PACIFIC NORTHWEST MAJOR LANDFORMS

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PACIFIC NORTHWEST MAJOR LANDFORMS

• A Brief Geological History

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700 to 800 million years ago

• Western border of Idaho was coastline, all areas
  to the west were open ocean. Inland areas were
  deserts
• This arrangement survived until 200 million
  years ago

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During Last 200 million years

• Northern Rocky Mountains rose along the western
  margin of the continent, areas E. of Rockies were
  inland shallow sea. (handout 1)
• Large and small scraps of coastal rocks attached
  themselves to the W. Coastline, assembling the
  geologic mosaic of the Pacific Northwest. Key
  features
• Klamath, Blue and Wallowa Mountains were groups
  of islands (terranes) far out in the Pacific that
  lodged against the western edge of North America
  (100-90 mya)
• At same time, other terranes jammed against the
  western edge of the continent, forming the North
  Cascades one large mass that became Vancouver
  Island and W. Coast of B.C.

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More recently

• N. America collides with Pacific seamounts
  (undersea volcanoes) to form core of Coast Range
  (OR) 40 mya
• Eocene (35-55 mya) tropical period leaves world
  class fossils (John Day formations)
• Western Cascades (volcanic) form 42-17 mya, as
  the oceanic plate sank through the trench off the
  west coast.

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Major Events 17 Million Yrs. Ago

• Western Cascades volcanoes go extinct
• Faults began to break former plains of Nevada,
  Utah into broad valleys jagged mountain ranges
  (Basin Range Province) see animation
• Immense flows of basalt spread across E. WA and
  OR and into Willamette Valley Coast (Cape
  Lookout is example)
• 15 million years ago, Olympic Mountains rise,
  mostly of oceanic crust

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lt17 million years ago (cont)

• Climate transitions from dry to wet

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12 million years ago

• High Cascades rise in line east of Western
  Cascades (Oregon) and on nearly same axis
  (Washington)
• 15,000 years ago The Great Lake Bonneville
  Flood

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Glacial Lake Missoula Floods

• Purcell lobe of regional ice blocks Clark Fork
  River, then breaks, at least 41 separate events
• Lake Missoulas volume exceeded that of Erie or
  Ontario
• Wall of water gt2000 high in places, moving
  perhaps 45 mphevidence?
• Erratics in high and far away places, shorelines
  of Lake Missoula, Channeled Scablands

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This is site of the Purcell Lobe, now Lake Pend
Orielle, Idaho (Sandpoint in the distance). In
some places the Lake is more than 1000 deep. At
the southernmost extent of the lobe, a large dam
known as a moraine was left behind, blocking the
St. Joe River and thus forming Lake Coeur d
Alene.
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