The geological story may be told in a few untechnical words. As those folds in the earth's crust which parallel the coast were slowly formed by the lateral pressure of sea upon land, fractures often occurred in the general incline thus created. Through the fissures that resulted the subterranean fires thrust molten rock. In many cases, the expulsion was of sufficient amount and duration to form clearly defined volcanic craters. The most active craters built up, by continued eruptions of lava and ashes, a great series of cones now seen on both sides of the Cordillera, that huge mountain system which borders the Pacific from Behring sea to the Straits of Magellan. Tacoma-Rainier is one of the more important units in this army of volcanic giants.
Mazamas rounding Gibraltar—a reminiscence of the ascent by the Portland club in 1905. The precipice rises more than 1000 feet above the trail which offers a precarious footing at the head of a steep slope of loose talus.
Unlike some of its companions, however, it owes its bulk less to lava flows than to the explosive eruptions which threw forth bombs and scoriae. It is a mass of agglomerates, with only occasional strata of solid volcanic rock. This becomes evident to one who inspects the exposed sides of any of the canyons, or of the great cliffs, Gibraltar Rock, Little Tahoma or Russell Peak. It is made clear in such pictures as are on this page and the next.
This looseness of structure accounts for the rapidity with which the glaciers are cutting into the peak, and carrying it away. Most of them carry an extraordinary amount of debris, to be deposited in lateral or terminal moraines, or dropped in streams which they feed. They are rivers of rock as well as of ice.
Under the walls of Gibraltar.
That the glaciers of this and every other mountain in the northern hemisphere are receding, and that they are now mere pygmies compared with their former selves, is well known. What their destructive power must have been when their volume was many times greater than now may be judged from the moraines along their former channels. Some of these ridges are hundreds of feet in height. As you go to the Mountain from Tacoma, either by the Tacoma Eastern railway or the Nisqually canyon road, you find them everywhere above the prairies. They are largest on the north side of the Mountain, because there the largest glaciers have been busy. Many of them, on all sides, are covered with forests that must be centuries old.