San Diego Ancient Shorelines
For the story of ancient shorelines, there are also important clues to be found at the modern shoreline. After all, it is important to know what features are characteristic of shorelines before we go looking for them in the geologic record. We all know that the shoreline is the place where the ocean and land meet. For geologically active coasts like California, this generally means that the shoreline is a zone of dynamic interactions of erosion and deposition, of sea cliff failure and sand buildup. As ocean waves wear away at the base of the sea cliff, the cliff face fails and the shoreline moves (or retreats) landward. As the sea cliff retreats, a broad nearly flat surface called an abrasion platform is formed. This abrasion platform is carved directly into bedrock and can be thought of as the "shallow end" of the sea floor. The point at the toe of the sea cliff where the nearly horizontal abrasion platform meets the nearly vertical sea cliff is technically referred to as the shoreline angle and is generally coincident with sea level. Given the condition of a stable global sea level, wave erosion and sea cliff retreat will continue to cause the shoreline angle to move steadily inland and the rocky abrasion platform to widen.
Just as erosion is a major factor in shaping our shoreline, so is deposition. When there is an adequate supply of sediment (derived from sea cliff failures and rivers), the abrasion platform will be covered by a veneer of sedimentary material. This sedimentary veneer is typically sand, but it can also be made up of pebbles, cobbles, and boulders. Anyone that has dug a hole at the beach has probably noticed the fine alternating light and dark layers of sand that underlie the beach surface. These light and dark sand laminations are not horizontal, but instead tilt slightly seaward. They are formed by waves washing up and back down the beach face, and serve as a record of the last few years of summer and winter beaches. Generally speaking, large winter storm waves carry sand offshore, while smaller summer waves bring sand onshore. Add to this the daily cycle of high and low tide, and you have a mechanism for moving sand up and down the beach face.
Now that we have an idea of what features make up a modern shoreline (sea cliff, shoreline angle, abrasion platform, and veneer of finely laminated sand or rocky rubble), we can begin to look for San Diego's ancient shorelines. The coastal plain of San Diego County is characterized by a series of broad planar surfaces, or mesas, that extend from the foothills of the Peninsular Ranges west to the modern shoreline. Within metropolitan San Diego the broadest and most continuous mesas underlie the communities of Mission Hills, Hillcrest, North Park, Normal Heights, Talmadge, and Golden Hill on the south side of Mission Valley and Linda Vista, Serra Mesa, Clairemont, Kearny Mesa, University City, Miramar, and Mira Mesa on the north side of Mission Valley. In places where canyons or roads have cut down through these broad mesa surfaces, the resulting canyon slopes and road cuts expose a buried marine abrasion platform, generally positioned some 20 to 30 feet below the top of the mesa. Typically, the sedimentary layers resting on these abrasion platforms consist of the basal series of laminated marine sandstones discussed earlier, overlain by cross-bedded dune sandstones or cobble-rich river sandstones and conglomerates. The near horizontal nature of the mesa surfaces is merely mirroring the near horizontal nature of the underlying abrasion platforms.
Geologists refer to landforms like our coastal mesas as elevated marine terraces. In San Diego County a close look at the coastal mesas reveals not just one, but a stair-step series of at least 14 different marine terraces. A basic topographic distinction can be made for this marine terrace series using the 300-foot elevation contour line as a datum. Above this datum are at least eight marine terraces, each as much as 3 miles wide. Below 300 feet are at least four additional marine terraces, each averaging less than 0.3 miles in width, except on Camp Pendleton where the lowest marine terrace is up to 2 miles wide. A paleo-sea cliff backs each marine terrace on its landward edge. The shoreline angle at the back of each terrace can be used as a proxy for the position of sea level at the time of terrace formation. In terms of geologic age, the oldest marine terraces are the topographically highest, and age decreases with decreasing elevation. The lowest marine terrace (shoreline angle at 30 feet above sea level) is approximately 80,000 years old, while the highest marine terrace (shoreline angle at 510 feet above sea level) is estimated to be 1,200,000 years old both within the Pleistocene Epoch (10,000-1.8 millions years ago).
