 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 . 
