Nearby hills record a long history of volcanism, uplift, and erosion. Most were formed from abundant volcanic ash-falls and floods of lava over many millions of years. About five million years ago land-building slowed, and erosion cut down into earlier layers to create the landscape seen today. The ancestral Cascade Range, now reduced to hilly remnants to the west, once sent repeated clouds of ash across this area. Those ash-fall beds make up the John Day formations: the Big Basin, Turtle Cove, and Haystack Valley strata, which reveal ancient changes in regional geology and life forms. Later, massive forces tilted these layers downward to the east, and floods of lava spread across the surface as flat molten lakes. These basalt floods hardened and protected much of the softer layers below from erosion. The Picture Gorge Basalts formed sixteen million years ago, when more than sixty lava flows from nearby cracks in the earth covered over two thousand square miles, roughly one flow every eight thousand years, creating much of the John Day country’s rimrock. Haystack Valley formed twenty to twenty-two million years ago from episodic heavy ash-falls from a mature ancestral Cascade Range, and its soft yellowish rock commonly contains fossilized mammal bones. The Picture Gorge Ignimbrite formed twenty-eight million years ago, when an explosive eruption to the west sent a fiery tidal wave of volcanic ash and debris across the land, fusing into a hard layer too hot to preserve plant and animal remains. Turtle Cove, formed twenty-two to twenty-eight million years ago from abundant ash-fall from a growing ancestral Cascade Range, is named for a cove landscape where many turtle fossils have been found, and parts of its layers are rich in fossils. Big Basin, formed twenty-eight to thirty-nine million years ago, includes the vivid red and gold Painted Hills; its sporadic ash-falls from a young ancestral Cascade Range did not bury organic remains deeply enough to fossilize well, though nearby pond deposits of the same age are rich in leaf fossils.