Saddle Mountain was created around 15 million years ago in the Miocene epoch when lava flows poured down the old Columbia River valley. When the lava encountered the water at the Astoria Sea, great steam explosions and thermal shocks occurred to create a large pile of basalt rocks. The mountain consists of this volcanic breccia, which is a rock made up of broken basalt fragments that are fused together in a fine-grained matrix.