| Stops for this field trip are along a circular route (Highways 20, 395, and 26) in the vicinity of Vale and John Day, Oregon. The focus of the trip is the extensive middle to late Miocene rhyolitic ash-flow tuffs that erupted after the main stages of Columbia River Basalt volcanism to the north and Steens Basalt volcanism to the south including the youngest voluminous ash-flow tuff, the 70-Ma Rattlesnake Tuff. |