Malheur Butte

Malheur Butte is an extinct volcano located in Malheur County. Between 20 and 15 million years ago, the region from north-central Washington to northeastern California experienced a series of volcanic eruptions and basalt lava floods that covered thousands of square miles. These ancient lava floods often dammed streams, creating lakes and swamps that may have lasted for millions of years before filling with sediment or draining as erosion slowly lowered their outlets.
-from Wikipedia