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Latah Formation

The Latah formation has had an interesting geologic history. Floods of lava advancing over the Columbia Plateau were checked by a line of hills near the present site of Spokane. Unable to enter the valleys to the east, the lava flowed south, thus damming their drainage. In time layers of clay and shale to a depth of some 1,500 feet accumulated on the bedrock of the lakes formed in the valleys behind these lava dams. In these Miocene lake beds are preserved fossil remains. Glaciers, however, advanced as far as this region, diverting many of the streams, including Latah Creek, southward; and, except where dams held back the rush of water, most of the soft clay and shale were washed away.