Pounding Pacific waves shape the coast. Even the hard basalt eventually erodes, especially if it is cracked and fractured. Waves compress the air in fractures, forcing them to widen by chipping off pieces of rock, gradually enlarging them into crevasses, caves, and tunnels, finally grinding them down to sand and gravel. Where rock is less fractured, it endures as headland, or if cut off from land, as a sea stack.
About one-half mile off Oceanside are the massive, basalt arched sea stacks, the Three Arch Rocks. Once called the Three Brothers, these stacks, along with the other basalt headlands and offshore rocks, are composed of Miocene Columbia River basalts, specifically Grande Ronde basalts that erupted from volcanic fissures in southeastern Washington and northeastern Oregon 15.6 to 16.8 million years ago and flowed over large parts of Washington and Oregon, down what is now the Columbia River drainage, all the way to the coast. Three Arch Rocks were once part of Maxwell Point before erosion separated them into individual stacks.
The Arches make up fifteen acres of wildlife refuge that are home to over a quarter million roosting, breeding, and nesting seabirds that include common murres, tufted puffins, Brandt's cormorants, pelagic cormorants, fork-tailed storm petrels, guillemots, brown pelicans and other seabirds. Also found there are bald eagles, peregrine falcons, harbor seals and the huge Steller sea lions. The refuge consist of three large rocks and six small rocks, four of which Finley Rock, Middle Rock, Seal Rock and Shag Rock support breeding colonies of seabirds. Thirty percent of Oregon's Common Murres and 60