So where are good places to do some ancient beachcombing? Although urbanization is rapidly reducing the number of areas available for viewing San Diego's ancient shorelines, there are still some good places left. At Torrey Pines State Reserve, a walk south along the modern beach from the mouth of Los Penasquitos Lagoon will take you to the prominent landform known as bathtub rock (also called flat rock). From this point, the "Beach Trail" leads up into the park. The trail first traverses the steep face of the modern sea cliff before reaching the bluff top, which is on the seaward edge of a narrow marine terrace. Although the cliff face exposes hardened sandstone layers of the 48-46 million-year-old Delmar Formation and Torrey Sandstone (Eocene age), the bluff top itself is formed of layers of very friable, or loosely cemented, and laminated fine sand of the 120,000+/- year old Bay Point Formation (Pleistocene age). The contact between the hardened sandstone layers and the friable sands is an abrasion platform. The friable sands represent ancient beach deposits. A close look reveals the presence of well-preserved shells of fossil molluscs, including shells of Tivela stultorum (Pismo Clam) and Donax gouldii (Gould's bean clam)*. Both of these species are characteristic inhabitants of modern wave-swept sand beaches, and their occurrence as fossils on the sand-covered elevated abrasion platform at Torrey Pines is evidence of an ancient beach. A paleo-sea cliff is preserved at the landward edge of this abrasion platform and can be seen by walking east from the bluff edge across the platform for about 200 yards.
At this point the trial leaves the low relief region of the marine terrace and begins to traverse up a white sandstone bluff formed in the Eocene-age Torrey Sandstone. This bluff represents the ancient sea cliff and is associated with a shoreline angle at about 70 feet above modern sea level. If you continue walking up the Beach Trail you will eventually reach a second marine terrace with a shoreline angle at about 330 feet above sea level. The abrasion platform for this terrace is eroded into white and pale yellow sandstones of the Torrey Sandstone. Brick red coarse-grained sandstones of the Lindavista Formation cover the platform. The near horizontal contact between the red and white sandstones is quite dramatic and can be seen in other areas of the Reserve. including along Torrey Pines Park Road, along North Torrey Pines Road, and in the Reserve annex on the north side of Los Penasquitos Lagoon.
Local geologists have estimated the 330-foot abrasion platform and its associated marine terrace sandstones to be approximately 700,000 years old. This same platform is quite widespread and underlies much of Clairemont, University City, and Linda Vista. Higher (and older) abrasion platforms and marine terrace deposits occur to the east and, although more difficult to observe up close, offer a good opportunity to see paleo-shorelines. Along the Interstate 15 corridor, between State Route 52 and Mira Mesa Boulevard (U.S. Marine Corps Air Station Miramar), the freeway crosses a broad marine terrace surface that is underlain by the 430-foot and 460-foot abrasion platforms and their associated marine terrace deposits. On the east side of the freeway the nearly horizontal mesa surface ends rather abruptly at the base of a low line of bluffs that generally parallel the roadway. These bluffs, now dissected by modern erosion, are remnants of a sea cliff that formed at the landward edge of the 460-foot abrasion platform approximately 1,000,000 years ago. At that time, everything to the west of this sea cliff was under water-so hold your breath and imagine waves breaking over the HOV lanes!
Of course, there are many other areas in San Diego County where Pleistocene shorelines can be seen, including at Point Loma (the Coast Guard lighthouse is on the 80,000-year-old marine terrace and Cabrillo Road traverses the associated paleo-sea cliff), at La Jolla (Ellen Browning Scripps Park is on the 80,000-year-old terrace, while the Prospect shopping area is on the 120,000 year old terrace), at Carlsbad (the flower fields are on the 200,000 year old terrace and extend up the slope of the associated paleo-sea cliff), and at Camp Pendleton (Interstate 5 crosses the 120,000-year-old-terrace, which is backed by its associated paleo-sea cliff). So, this summer when the coast is socked in by a thick marine layer, take a trip inland to look for ancient shorelines.
* No fossil collecting is allowed in the reserve.
Text by Thomas A. Deméré, Ph.D., Curator of Paleontology; Torrey Pines photo by Jim Melli; Salton Sea photo by Michael Field; illustration by Bradford O. Riney