Geology Reference
In-Depth Information
Pleistocene Lake Labish northeast of Salem may
represent an older channel of the Willamette
blocked by sediments from the Western
Cascades (after Glenn, 1962).
Basalt.
The Pliocene lavas covered
the gravels in the old lake bed, and
today they make up many of the
buttes near Gresham and Boring as
well as capping a number of hills in
the vicinity of Oregon City. Rocky
Butte, Mt. Scott, Mt. Tabor, Kelly
Butte, and Mt. Sylvania contain cin-
ders and lavas of these flows, the most
westerly of which is along the slopes
of the Tualatin Valley near Beaverton
and Metzger. La Butte southwest of
Wilsonville in Marion County may be
the most southern eruption of these
lavas. The landscape in the valley and
around Portland shows clear evidence
of these late eruptions. Flows from
Highland Butte southeast of Oregon
City pushed the Willamette River ten
miles to the northwest near its pres-
ent course, whereas in Portland lavas filled old stream-
beds where Burnside Street and Canyon Road are now
located. Rocky Butte, a volcanic vent that erupted
approximately 1.2 million years ago, and Mt. Tabor,
with a small cinder cone projecting from its north side,
are presently in downtown Portland. Across the river in
Washington, Bobs Mountain northeast of Portland and
Battle Ground crater north of Vancouver still have
intact cones.
Pliocene
During the Pliocene epoch, continued subduc-
tion of the Juan de Fuca plate and growth of the
offshore accretionary wedge brought about the renewed
uplift and tilting of the Coast Range and gentle folding
of northern Willamette Valley rocks. The end of the
wide coastal plain marked the beginning of the Willam-
ette Valley as a separate physiographic feature once the
sea had withdrawn to its present position. At this time
major folds of the coastal mountain range formed, and
outer continental shelf environments were raised over
a mile from 1,000 to 2,000 feet below sea level to
elevations of 3,000 feet above.
About 5 million years ago in the northern
valley Boring Lavas erupted from over 100 small vents,
cones, and larger shield volcanoes as Mt. Defiance,
Larch Mt., Mt. Sylvania, and Highland Butte. Boring
Lavas can be readily distinguished from the older
Miocene Columbia River basalts by their fracture
pattern. The fine-grained, light-weight Boring lavas
break into large blocks and are seldom found in the
small columns so common to the Columbia River
Basalt.
Pleistocene Rivers
Just prior to deposition of the Willamette
Formation, up to 150 feet of coarse gravel and sand of
the Rowland Formation spread over a broad area in
the southern Willamette Valley. Divided into two
members, the Rowland represents glacial outwash from
the Cascades that was flushed into the valley by the
North and South Santiam, Calapooia, and McKenzie
rivers. This massive outwash unit thins toward the
northwest and forms a complex series of coalescing
alluvial fans that have bulldozed the Willamette
channel off to the western margin of the valley between
Eugene and Corvallis.
The valley beneath the Rowland glacial-alluvial
fan sequence is filled with almost 300 feet of proto-
