The Geologic Story of the Columbia Basin
The rock formations of the Columbia Basin were formed by some of the most unusual and
catastrophic processes in geologic history.
Early History - 40-60 million years ago:
During the early stages of the Columbia Basin formation, granite rock was slowly created by heat and pressure deep in the crust of the earth. Then the crust was uplifted, exposing the granite, creating mountains similar to the Okanogan Highlands north of Grand Coulee Dam.
Forty to sixty million years ago the formation of the outline of the Columbia Basin was complete. The land had subsided below sea level, and a large inland sea had formed.
The land was again uplifted and then, 10-15 million years ago, was flooded with volcanic lava. The boundaries of the flood lava were located in almost the same position as the former seashore. Many layers of lava were needed to build up to a 5,000 feet (1500 meter) thickness and form the smooth surfaced Columbia Plateau.
Ice Age - 2 million years ago
The Ice Age (or Pleistocene epoch) began at the end of the Pliocene epoch, 2 million years ago. Glaciers 5-10,000 feet (1500-3000 meters) thick in northern Washington pushed down the Okanogan Valley and crossed the Columbia River near the present site of Chief Joseph Dam. Ice filled the Columbia Valley and pushed onto the Waterville Plateau as for south as Coulee City. Water, backed up by the ice dam, spilled over the Columbia Plateau into the Columbia Basin.
During this time, the Grand Coulee began its process of formation since the original river channel had been lost by burial in ice. The length of time that it took to form the Grand Coulee has been the subject of much controversy. Some geologists think that it was formed by a succession of large floods; others believe that it was formed by a gradual process of erosion as the Columbia River sought to form a new river channel.
During the Ice Age, the old Cascade Mountains were also formed. Their outline still remains on the western slopes of the Cascades. The uplifting mountains were not able to block the flow of the Columbia River completely, and a deep Columbia River gorge was formed.
Near the end of the Ice Age the volcanoes of the high Cascades rose to elevations of 14,000-15,000 feet (4000-4500 meters). Older volcanoes, such as Mt. Hood and Mt. Rainier, were sculpted by glaciers of the Ice Age; others such as Mt. St. Helens remained unsculpted, retaining their original volcanic form.
Lake Missoula Flood - 18 thousand years ago
Eighteen thousand years ago the Columbia Basin was nearly covered by floodwaters when an ice dam at Lake Missoula in western Montana broke. Large boulders were strewn near the outlet of the Lower Coulee (Lake Lenore). Other boulders were carried in icebergs as far as western Oregon. The floodwaters were 800 feet (250 meters) deep near Pasco and 400 feet (125 meters) deep at Portland.
After the Ice Age, the Columbia River returned to its former channel. The channeled scab lands and large coulees that had been formed were left stranded 500-1600 feet (150-500 meters) above the present river floor and serve as a constant reminder of some of the most unusual episodes in geologic history.
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