Geology of Cabrillo National Monument
This article is text excerpted from “A Field Guide to Pacific Ocean Sea-Level History” by Phillip Kern, San Diego State University, in Environment Southwest, San Diego Natural History Museum, Winter 1987
On the eastern shore the entire length of the Pacific Basin is lined with prominent terraces from Alaska to Chile. The13-step-like raised shorelines on the Palos Verdes Hills in Los Angeles probably are the best locality on this shore, but scarcely a section of the California coast is without them. Among this great wealth of evidence of Pacific Basin sea-level fluctuations, however, San Diego County stands alone, for here we probably see the greatest number of shorelines and the most complete sea-level chronicle of all.
To appreciate fully the nature of this local sea-level record, you need to see the shorelines themselves, so several field trips are in order. Your first one should be to the shoreline that is now being carved, for there you can most easily see all the features of the sea-sculpted forms. A tide at least as low as zero feet is necessary to see the modern shoreline well; but minus one to minus two feet would be much better, so check the tide tables first (on the weather page of the newspaper, for example) for a suitable time. Then bring this guide with you on a trip to Point Loma.
Plan your departure to arrive there an hour or so before low tide, in order not to be swamped by the returning sea. Leave our vehicle in the first parking lot past the new lighthouse at the south end of the point. From there you can follow the Park Trail to the tidepools. Walk out across that low rocky platform, way out, as far as you can go to the southwest. Take time to admire the creatures that inhabit the rocky knobs and pools; they are part of the shoreline environment. Look for the heaps of their shells in pockets of sand and gravel that the high-tide waters have deposited here. If the tide is well out, you may be able to walk with dry feet a hundred yards or more beyond the southwestern most point of permanent land to the place where the gently sloping platform is swallowed by the sea.
This rocky surface you have crossed is properly called a marine abrasion platform, as it is now being carved by the sea. You would not want to walk here when the tide is high, for this is a place of great turbulence and the rocks are hard and sharp. The pounding waves and currents wield those boulders, shells, sand and other bits of abrasive grit that lie at your feet and use them to grind the surface down as if with sandpaper. That same abrading action continually undermines the adjacent sea cliff, frequently tumbling it down and eroding it landward. So long as the level of the sea does not change, the platform will grow progressively wider as well as deeper.
You stand here now on the shore of an interglacial sea, one that has a high level because of the melting of the great continental glaciers 15,000 years ago.
What would happen, though, if the climate were to deteriorate again, as it has repeatedly during the past million years and longer? As the atmosphere grew colder, snow would begin to accumulate at high latitudes. Eventually the vast sheets of ice would once again descend from the poles into North America, Europe and Asia, scraping away all that lay in their path. As hundreds of thousands of cubic miles of water gradually congealed into ice over a period of thousands of years, the level of the oceans would be drawn down. That shoreline at your feet would retreat from the land, and the platform would rise slowly above the waves.
After thousands of years, sea level might have fallen 300 feet or more below its present height, leaving your present low-tide vantage point two miles or more from the sea. The scattered pockets of sand and gravel might be spread by rain and wind into a more uniform layer, but soon they would be covered by more sand and mud washing down from above the former sea cliffs. The platform would become a true marine terrace, standing high above the sea, and it would remain in that position as long as the global glaciers reigned, probably for tens of thousands of years.
Eventually, of course, the climatic cycle presumably would swing back toward the present warmer conditions. The glaciers gradually would recede over more thousands of years, their melt-water flooding the seas aback toward their present level. Perhaps our descendants would stand on this shore when that encroaching coastline again became stabilized, and they – as we do today – might watch the surf renew its relentless sculpting of the shore.
The final outcome of this cycle, though, depends on yet another geologic factor. Different regions of the Earth’s surface differ in temperament and behavior. In stolid stable lands all those monotonously recurring high seas rise to about the same height, and their successive abrasion platforms commonly coincide with or obliterate earlier ones.
Southern California, on the contrary, is geologically hyperactive, and presently the land here is rising independently of the sea. By the time the next glacial-interglacial cycle is complete and the sea again has approached its present level, this platform probably will have risen beyond the reach of hose rasping waves. While another abrasion platform begins to grow, a new marine terrace will have been added to those that already encumber the San Diego coastline. That terrace would mark the latest advance of the land into the sea, as it begins its own escalator-like ascent of the coastal slope. Exactly this sequence of events has occurred here many times during the past thousand millennia. The San Diego coast has advanced miles into the sea, its carved landforms bearing witness to some of that history.
Now we walk back to the seacliff west of the light station. You will see that the low bench on which the buildings stand is just such a raised terrace. The base of the cliffs is carved in alternating beds of hard brown sandstone and softer gray mudstone. These rocks were deposited in a former sea that occupied this region 70 million years ago (late in the Cretaceous Period). Clearly visible in the cliff five to 10 feet below its top is the truncated edge of a very flat surface of abrasion like the one on which you are standing. The pockets of sediments and shells on the modern platform behind you are represented on this raised terrace by the immediately overlying thin layer of clean sand that washed down from the slopes to the east after the sea receded.
“But where is the sea cliff?” you ask, that should stand behind that platform? Walk south around the Point and follow the line of the raised terrace eastward past the lighthouse (Ed. note: this area is now in Zone 3 which is closed). The platform you are pursuing rises gradually to an elevation of 30 feet at a point about 200 feet east of the light; it ends abruptly at the base of its own steep sea cliff.
You have traced a complete cross-section through the Bird Rock terrace (named for a poorer exposure at Bird Rock in La Jolla). Its abrasion platform was carved during an interglacial high sea level 85,000 to 105,000 years ago. The fossil species that may be seen on its platform reveal that the ocean then was a bit cooler than it is here today, and sea level may have been correspondingly a few feet lower. After being abandoned by the sea, this surface subsequently has been carried 30 feet above its original level along with the rising land.
You are standing at an extraordinary geologic site. This terrace cross-section is without comparison in San Diego County, possibly in Southern California. Such a fine exposure of the one terrace alone would be exceptional, but there is a second one here as well. Look back to the outcrop. Follow upward with your eyes that raised sea cliff behind the Bird Rock terrace. At an elevation of 90 feet that cliff is truncated by a second abrasion platform. This one is not quite as conspicuous, but the large boulders on its surface should locate it for you. Trace the line eastward and in a short distance you will encounter the sea cliff backing that platform.
There you have the Nestor terrace (named for the community of Nestor, which is south of San Diego Bay). It was abraded 125,000 years ago by a high sea level at the peak of the last major interglacial interval (Sangamon). The fossil species it carries tell of ocean waters somewhat warmer than today’s suggesting that sea level – when the terrace was cut – may have been slightly higher than today. Still, its present height shows that Point Loma has been raised nearly 90 feet here since that terrace was formed.
As you walk back around the Point and toward the north, you will be able to see that these two terraces continue on toward Sunset Cliffs, where the Nestor terrace is equally conspicuous. Before you leave Point Loma, stop in the Visitors Center parking lot at the top of the ridge and walk toward the old lighthouse. Keep to the right and on the flat ridge-top north of the Park Service garage you can walk out across still another raised marine terrace. Peer over the edge to see the surf-rounded boulders embedded atop the Cretaceous sandstone in which this platform was long ago abraded. Though its age is not known, at least several hundred thousand years must have been necessary for buoyant forces in the Earth to raise this terrace 400 feet above the level of the sea.
Text excerpted from “A Field Guide to Pacific Ocean Sea-Level History” by Phillip Kern, San Diego State University, in Environment Southwest, San Diego Natural History Museum, Winter 1987
Last revised 29-May-13