Geology Reference
In-Depth Information
Cascade Mountains
Physiography
With its high glaciated volcanic peaks, the
Cascade Mountain physiographic province is easily the
most dramatic and scenic in the state. Extending south
into California and north into Washington and British
Columbia, the Cascade range is actually two parallel
chains. The older, deeply eroded Western Cascades
sharply contrast with the more recent snow-covered
High Cascade range that sits on the eastern flank of
the older range. The Western Cascades, with heights of
1,700 feet at the western edge to 5,800 feet on the
eastern margin, are only half the elevation of the High
Cascade peaks which reach altitudes of more than
11,000 feet. This extensive province divides the state
into western and eastern parts and touches all of the
physiographic regions in Oregon except the Blue
Mountains and Coast Range. The Cascades are bound-
ed by the Willamette Valley on the west, by the
Deschutes-Columbia Plateau on the northeast, by the
High Lava Plains in the central region, with the Basin
and Range to the southeast, and the Klamath Moun-
tains to the southwest.
Around 60 to 100 inches of rainfall annually
result in deep weathering that is a characteristic of the
Cascades where major westward-flowing rivers drain
the range. North of the Calapooya Mountains, the
Sandy, Clackamas, Santiam, McKenzie, and Middle
Fork rivers all wind their way from headwaters in the
mountains through an irregular pattern of dissected
ridges and valleys before entering the Willamette River.
South of the Calapooya divide, streams as the Umpqua
and Rogue flow into the ocean. Across the Cascades,
streams flowing eastward are small in comparison.
Toward the north end of the range, creeks such as
Fifteenmile and Threemile as well as the Warm Springs
and Metolius rivers all reach the Deschutes River
which has its headwaters near Mt. Thielsen. The few
creeks that occur south of Mt. Thielsen drain into the
Great Basin and ultimately empty into Upper Klamath
Lake.
At that time the coast ran northwest through the
region of the Willamette Valley. Triggered by move-
ment of enormous crustal plates, intense volcanic
activity produced thick accumulations of lava and ash
that built up the older Western Cascade volcanoes.
Between the eruptions, erosion, assisted by subtropical
climates, stripped volcanic material from the slopes to
redeposit it along the coastal plain and in the shallow
near-shore waters of the ocean.
With uplift of the landmass, the Tertiary sea
retreated progressively westward. Volcanic activity
shifted to the east in the Miocene and Pliocene when
folding and tilting were followed by massive outpour-
ings of lavas in the range as well as throughout Ore-
gon. At this time, large Cascade stratovolcanoes
extruded thick lavas. About 5 million years ago the
Western Cascades were again tilted creating a sloping
ramp on the west side and a steep face on the east. It
is this ramp of the Western Cascades and not the
dominant and more obvious peaks of the High Cascade
range that casts a rainshadow over eastern Oregon,
dramatically changing the climate.
As the westward tilting took place, another
episode of volcanism from local vents resulted in the
more pronounced cones that form the High Cascades
today. The High Cascade peaks appear in sharp con-
trast to the flat lava plains of eastern Oregon and the
Geologic Overview
The geologic story of the Cascades begins
around 40 million years ago with eruptions from a
chain of volcanoes just east of the Eocene shoreline.
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