Picture Gorge’s dark layers were formed by seventeen distinct floods of lava that flowed from nearby cracks in the earth and joined other basalt flows covering much of eastern Washington and Oregon and northern Idaho, beginning about 16 million years ago. Powerful forces later broke and tilted the land, and erosion shaped it as the river cut the gorge. The bottom two layers of the gorge are the same two that cap Sheep Rock peak behind it. In recent centuries, American Indian pictographs drawn on the rock gave the gorge its name. As each new lava flow rapidly cooled at the surface to a crust, its interior remained hot and somewhat fluid for many years. Heat escaped far faster upward into the atmosphere than downward into the earth, producing a jumbled, shattered upper layer through quicker cooling and later cracking from the top downward. More than 70 miles to the south, a violent volcanic explosion sent a fiery torrent of superheated gases, ash, and large particles across the land; when the surge slowed and settled, it welded into hard rock called ignimbrite. The high, flat-topped mesa capped with these ignimbrite layers is part of the Rattlesnake Group. Vertical columns formed where molten lava slowly cooled, contracted, and cracked uniformly upward from the bottom of a layer, creating six-sided pillars. In this region, basalt layers commonly show both jumbled and columnar patterns, making each layer look like two layers.