Geology Reference
In-Depth Information
sediment of a large-scale flood. Extending over 300
square miles of the southern valley, the Irish Bend silt
reaches a maximum thickness of nearly 50 feet just
south of Corvallis.
With the separate silt layers in the Willamette
Valley suggesting multiple periods of flooding, the
precise number of floods is in doubt. Figures proposed
are "many floods", "35 floods", or "7 to 8 floods".
Whether the floods occurred in "two cycles", "annually",
"every 175 years", "over an extended period of time", or
were "short-lived" is still conjectural. These floods
continued over the 2,500 year interval until the ice
sheets permanently retreated northward with a warming
climate.
Varying in composition and size, the erratics
are granite, granodiorite, quartzite, gneiss, slate, and a
few of basalt. With the exception of basalt, these rocks
are common to central Montana and not the Willame-
tte Valley. Because of the exotic composition of the
erratic material, the path along which they were rafted
into the valley can be traced down the Columbia River
channel. Erratics were deposited through Wallula Gap
to The Dalles where they are found up to 1,000 feet
above sea level
.
In the Willamette Valley erratic
fragments are imbedded in the top of the Willamette
Silt.
The largest known erratic lies in the valley
between McMinnville and Sheridan in Yamhill County.
Composed of the metamorphic rock argillite, the
boulder originally weighed about 160 tons, but over 70
tons have been removed by tourists. Perhaps the most
famous erratic is the Willamette meteorite which may
have fallen in Montana only to be transported by
floodwaters to where it came to rest near West Linn in
Clackamas County. The meteorite was subsequently
purchased for $20,000 in 1905 by Mrs. William E.
Dodge who donated it to the American Museum of
Natural History in New York.
Erratics
Flood waters spilling into the Willamette
Valley carried large blocks of ice borne on the torrent.
Atop and within the icebergs, rocks and sediment were
transported all the way from Montana. Once the ice
melted, the stones were dropped as glacial erratics in
a wide pattern across the valley. Although more than
300 occurrences of these erratics have been recorded,
thousands more lay unrecognized. More than 40
boulders over 3 feet in diameter have been located, and
many smaller stones as well as chips and pebbles of
foreign material have been noted in farm fields, road-
cuts, and along old river terraces.
Structure
Uplift, tilting, and folding in the Coast Range
and Willamette Valley are attributed to the continued
subduction of the Juan de Fuca plate. Gentle folding
accompanied by faulting with as much as 1,000 feet of
vertical displacement produced deep valleys. Columbia
River basalts, that had filled the bowl-shaped Tualatin,
Wilsonville, and Newberg valleys in the middle Mio-
cene, were depressed to 1,300 feet below sea level by
steady eastward tilting of the Coast Range block.
Within the Willamette Valley, broad northwest trend-
ing anticlinal folds are interspersed with parallel,
subdued synclines. The dominant folds are the Portland
Hills, Cooper Mountain, and Bull Mountain, as well as
Parrett and Chehalem mountains. Separating the
anticlinal hills, wide valleys of Tualatin, Newberg, and
Wilsonville are gentle synclines or downfolds.
The Portland Hills anticline that formed
during the late Miocene and Pliocene is steepened on
the east side by a large fault that stretches for over 60
miles northwest and southeast of Portland. This fault
system extends southeastward to join the Clackamas
River lineament, an alignment of surface fractures on
a grand scale that traverses Oregon all the way to
Steens Mountain as part of the Brothers Fault zone.
Running parallel to this immense feature, the Mt.
Angel fault trends northwestward under Woodburn to
project into the Gales Creek fault zone of the Coast
Range. Northeast of Salem, the butte at Mt. Angel
The Willamette meteorite from near West Linn may
have been carried into the valley by an iceberg
during flooding (photo courtesy of Oregon
Historical Society).
