Mount Rainier is built on the foundations of much older mountains. Long before Mount Rainier formed, volcanoes 26-28 million years ago spread great blankets of hot pumice and ash over the landscape. Some flows were 350 feet thick. Heat trapped in the flows remelted the particles, forming the hard rock, known as welded tuff, of the Stevens Ridge Formation, which forms the peaks of the Tatoosh Range visible to the south. From beneath these rocks a large blob of magma many miles across pushed upward about 18-14 million years ago. Before it reached the surface, the molten rock cooled and hardened into a "salt and pepper" crystalline rock, known as granodiorite – a relative of granite. Grano­diorite forms the base rocks under both the Tatoosh Range and Mount Rainier. As the entire Cascade Mountain Range rose, severe erosion carved rocks of the Tatoosh into the rugged ridges and deep valleys we see. In the last million years, repeated glaciation shaped the range into a series of horns and cirques.