Wednesday, February 16, 2005

Some places show off this history especially well. "Trinidad is a remarkable place geologically," Sue remarked. "The bedrock that underlies the region of Trinidad and the eastern part of McKinleyville, and Blue Lake to Willow Creek, consists of what's called the Franciscan Formation. Its name comes from the Bay Area, and it describes a very mixed assemblage of rocks, including those that formed in the deep ocean, the shallow ocean margin, volcanic rocks we think formed on the sea floor, rocks that formed deep in the earth's crust---all mixed together."

Many of the large offshore rocks and seastacks at Trinidad are examples of Franciscan Formation, and they are hundreds of millions of years old. Some may have been pushed up from the ocean floor and have been standing where they stand today for tens of thousands of years.

"This very mixed sandwich of rock we think were gradually accreted over long periods of time at the continental margin, where the oceanic crust is thrusting under the edge of the continent." But there's also part of Trinidad that's considerably younger.

"Marine terraces were formed by the ocean, both beveling off a flat surface because of wave action, then depositing sand on top of it. As the crust is being worked in an active tectonic setting, these old wave-cut platforms are raised upward. Basically these are old beaches that have been moved above sea level---and the town of Trinidad is sitting on one of them." So while some of the offshore rocks at Trinidad are 70 to 150 million years old, the flat surface where the town of Trinidad sits is about 60 thousand years old. "That's two different time frames contributing to Trinidad's interesting geologic history."

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