 Approximately seven million years ago in the upper Miocene, a 44-mile (71 km) long trachyandesitic lava flow that likely came from Olson Mountain near present-day Lost Creek Lake flowed down the Ancestral Rogue River and its tributaries and spread throughout the valley. This lava formed a hard cap over the Payne Cliffs Formation. At Lost Creek Lake, the lava attained its maximum thickness of 730 feet (220 m) and thinned to about 100 to 200 feet (30 to 61 m) to the north of Medford.
