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Sea Cliffs, Beaches, and Coastal Valleys of San Diego County
Some Amazing Histories and Some Horrifying Implications
By Gerald C. Kuhn Francis P. Shepard University of California Press
Copyright © 1984 Gerald C. Kuhn and Francis P. Shepard
All right reserved. ISBN: 0-520-05118-1
Chapter One
General Description of Coastal Area The coast of San Diego County is a combination of both sea cliffs and lowlands, the latter occurring where valleys enter the sea and estuaries are cut off by barriers (fig. 1). Thus we have few embayed coasts, which are so common on the east coast and Gulf Coast as well as in the northwestern United States. The cliffed coasts are, for the most part, relatively straight, although considerable irregularities exist off Point La Jolla. This irregularity is not as pronounced now as it was a half-century ago, before waves had cut back some of the projecting spurs. The cliffed coasts are cut mostly in consolidated rock of somewhat uniform material, which usually results in straight coastlines. The cliffs vary in height; the highest portion is between La Jolla north to Sorrento Valley, where some vertical cliffs reach 350 feet. Except where the cliffs are on projecting points, they are bordered by sand beaches which are denuded from time to time, exposing underlying gravel and cobbles.
Another type of cliffed beach consists of unconsolidated alluvium. These cliffs are usually quite low, rarely exceeding twenty feet. Like the rock cliffs, the alluvial ones are quite straight and are bordered by sand beaches, except when the sand is temporarily stripped away to expose gravel or underlying rocks. The most extensive stretch of high alluvial cliffs is south of San Onofre, extending south through Camp Pendleton almost to Oceanside. Another example, of low alluvial cliffs extending a third of a mile, is at Scripps Institution of Oceanography in La Jolla.
The low coasts occur at the mouths of major river valleys. These lowlands were estuaries in the past, after the sea level rose owing to melting continental glaciers, which raised the level of the ocean by some 400 feet and flooded all these valleys, except where the rise in the land exceeded the rise in sea level. Considerable erosion from the highlands filled these estuaries sufficiently in recent years so that longshore currents were able to build barriers across their mouths; in the case of San Diego Bay, the entrance has been kept open by the strong tidal currents and probably to some extent by earth movements. Other estuaries are opened artificially to let floodwater out from time to time (fig. 2). The barrier beaches include one south of San Diego Bay, which extends all the way from the U. S.-Mexican border north to what were formerly the Coronado Islands (not to be confused with Los Coronados located off Baja California). The U. S. Coronado Islands have been tied to the mainland by predominantly north-flowing, sediment-laden currents (fig. 3).
A large U. S. Navy base and airfield on Coronado has considerably changed the contour of the former islands by ground-filling; the breakwater at the entrance to the bay has also played an important part, as has the building of high-rise condominium apartments at the north end of the spit. The importance of artificial earth movements, however, is not known.
One special type of coast is represented in the San Diego area along the north side of Point La Jolla. A row of sandstone bluffs with deeply penetrating caves at their base probably represents a fault scarp. There is no wave-cut terrace directly at the base of the cliffs, as would be produced by marine erosion. The cliffs descend to a moderate depth before encountering the rock shelf off the coast to the south. This certainly suggests down-faulting, which is entirely likely in view of the location in line of continuation with a fault at the north base of Mount Soledad.
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Excerpted from Sea Cliffs, Beaches, and Coastal Valleys of San Diego County by Gerald C. Kuhn Francis P. Shepard Copyright © 1984 by Gerald C. Kuhn and Francis P. Shepard. Excerpted by permission.
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