In 1958, a geomorphic study of the coastal sand dunes of Oregon and Washington by this author was published as Memoir 72 of The Geological Society of America; it was part of a project designed to cover the entire Pacific coast of the United States. The present work, dealing with the coastal dunes of California, completes the geomorphic investigation; five localities in northern Baja California are included.
In the geomorphic history of these dunes the most significant event is the eustatic rise of sea level that accompanied the final waning of the continental ice sheets, known as the Flandrian transgression. The processes of the post-Flandrian period—the last few thousand years, characterized by comparative stability of sea level—have been essentially a continuation of those of Flandrian time.
The rising gradient of temperature southward on the coast of California is more than twice as steep as on the Washington-Oregon coast. Precipitation at Crescent City in extreme northern California, 74.87 inches average annual (1902 mm), is as high as in Oregon. Between Crescent City and Eureka it is more than cut in half. Southward from Eureka the decrease is gradual to 10.86 inches (276 mm) at San Diego and 5.2 inches (132 mm) at San Quintin, Baja California. Winter concentration is characteristic every-where; the summer dry season increases in length and intensity southward.
The most striking change in wind regime as one goes southward is the increase in all-year importance of the northerly components. Increase of northerlies coupled with decrease of southerlies in winter is added to universal predominance of northerlies in summer. The resultants of those winds that exert control over the orientation of dune features—the “effective winds”—are regionally closely similar; they lie within the octant centered on the northwest compass direction. The predominant direction of coastwise transport of sand by longshore currents is southward. Dune masses occur where the coastal trend jogs westward, presenting receptive shore to littoral drift from the north.
Over much of the coast there is an imperfectly developed foredune zone consisting of hillocks originating from single plants of more or less succulent species, which are coalescent to some extent. The zone often acquires a pattern of ridges and troughs oriented with the “effective wind.” The exotic marram grass is extremely important where it has become established. Active dune masses commonly lie behind the foredune zone or extend to the beach. If considerable in extent, they acquire a transverse-ridge pattern. In the extreme north precipitation ridges develop where dunes advance against a forest barrier; in time, these become stabilized, still retaining their characteristic form. The parabola form occurs on the major dune sheets, where it forms a pattern; its units, after stabilization, are narrow, commonly overlapping, and frequently very long. Giant parabolas, which in Oregon occur singly or in small groups, are absent in California.
Twenty-seven dune localities were investigated on the coast of California and five in northern Baja California. Special field study was given to two dune areas: at Monterey Bay and Santa Maria River, because of their great extent and variety of features. A third locality, San Francisco Peninsula, was originally comparable in magnitude, but urbanization has little by little obliterated the dune features; it was necessary here to depend in the main upon accounts by early visitors, early maps, and recent studies by geologists involving numerous borings.
There are two major classes of dunes on the basis of age: Flandrian and pre-Flandrian. Flandrian dunes are present in every locality. They represent the contemporary phase in a process that has continued through the period of the Flandrian transgression and post-Flandrian time. Their forms are those characteristic of retrograding shores. Building, destruction, and rebuilding have gone on constantly, while the dune belt as a whole has moved inland.
In eight well-distributed localities the Flandrian dune belt comprises two waves of invasion, termed “episodes.” The dunes of Episode I, the inner and older, are at present stabilized, except for small areas still active, by forest in the extreme north and by the “dune shrub community” on the remainder of the coast. The dunes of Episode II, active and with little vegetation, extend to the beach or merge with an imperfect foredune. They are invading the masses of Episode I where these are present. In several localities dunes of Episode I are perched at the tops of cliffs. There being no receptive shore, Episode II is absent.
In four localities, Monterey Bay, Morro Bay, Santa Maria River, and El Segundo, there are extensive dune fields manifestly of much greater age than the Flandrian dunes. From the inner margin of the latter they extend several km inland. Their topography is, in most areas, definitely of dune character; there are occasional unmistakable parabola forms. All these features are much softened; in some places the surface is merely gently rolling, with numerous undrained depressions the only manifestation of eolian origin. Stabilization is complete; the prevailing vegetation type is chaparral. A definite soil profile commonly is present. The history of the existing pre-Flandrian dunes is obscure; at Santa Maria River it may extend back through more than one glacio-eustatic cycle.
The final section presents a general geomorphic survey of the dunes of the Pacific coast of the United States, covering the materials of the present work for California and northern Baja California and of Memoir 72 (Cooper, 1958) for Oregon and Washington. The influence of climatic factors on dune phenomena is considered for various parts of the coast. The dune forms occurring on this coast are described and their geographic distribution indicated. The general dune history of the coast is summarized: an essentially uniform sequence of events has prevailed over the entire length of the Pacific coast of the United States. It is assumed that these events, in the main, have been corollaries to glacio-eustatic changes of sea level. In support of this assumption several similar dune sequences on other widely distributed coasts are cited.