PACIFIC NORTHWEST MAJOR LANDFORMS A Brief Geological History

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

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 (~ 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.

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.

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

<17 million years ago (cont) Climate transitions from dry to wet

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

Glacial Lake Missoula Floods Purcell lobe of regional ice blocks Clark Fork River, then breaks, at least 41 separate events Lake Missoula’s volume exceeded that of Erie or Ontario Wall of water >2000’ high in places, moving perhaps 45+ mph…evidence? Erratics in high and far away places, shorelines of Lake Missoula, Channeled Scablands…

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.