Page 27 - (DK Eyewitness) Travel Guide - Pacific Northwest

A POR TR AIT OF THE P A CIFIC NOR THWEST  25

Glaciers are masses of
ice that advance and Plate Tectonics
retreat, scooping out
deep gorges and Three main forces are responsible
for the formation of mountain
sculpting jagged ranges such as the Rockies or the
mountain peaks.
Continent-sized glaciers Cascades. First, large areas of the
Earth’s crust (known as tectonic
are known as ice sheets. plates), constantly moving
About 15,000 years ago,
the Cordilleran ice sheet together and apart, created
uplift. Second, the North
covered much of American plate, subducted by
Washington and British the Pacific plate, caused a chain
Columbia; it was 4,000 ft of volcanoes to form from the
(1,219 m) thick in places. molten rock of the oceanic crust.
When it melted, Third, erosion caused by ice ages
the raised water levels deposited sedimentary rocks on
of the Pacific Ocean the North American plate, which
filled two of the deepest was then folded by more plate
gouges, creating Puget movement between 50 and
Sound and the Strait of 25 million years ago.
Juan de Fuca.
Volcanoes North America plate
Pacific plate

Some 150 million years
1ago, the Pacific plate
moved east, adding to the
molten rock from great
depths of the North
American plate. This then
rose up to form the Western
Cordillera Mountains.
Pacific Sediments
plate

The Cordilleras were eroded
2over millions of years and
during various ice ages.
This led to sediments being
deposited in the sagging,
wedge-shaped crust east
of the mountain range.
Cordillera Rockies
Mountains
Gorges were formed at
the end of the last ice age,
when massive floods were
triggered periodically by
melting glaciers. These
floods etched out deep Around 50 million years ago,
narrow chasms such as 3the Pacific plate continued
the one shown here, or to push east, forcing the
much wider ones such Cordillera range eastward,
as the Columbia River compressing sedimentary
Gorge, which forms rocks, folding and uplifting
the boundary between them to form the Rockies.
Washington and Oregon.

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