Geologists are learning all the time, too. As certifiable human beings, they occasionally make mistakes and frequently disagree with one another.
In my 1990 book, Idaho Snapshots, I joked about oceanfront property in Riggins, because at one time—not exactly last week—the area around Riggins would have overlooked the Pacific Ocean. This was a long time ago, in the Middle Permian to Early Cretaceous periods. I noted that what is now Washington and Oregon would have been an island before tectonic plate movement kissed that prehistorical land into what would become Idaho.
Lands roughly west of Riggins along a north-south squiggly line (not an official term of geology) from Alaska to Mexico did form at different times, play their roles as islands, then snuggle up against the North American continent, adding much beloved land to these United States, including California (see graphic).
Most of these islands were formed by volcanic activity or started as mountain ranges beneath the Pacific. They rode oceanic plates on their journey to their present position. The result is called accreted terrane, meaning that land masses formed somewhere else found a different home. Geologists generally agree that a wide swath of the west coast is accreted terrane. They disagree on exactly when and how it all came together.
Evidence that those islands piled up against the rest of the continent is on display in the Riggins area. The photo below, generously contributed by Terry Maley from his book Idaho Geology, shows where two radically different kinds of rocks are squished side-by-side to form part of what we call Idaho.
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