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We are interested here in late Holocene sea levels and their effect on human geogrpaphy. A recent survey of 36 world wide Holocene deltas indicates that delta formation began about 8000 (cal) years ago (6000 BC)when sea level was roughly 50 ft. below present level. (1) The isotope and sea level curves indicate a sudden drop in level (values of 5-15 cm/yr) of about 14 meters (more than 40 ft.) at this time. Subsequent smaller (1 to 5 m) catastrophes have occurred since this time and show up on abandoned beach strands in Denmark. (2) Presumably life in lower river valleys was precarious until the rise
in sea level deccelerated at about 4000 BC. Along the Belgian coast, recent work shows that "two rather distinct retardations appear to be present; a more marked one at about 7500-7000 cal yrs. B.P. and a second one at about 5500-5000 yrs. cal B.P."(3)In other words, sea level rise was temporarily reversed at these times. The idea of irregular sea level rise, introduced my Fairbridge (1961), and subsequently dismissed by uniformitarian interpretations, has recently been reinforced by analysis of Australian coral reefs(4). Fairbridge's sea level curves are discussed in the Encyclopedia Brittanica. They had been suppressed in favor of the more uniform curve of Shepard, thought the irregular model is now coming back into vogue with the return to respectability of more catastrophic ways of looking at the data. The upper graph shows C14 dates for muds and vegetation in the San Francisquito
Bay and delta; C14 dates have been corrected to give calendar years, and
the effects of autocompaction have been removed. Comparison of these data
with the world wide data shown in the lower curves provides reasonable
latitude for short term sea level regressions without proving them. It
also appears that the the data are in close enough vertical agreement to
suggest vertical crustal stability in the late Holocene in San Francisco
Bay, though this is not the conclusion of Lajoie and the other authors
of this USGS study.
(1) Stanley, D.J. and Warne, D.F. (1994), Science vol. 265, p. 228. (2) Tanner, William F. (1995), AAAPG Bulletin 79/10, p. 1568. (3) Denys, L. and C. Baeteman (1995), Marine Geology vol. 124, p. 16. (4) Jarcombe, P., et al (1995), Marine Geology vol. 127, p. 1-44. (5) van de Plasshe, O. (1995), Marine Geology vol. 124,
p. 117.
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