Geology Reference
In-Depth Information
debris to become a marsh. Thick peat deposits in the
old lake reflect a long period as a swamp and bog. In
this organic layer, bones of Ice Age mammals such as
mammoths, mastodon, giant sloth, and bison are
frequently found. Unlike the Pleistocene LaBrea tar
pits of Los Angeles, the animals were probably not
mired in the peat, but carcasses were washed in and
covered allowing the remains to be preserved in the
oxygen-poor bog away from the attentions of scaven-
gers. Today the fertile soils of Lake Labish support a
thriving onion industry.
strata in southwest Washington indicating that the
sediments are wind blown in origin. The Yellow River
in China derives its name from comparable Pleistocene
loess deposits that blanket vast areas in the northern
part of the country. Ground up rock flour, produced by
the crushing and milling action of glacial ice, was trans-
ported by water to be deposited along flood plains of
the Columbia River. Strong Pleistocene winds, collect-
ing the fine dust, carried it aloft in enormous clouds to
cover the Portland Hills. Four different layers of silt
are separated by three soil horizons. Interglacial, warm
intervals are represented by the silt deposits, whereas
soil horizons reflect times of glacial advance.
Pleistocene
Covering the Boring lavas and Troutdale
gravels, a gritty, structureless, yellowish-brown sediment
called the Portland Hills Silt was deposited within the
last 700,000 years. Commonly 25 to 100 feet thick, the
layer mantles much of the Portland area from the
Tualatin and Chehalem mountains on the west all the
way to the Gresham Hills and Ross Mountain on the
east. Microscopically the silt is remarkably uniform,
and the identified minerals include many which must
have been derived from terrains as far north as Canada.
This unique formation has long been something of a
geologic puzzle and has been designated as both a
wind-blown and water-laid deposit. The physical
characteristics of the silt are similar to the palouse
Ice Age Floods
Beginning about 2 million years in the past,
the Ice Ages mark the advance and retreat of continen-
tal glaciers, an event that triggered one of the most
catastrophic episodes in Oregon's geologic history.
When first proposed in the 1920s by J. Harlen Bretz,
the theory of an enormous flood washing across
Washington and through the Columbia River gorge
was not readily accepted. Careful work by Bretz,
however, built up a body of evidence that could not be
ignored. Between 15,500 and 13,000 years ago, the
Columbia River drainage experienced a series of
spectacular floods from ruptured ice dams along its
Four large lakes mark the source and pathway of
the catastrophic glacial floods (after Allen, 1986).
