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| Sheep Rock Peak (the highest peak on the right) provides one of the most complete stratigraphic sections of the John Day Formation in the region including its three members: Big Basin Member, Turtle Cove Member, and Haystack Member, oldest to youngest respectively. The lower slopes of the hill consist of the Big Basin Member. The ledge-forming brown layer running diagonally across the mountain is the Picture Gorge Ignimbrite (a massive volcanic tuff unit in the middle to lower part of the Turtle Cove Member). The upper part of the peak consists of the slope-forming Haystack Member. The highest point of the mountain consists of Picture Gorge Basalt and beds of the Late Tertiary Rattlesnake Formation. The terrestrial-deposited stratigraphic sequence spans the period of time from roughly 40 to 7 million years ago, representing one of the most complete and accessible Tertiary sections in western North America. |
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