4 - Geology at the Edge

Transcription of accompanying audio.
Woman's Voice.

This is stop 4.

Now that you’ve met the plants and animals of Lands End, we’ll put them in their geological context. The first panel is called: Geology at the Edge: A Land in Motion.

A color photo at far left shows the rocky coast of the Pacific, with some green cliffs at left.

Land’s End is aptly named. This is the westernmost tip of San Francisco, the place where the land ends and the vast Pacific Ocean begins.

To the right, is another section. It shows a map of Marin County and San Francisco, with the San Andreas Fault and the Pacific Ocean. The maps is titled, "Northern California's Shifting Coastline".

Two miles offshore, the San Andreas fault runs in a north-south direction beneath the Pacific Ocean. The North American and Pacific plates meet along this famous fault, slipping slowly past each other at the rate of two to four inches, or five to ten centimeters, each year.

In geological terms, San Francisco's coastline is relatively young. Once part of the ocean floor, the coastal mountains were raised above sea level several millions of years ago as the Earth's crust was compressed along the San Andreas Fault. As sea levels rose and fell during past glacial periods, Lands End was repeatedly covered by, and then exposed to, the ocean's force.

Twenty thousand years ago, during the last ice age, the California coastline was more than twenty-five miles farther west, about where the Farallon Islands are today. When the ice melted, sea level eventually reached today's Lands End and flooded the inland valley, which became San Francisco Bay.

On the right side of this panel are three photographs. The largest titled, “Greywacke sandstone outcrop at Lands End”, shows outcroppings of rock along the ocean's edge, rising up into cliffs.

The next two, are one on top of the other. The top photo shows the Golden Gate Bridge at right, the Marin County hills on the horizon, and a close-up view of greenish rocks near the shore on the San Francisco side.

The photo is titled: Serpentine outcrop near the Golden Gate.

The lower photo shows a close-up of much darker, charcoal-colored shoreline rocks. It’s captioned: Pillow basalt at Lands End.

Now we’ll talk about the slanted panel, to the right of this one.