Site: Corona Heights.

Corona Heights began to develop about 200 million years ago in the middle of the Pacific Ocean. Magma emerging through the earth’s crust at the “spreading center” between the Pacific and Farallon plates contacted seawater and cooled to form “pillow basalt”, the bedrock of oceans everywhere. Over the next 75 million years, “radiolaria” (tiny protista organisms similar to amoebae but having silica-rich spiculed exoskeletons) lived, died and drifted down through the deep ocean waters to deposit on the ocean floor. Eventually the combination of pressure and time turned these microscopic organisms into rock called “radiolarian chert”.

The movements of the tectonic plates eventually brought the Corona Heights rock close to the North American continental shore several tens of million years ago. Sand eroding from the Sierras washed into the ocean and turned to sandstone layered atop the chert. As the Farallon plate subducted under the continental plate, the rocks of Corona Heights were scraped off, scrambled up and pitched upon the land – forming the landscape you see in the park now. Formally, the park is part of the “Marin Headlands terrane” of the “Franciscan Complex.”

The majority of the rock visible at Corona Heights is chert. It is reddish-brown and forms distinctive sedimentary layers. Each meter represents the deposition of about one million years’ worth of radiolaria; each millimeter of rock took about a thousand years to accumulate. Chert is a very hard rock that resists erosion better than other types of rock; that is why Corona Heights stands above the surrounding landscape. The best exposures of chert are above the tennis courts on the lower level, behind the museum, and at the summit. At the base of the hill on the northeast side is an excellent example of “slickensides”, a smooth, polished surface between two masses of rock created by tectonic sliding of one past the other. Usually slickensides are found in softer rock. This example in chert is highly unusual.

Pillow basalt is a dark brown rock that forms in the ocean in the form of “pillows” when the magma contacts seawater. Once exposed to weather, it wears down to a variety of different colors from yellow to greenish gray. Exposures of basalts are west of the tennis court on the lower level and at the west end of the museum parking lot along Museum Way.

Small areas of graywacke sandstone are visible near the east tennis court and in the northwest corner of the park by the dog run.

During the late 1800s and early 1900s, Corona Heights was heavily quarried. The steep depression on the north side of the hill and the flat areas under the museum and tennis courts are the remnants of these old quarries. An old brick factory stood where the museum is but burned down in the 1906 earthquake.

Created: 27 January 2008 - 18:04
Last updated: 27 January 2008 - 18:04

Comments

There are no comments so far.

If you signup and login, you can post comments.