Geoscience Reference
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Ice Age (AD 1418 and 1605). h e thinnest l ood layer occurred during the
Medieval Climate Anomaly, in A D 1029 (see i gure 28). h ese results suggest
that periodic megal oods may be a normal part of the larger cycle of climate
in this region, and the Medieval Climate Anomaly may have been anomalous
in part for its absence of such l oods.
h e largest l ood to hit the Santa Barbara Basin over the past two millen-
nia occurred in AD 1605, leaving a sediment layer two inches thick. Almost
as large, the AD 440 and 1418 l oods each had a thickness of one and a quar-
ter inches. For perspective, consider that the two major l oods that struck
the region during the twentieth century—the l oods of 1958 and 1969—let
sediment layers that were only 0.24 and 0.08 inches thick, respectively. In
its entire history as a state, California has yet to experience one of the mega-
l oods that Schimmelmann found to be a repeating part of the region's natu-
ral climate. Schimmelmann points out that the periodicity rel ected in his
record suggests the region is due for another one soon; it has been more than
four hundred years since the last megal ood, which occurred in AD 1605.
the mojave desert under water
For most of the Holocene, the Mojave Desert of southeastern California has
remained one of the driest places in the West, with annual rainfall less than
four inches per year in recent times. Only during the wettest years of the
twentieth century (every i t een to thirty years or so) did small ephemeral
lakes form in the region, lasting just two to eighteen months before dry-
ing up. However, paleoclimate researchers have examined sediment cores
extracted from a playa located in the Mojave and found evidence of past
periods that were wet enough to form much larger and long-lived lakes. h is
playa, called Silver Lake, lies at the terminus of the Mojave River. During
the late Pleistocene, Silver Lake was much larger but has remained mostly
dry since about 8,000 years ago. Only during two periods since the late
Pleistocene were conditions wet enough to create a lake that lasted several
decades.
Cores taken from the Silver Lake playa reveal two layers of clay sediments
deposited within the lake. Within those cores, the lake sediment layers
lie between sediments deposited by the Mojave River on the desert l oor.
Radiocarbon dating of the lake sediments reveals two extremely wet periods
in the past: the Neoglacial (about 3,600 years ago), and the Little Ice Age
