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| Steamboat Rock is a mesa of Columbia River Basalt that rises in the middle of the Grand Coulee (now flooded by Banks Lake Reservoir). Note the light-colored granitic rock outcrops along the shore. These granitic rocks formed in association with a great volcanic arc that formed in the region during the middle to late Mesozoic (Jurassic and Cretaceous time, roughly between 200 and 100 million years ago). The volcanoes in the region eroded away long before the basalt flows flooded the region in late Tertiary time. |
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