Some 200 million years ago the western edge of the North American continent was about 60 miles west of where Spokane is now. As the Atlantic Ocean bottom split through volcanic activity the continent was pushed to the west at the rate of 2 or 3 inches per year. This action allowed the continent to collide with smaller island continents which added to the area we call Washington state. Over 180 million years or so the process continued until the current landscape was formed.
"About 20 to 30 million years ago, during the Miocene time, the landscape of central Washington consisted of mountains, valleys, streams and lakes. Trees, shrubs and plants flourished in a moist temperate climate. Sequoia, oak, elm, hickory, cypress, chestnut and other large trees, as well as the as the sacred tree of China, the ginkgo, grew in profusion.
During late Miocene and early Pliocene times, one of the largest basaltic lava floods ever to appear on the earth's surface engulfed about 63,000 square miles of the Pacific Northwest. Over a period of perhaps 10 to 15 million years lava flow after lava flow poured out, eventually accumulating to a thickness of over 6,000 feet. As the molten rock came to the surface, the earth's crust gradually sank into the space left by the rising lava. The subsiding of the crust produced a large, slightly depressed lava plain, now known as the Columbia Basin (Plateau). The ancient Columbia River was forced into its present course by the northwesterly advancing lava."
The image at left is basalt covered by caliche near Spokane. Caliche is calcium carbonate similar to hard water deposits on drying dishes.
"The lava, as it flowed over the area, first filled the stream valleys forming dams that in turn caused impoundments or lakes. In these ancient lake beds are found fossil leaf impressions, petrified wood, fossil insects, and bones of vertebrate animals. Between one and 25 million years ago, during Miocene and Pliocene times, several types of animals existed in the Columbia Basin area. Among these were the sloth, along with perhaps thousands of varieties of insects and fish.
Folding of the Plateau
With the end of the outpouring of lava, tremendous forces deep within the earth began to warp the plateau in several places. With a general uplift of the mountainous region in the north, the entire Plateau was tilted slightly to the south. This tilting and associated stairstep rock folds, called monoclines, in the vicinity of Coulee City and Soap Lake, played an important role in the formation of the Grand Coulee.
The Ice Age
With the beginning of the Pleistocene time about one million years ago, a cooling temperature provided conditions favorable for the creation of great sheets of moving ice called glaciers. Thus began the Ice Age.
Over the centuries, as snowfall exceeded melting and evaporation, a great accumulation of snow covered part of the continent resulting in the formation of extensive ice fields. Sufficient pressure on the ice caused it to start its outward flow as a glacier. This vast continental ice sheet reached a thickness of about 4,000 feet in some areas. The glacier moved south out of Canada, damming rivers and creating lakes in Washington, Idaho and Montana.
One especially large lake, covering a portion of Northwest Montana, played an important role in the formation of coulees (dry water channels). As this lake (Glacial Lake Missoula) grew in size it would eventually break through an ice dam, allowing a tremendous volume of water to rush across northern Idaho and into eastern Washington. Thus, catastrophic floods raced across the southward dipping plateau a number of times, etching the coulees which characterize this region today. The entire area is now known as the channeled scablands.
The Ice Recedes
With a moderation in the climate, the ice slowly retreated back to the north. The Columbia eventually returned to its original channel around the edge of the lava plateau in the Big Bend Country. The Grand Coulee and the network of other watercourses across the Plateau were left high and dry several hundred feet above the Columbia River."
As all this was happening the Cascade Mountains slowly rose higher and rainfall was less and less in the eastern plateau. This effect is called a "rain shadow" as in blocking rain clouds just as an object might block sunlight to form a shadow.
*-Excerpts from The Dry Falls Story, Washington State Parks.
The map at right by Ray Sterner, Johns Hopkins University shows the mountains and rivers (dark blue) of the area. The different colors indicate elevation and the red lines show state boundaries.
A full page map is also available.
You can see the channels and flat area of the plateau (orange,
yellow and green). Spokane, near the eastern edge of Washington, is where we begin. The Spokane River flows through the city and on to Franklin D. Roosevelt Lake, a part of the Columbia River behind Grand Coulee Dam.
For full text, see the Relief Map Text page.
Here are some basic terms used in the tour. Find more geology terms in the Glossary.
- Volcanic rock caused by partial melting of the Earth's crust.
- Solid layers of rock in the Earth's crust beneath soil.
- One or more layers of calcium carbonate deposited as water holding the carbonates in suspension evaporates. This is similar to hard water deposits on drying dishes.
- Long winding channel cut through lava formations.
- The study of the changes in landforms due to volcanoes,
earthquakes, weather, floods, etc.
- Metamorphic rock with bands or or streaks.
- Coarse-grained igneous rock usually without obvious bands or markings.
- Fine dirt deposited by wind usually from arid or glaciated areas.
- The period of geologic time that began about 24 million years ago and ended approximately ten million years ago.
- The period of geologic time that began about two or three million years ago and ended approximately 8,000 years ago.
- The period of geologic time that began about ten million years ago and ended approximately two or three million years ago.
- Rain Shadow
- A mountain or mountain range that blocks rain clouds just as an object might block sunlight to form a shadow. Areas in the shadow are more dry as a result.
- Collection of sand, silt, gravel and organic material that sinks to the bottom of a river, lake or ocean. Some or all of these materials may be present.
- A vent at the surface where magma, gas and steam erupt. Also, the landform constructed by volcanic material.
Kids' Cosmos… Expanding Minds Beyond the Limits of the Universe
P.O. Box 14077, Spokane, WA 99206-4077
© 2011 Kid's Cosmos
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