You probably know that the bedrock along the entire coast of California and southern Oregon is comprised of a chaotic mix of rock types, generally referred to in the literature as Franciscan mélange.
Pillow basalts are among the most common of these types and are recognized by their distinctive pillow-like shape, glassy margins and interstitial calcareous mud. They form under water, in this case the Mesozoic ocean floor prior to subduction.
In addition to these rocks, there are so-called exotic blocks within the mélange. Their origin is much more problematic because they have been thoroughly metamorphosed at great depth during the subduction process. They have since been uplifted and are now found in various localities including the Bandon area. The minerals in these blocks, some as large as houses, are hard and dense, imparting great resistance to erosion. They form some of the best sea stacks along the coast.
The second image here is a close-up of some of these minerals. The blue mineral is glaucophane, the spherical red mineral is garnet and the green mineral is pyroxene that is rich in jadeite. People occasionally find gem quality jadeite along the beaches there. All of these minerals are strictly metamorphic, having formed at depths on the order of 20 km and temperatures on the order of 500 degrees Celsius!
(Thanks to Dr. Jim Stout for his insights into local geology!)
You can enhance your next visit to the coast by hiring a professional guide to reveal some stories of the fascinating coastal rock formations! One such company is Wavecrest Tours in Coos Bay