This study involves the examination of lithologic and microfossil samples combined with detailed stratigraphic observations from the Otay, San Diego, and Linda Vista Formations in southern San Diego, California. The Otay Formation occurs from the community of Lemon Grove south to the U.S.-Mexican border and has a slight dip to the south. A suggested type section is along Otay Valley and Heritage Roads. The formation consists of a lower pebble to cobble conglomerate and associated coarse sandstone of lithic arkose composition, which grades upward into a medium to very fine-grained feldspathic litharenite with much montmorillonite clay matrix. The lithologic composition, suggests a dual source area from both the northeast and south. Badly weathered microfossils (Cibicides sp.) from one locality, and stratigraphic position, indicate an age for the formation of somewhere between Miocene and late Pliocene. Microfossils also suggest the formation is at least in part of shallow marine origin. The Otay Formation is overlain with a slight angular unconformity by the San Diego Formation, which consists mostly of a medium-grained biotite-hornblende lithic arkose. The formation extends from Mission Valley to below the U.S.-Mexican border and is extremely wedge-shaped, thickening to the southwest with a source area to the northeast. Microfossils indicate this formation is a shallow marine deposit which is latest Pliocene to earliest Pleistocene in age. The formation has a depositional dip of about 4° to the southwest. The shallow depth, stratigraphy, and sedimentary structures indicate the formation is a delta slope deposit. The San Diego Formation is in part gradational into the overlying Linda Vista Formation which is also latest Pliocene to earliest Pleistocene in age. The Linda Vista Formation is exposed from north of the study area to south of the U.S.-Mexican border and thickens slightly in a southwest direction. The unit is a red-stained cobble conglomerate with sandstone of feldspathic litharenite composition. The source area is also from the northeast. Stratigraphic, lithologic, and fossil evidence indicate the formation is a marginal fluviomarine clastic fan deposit. The upper surface of the Linda Vista Formation is a flat, wave cut marine terrace. The possible Neogene geologic history of the area involves at least partly shallow marine deposition with a dual source for the Otay Formation sometime be-tween the middle Miocene and late Pliocene. This was followed by a short (?) hiatus and then contemporaneous deposition of the San Diego and Linda Vista formations from a single northeast source during latest Pliocene to earliest Pleistocene time. The San Diego Formation is the marine delta slope facies and the Linda Vista is the marginal fluviomarine facies of a clastic fan which prograded from northeast to southwest. Ocean waves then cut and leveled the Linda Vista beds, and the whole area has been uplifted and eroded since the early Pleistocene.