Geology Reference
In-Depth Information
With uplift and intensive erosion of the land
during glacial periods when sea level was lowered,
rejuvenated streams began to cut deeply into flood
plains. Terrace gravels, recording the many changes in
stream levels, can be seen along the Sandy and Clacka-
mas rivers and in eastern Portland where the streets
rise in elevation toward Gresham. At that time the
ancient Willamette River ran southeast of Oregon City.
Here it was joined by the ancestral Tualatin River
flowing along the western margin of the Portland Hills.
Both rivers wandered over a wide floodplain resulting
from thick alluvial deposits which first filled the
Portland and Tualatin basins, then backed up the
stream valleys, and finally, in places, covered the
divides between the streams. As sea level continued to
drop, the Tualatin flowed through the channel at Lake
Oswego while the Willamette established its present
pathway in the Columbia River basalts at Oregon City
where cutting action of the river produced the falls. In
the final stage, flood waters flushed out the sands of
the previously abandoned channel, diverting the
Tualatin back into its original southward pathway to
merge again with the Willamette. Today water from the
Tualatin River is channelled into Lake Oswego by
means of a low dam and ditch.
The rich dark soils of what may be a former
channel of the Willamette River, now called Lake
Labish, can be seen in a straight strip extending for
almost 10 miles northeast of Salem in Marion County.
The former course of the river was cut off during the
Pleistocene when a natural dam of sand from Silver,
Abiqua, and Butte creeks blocked the channel. The
resulting shallow lake slowly filled with silt and organic
Crown views of elephant teeth clearly distinguish
the low cusps of the browsing mastodon (top) from
those of the grazing mammoth (bottom). (Fossils
courtesy of the Thomas Condon collection, Univ. of
Oregon)
Willamette River sands and gravels. Bordered on both
sides by bluish clay overbank deposits, these coarse
pebbles trace the path of the ancient river as it began
filling the rugged erosional surface on top of the
Columbia River basalts and older Tertiary units in the
valley. Prior to the emplacement of the alluvial fans,
the Willamette River may have followed a more
easterly pathway in the valley. There is evidence that
the old channel between the southern and northern
valleys ran through a narrow gap at Mill Creek be-
tween Salem and the Waldo Hills.
About the size of a modern Indian
elephant, mastodon roamed in
herds throughout the Willamette
Valley during the Pleistocene.
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