Picture Gorge Basalts

Picture Gorge Basalts. John Day Fossil Beds National Monument. The dark layers of Picture Gorge were formed from seventeen distinct floods of lava flowing from nearby cracks in the earth. These basalt flows joined with others covering much of eastern Washington and Oregon, and northern Idaho, beginning about 16 million years ago. Powerful forces have since broken and tilted the land. Erosion has shaped it, the river cutting this gorge. , , The bottom two layers of the gorge are the same two that cap Sheep Rock peak, behind you. In recent centuries, American Indian pictographs were drawn upon the rock, giving the gorge its name. , , , As the surface of each new lava flow rapidly cooled to a crust, the interior remained hot and somewhat fluid for many years. Heat released above the hot flow into the atmosphere was much faster than heat released from the bottom into the earth. A jumbled, shattered appearance in the hardened, upper layer resulted from the quicker cooling, and subsequent cracking of the lava flow downward from the top. , , , Over 70 miles to the south, a violent volcanic explosion sent a fiery torrent of superheated gases, ash, and large particles sweeping across the surface of the land. This surge slowed and settled, welding into hard rock called ignimbrite. The high, flat-topped mesa, capped with his ignimbrite layers, is part of the Rattlesnake Group. , , , Vertical columns formed when the molten lave slowly cooled, contracted and cracked uniformly, upward from the bottom of the layer, forming six-sided pillars. In this region, basalt layers commonly feature both jumbled and columnar patterns, making each layer look like two layers.