Articles - MALIBU'S RISING COASTAL SLOPE

It is well understood from geomorphic features, i.e., landforms, and other evidence that the Santa Monica Mountains are rising in relation to sea level. Direct temporal evidence is, however, confined to the coastal slope in Malibu. Of the few authoritative studies of the coast, the first was that by William Morris Davis. Davis was referred to as the "Father of American Geography," at a time when the term "geomorphology," meaning the study of landforms, had not yet been adopted. Davis graduated from Harvard University in 1869 and thereafter pursued a lengthy professional and largely academic career. Toward its end, he became associated as a guest lecturer with the California Institute of Technology, Stanford University, and the University of California, Berkeley.

While on a pleasure drive in 1927 along the newly opened State Highway in Malibu he became interested in the geomorphic character of the coast. Pursuing that interest, in 1929 he became aware of marine terraces in the Point Dume "triangle" which became the basis of a paper an early draft of which he presented at annual meetings of the Geological Society of America in 1931. Later published (Davis, 1933), that paper contains fascinating photographs, sketches of the local coastal localities, and considerable analytical data, a review of which is well worth the time of the student of Malibu history or geomorphology. Essentially, Davis postulated three abrasional features in the vicinity of Point Dume he called the Malibu, Dume, and Monic platforms. For present purposes, "platform" is to be considered synonymous with "marine terrace."

We now know that there is a fourth such feature, called the Corral terrace, evidence for which also is present at Point Dume of which Davis was not aware. For this, he is not to be criticized because during his field work he was not permitted entry to the Point Dume area which was fenced off south of the State Highway. He notes that despite "... urgent and repeated request for admission .... to the jealously guarded pastures that extend from the highway to the extremity of Point Dume, permission to cross them has not been granted by the owners of the extensive estate in which the Point is included..." (op. cit., p. 1090). He of course is referring to the Rindge interests at that time, either May Knight Rindge herself or managers of her Marblehead Land Company.

Davis is best known for his work, the first of its kind, which explains the cycle of stream development from youth to old age assuming a static landscape. Today, his theory is challenged as simplistic because it does not take into account tectonic and other dynamic forces that could affect the natural processes of stream development dependent only on gravity. Nevertheless, this failing, such as it is, does not detract from his acute observations of geomorphic features in Malibu.

Clarification of Davis' work had to wait some 30 years for the development of the uranium-thorium technique for dating carbonate-bearing materials such as sea shells. In the 1960s, with growing national concern about energy and the environment, R.F. Yerkes and C.M. Wentworth of the U.S. Geological Survey were assigned the task of studying the area at the mouth of Corral Canyon for the purpose of determining whether it would be an appropriate site for a nuclear power plant. As ill-advised as this proposal was, their study (Yerkes and Wentworth, 1965) is a valuable example of geologic methods of site examination. The most important result of their study, other than demonstrating that the site was inappropriate for a nuclear plant site, was the discovery of a marine terrace intermediate in elevation between the Malibu and Dume terraces Davis had recognized. They designated this feature "terrace C," and Davis's Dume terrace, "terrace D." Shells from marine deposits on these terraces gave ages of 280,000 and 130,000 years, respectively (op. cit., p. 158; App. C). However, it remained for Birkeland (1972) to show that terrace C, which he called the Corral terrace, occurs in a limited exposure close to the seawardmost extent of Point Dume (op. cit., Fig. 4; p. 436). Birkeland gives an average age for the Dume terrace as 104,000 years and for the Corral terrace as 131,000 years (op. cit., Fig. 8, pp. 436-437; 443). He does not address the age differences with respect to the ages reported by Yerkes and Wentworth which probably can be attributed to advancement in the dating technique that was used.

The Malibu marine terraces can only be explained as a result of a rising coast not necessarily, but probably, coupled with changing sea levels responding to glacial and interglacial epochs. These ages in years before the present, together with the dates for the Corral and Dume terraces suggest the events noted in Table 1.

Table 1. Postulated Glacial-related Events, Malibu.

Years BP	Epoch	Comments
Earlier than 198,000	Late Yarmouth Interglacial	Deposit of alluvial cover over previously abraded Malibu terrace.
198,000 -134,000	Illinois (Riss) Glacial	Corral terrace date of 131,000 yrs. bp requires coastal downwarp necessary to allow for abrasion even though sea level was relatively low because glaciation still in progress. Explains the narrow width of the Corral terrace.
134,000 - 95,000	Sangamon Interglacial	Uplift maroons the Coral terrace. Advance of sea from melting glaciers causes post-uplift abrasion of the Dume terrace during much of the Sangamon Epoch.
95,000 - 10,000	Wisconsin Glacial	Cessation of Dume terrace abrasion either due to sea retreat or uplift, subsequent renewed wave attack begins forming the modern sea cliffs and abrasion of the Monic terrace now in progress. At 18,000 BP, beginning of Flandrian transgression.
10,000 - present.	Holocene Interglacial	Sea advance now continuing with seacliff height increase and Monic terrace enlargement. Continuation of Flandrian transgression.

The elevation of a marine terrace is taken as that of its shoreline angle, i.e., its landwardmost edge at the base of the sea cliff formed by wave erosion. At Point Dume, the mean sea level elevations of the Malibu, Corral, Dume, and Monic terraces are about 200 - 250 feet, 150 feet, (Birkeland, 1972, p. 435), 50 feet (op. cit., p. 436) and zero, respectively. Such elevations vary considerable in other areas of Malibu because of warping, i.e., differential elevation changes in response to a single tectonic episode. The available evidence strongly suggests that the Malibu coast is still rising. At Point Dume, the 150-foot elevation of the Corral terrace, which has occurred over the previous 131,000 years, suggests a rate of rise of 0.001 feet per year. This would mean a period of 1.2 and 1.5 million years during which the Las Virgenes dissected plain has reached its present elevation of between 1200 to 1500 feet.

Other than a rising coast, only two other conceivable might be suggested: [i] fluctuations in sea level due to a constant change in sea bottom elevations which, independent of changes in the volumes of water periodically entrapped in glaciers, are large enough to cause a constant lowering of sea level; [ii] insignificant changes in sea bottom elevations, but rather a constant lowering of sea level due to increasingly large glacier volumes. However, there is no demonstrable mechanism for [i]. Although this possibility cannot be entirely discounted, for such to have been occurring even during historic time, it should not have escaped notice somewhere in view of extensive available bathymetric data. Condition [ii] clearly does not apply, because the advent of the Holocene interglacial epoch and generally a recession of glaciers since the end of Wisconsin glacial epoch are beyond question.

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