The twin sea stacks of Twin Rocks were connected at one point to form a single structure. Prior to 1925, the Twin Rocks were called the "Profile Rocks" until they changed names due to a conflict with another set of profile rocks in the region. When viewed from above, the rocks are in affect a wedge-shape perched on top of a rocky island.
Voluminous floods of lavas flowed westward along a broad Columbia paleochannel, through a gap in the Cascade Range and into the Willamette Valley. The flow advanced seaward pooling in marine bays and estuaries. The dense flow penetrated deeply into the soft sediments. These basalts are “rootless” which reinforces the idea that they are distant in origin rather than of a local volcanic source.
The Twin Rocks formation (and others like it along the northern Oregon coast) are all headlands of erosion resistant Columbia River basalt formed about 20-30 million years ago. Most of the offshore stacks and arches are vestiges of the same flows. These features have been isolated from the mainland by erosion.
To log this cache email the cache owner the answers to the following questions:
1) From the posted coordinates, what is the estimated length of the arch rock?
2)From the waypoint, compare the width of the arch rock to its length. Where do you think the erosive force of the sea will break the arch?
3) What will the feature look like then? Name it!