Rancho La Brea - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 252

287 onset of this glaciation exposed a flat

plain between the reduced Pacific and the
Santa Monica Mountains, on which were
numerous interconnected freshwater
lakes. Erosion of the mountains by rivers
led to the accumulation of fluvial sands,
clays, and gravels between 12 m (40 ft)
and 58 m (190 ft) thick, which gradually
raised the level of the plain.
Beneath this alluvial plain Tertiary
marine sediments of the Fernando Group,
consisting of shales and sandstones
interbedded with oil sands, acted as a
reservoir for the Salt Lake oilfield. These
sediments had been faulted, folded, and
eroded during the early Pleistocene to
form a north-east-south-west trending
anticline, and from around 40,000 years
ago crude oil began to seep upwards
towards the crest of the anticline and
into the overlying horizontally bedded
Pleistocene fluvial deposits. The lighter
petroleum evaporated, leaving sticky pools
of natural asphalt at the surface. Many
of the asphalt pools within Hancock Park
( 286 ) are aligned along a north-west-
south-east axis, suggesting that the oil
seepages may have originated from a
subsurface fault.
These shallow asphalt pools formed
natural traps for animals and plants
especially during the warm summers when
the asphalt would have been viscous.
Cooler winters may have solidified the
asphalt and covered it with river sediments
before the trap was reset the following
summer. Repetition of this annual cycle
produced conical bodies of asphalt (Shaw
and Quinn, 1986). Carcasses accumulated
in large numbers and it is likely that many
scavengers were lured to the pools by an
initial victim.
The excellent preservation of the
biota seems to be the result not only of
rapid burial, but also, and unusually, of
impregnation of the bones by asphalt.
Soft tissues are generally not present so
the deposit cannot strictly be considered
as a Conservation Lagerstätte, but the

Recent

? ?

Rancho la Brea
Biota

Wisconsinan
Glaciation
4

? ?

Minor
interglaciation 3

Illinoian
Glaciation
3

Major
interglaciation 2

Kansan
Glaciation
2

Minor
interglaciation 1

Nebraskan
Glaciation
1

End of Pliocene

287 Diagram showing the glacial and
interglacial stages of the Pleistocene, with
the position of the Rancho La Brea fauna
indicated after Stock, 1930.