Delta Environments

Stillstands (such as we have experienced for the past millenium) or reversals of sea level rise produce seaward delta buildup and static or temporarily lowered groundwater levels, leading to desiccation and subaerial weathering with profiles, often littered with cultural remains, in lower river valleys as shown in the lower Rhine-Meuse delta profiles below. Our late holocene civilization is currently producing such a soil horizon.

Previous cultures may be associated with similar stillstands or regressions; for example, the red bottom profile shows the probable profile of the lower Rhine valley at about 3000 BC.(5)

Similar lower-river sequence of sea level and sediment can be observed in San Francisco Bay. In these (and presumable all other deltas in regions of crustal stabilty) we should expect the occurrence of a soil surface with archaeological materials corresponding to the 3000 BC sea level reversal (regression) at a depth of 10 to 20 ft below existing sea and ground levels.

In fact the discovery of a skull radiocarbon dated at 3000 BC at the base of the recent alluvium in San Francisquito Creek conforms to the surface of late Hilocene alluvium dated at about 3000 BC (profiles taken from Lajoie's USGS study of holocene sediments in San Francisco Bay.

Global warming will presumably bury "our" horizon in much the fashion that the events of 3000 BC buried the mid-holocene culture.

(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.