968, Upper Cretaceous Foraminifera from southern California and northwestern Baja California, Mexico: The University of Kansas Pubs., Art. 7, Ser. no. 49, p. 141.