A previously undescribed conglomerate unit is discontinuously exposed over an area of twenty-five square miles near Rancho Santa Fe, California. This formation unconformably overlies the Late Jurassic to mid-Cretaceous basement rocks, and is unconformably overlain by the Eocene La Jolla Formation. Although the unit is unfossiliferous, it is thought to be Late Cretaceous in age because of similarity in clast content and other physical characteristics to known Cretaceous units nearby and a distinct dissimilarity to other Cenozoic conglomerate units. On the basis of stratigraphic position and lithic character a tentative correlation is made with the Trabuco Conglomerate of the Santa Ana Mountains. The unit is predominantly a reddish-brown boulder conglomerate with a sandstone-siltstone matrix. Clasts locally range up to 20-25 feet in diameter. The dominant clast types include plutonic rocks, volcanic and volcaniclastic rocks, hornfelses, and quartzites, which were derived from the nearby basement rocks. Thin lenses of well-sorted, medium- to fine-grained sandstone are locally present in the otherwise massive conglomerate and appear to represent channel deposits. The large size of the clasts, their local provenance, and the coloration of the unit suggest terrestrial deposition, possibly as a fanglomerate. Many of the clasts are soft and friable and readily disintegrate upon exposure; this condition indicates lengthly subaerial weathering at the site of deposition prior to deposition of the overlying unit.