5 - Rocks at the Edge

Transcription of accompanying audio.
Woman's Voice.

This is stop 5.

This is called: Rocks at the Edge.

It begins with this text, at left: The forces that helped build Lands End are now tearing it down. Wind, rain, and waves relentlessly chew at the bluffs, washing away softer rocks while leaving behind the more resistant outcroppings of harder rocks. Isolated harder blocks form the solitary "sea stacks" that dot the shoreline. Seal Rocks are good examples of sea stacks.

As the cliffs erode, some of their debris is carried by southbound ocean currents along Ocean Beach and deposited there as sand. However, most of the sand at Ocean Beach originated in the Sierra Nevada, where rain and freezing weather erode stone and rock, and a system of streams and rivers carries the particles hundreds of miles to San Francisco Bay and then through the Golden Gate Strait.

Climate change may well bring a further rise in the sea level. Many low-lying areas of San Francisco-along with the entire Pacific Coast-are at risk of inundation.

To the right of this text is a color photograph of "sea stacks" in the ocean, the tall, hard rocks left behind after softer rocks erode.

Next to this is, a photo on top and a chart below.

The photo is titled, “Layered chert in a road cut”. It shows rows of brick-like rocks compressed on a hillside.

The chart is titled, "Subduction Process from 160 to 70 million years ago." It shows the ocean with different rock types beneath the surface, the lithosphere and the earth's mantle, as well as the sierra arc.

The caption reads: "Ocean rocks were added to the continent's edge as the Pacific seafloor slid, or subducted, under the North American Plate. The plates now slide past each other along the San Andreas Fault, causing California's notorious earthquakes."

The rest of this panel shows and describes four types of rocks.

The first is basalt. An igneous rock formed by rapid cooling of lava on the sea floor. Usually grey or black, it is sometimes found in smooth, rounded, pillow-like formations. Ohlone people used chunks of heated basalt for cooking.

Next is Serpentinite rock. A metamorphic rock sometimes called “serpent rock”. Shiny green, it contains asbestos in its natural mineral form. Serpentinite is California's state rock.

Next is a reddish rock called chert. A sedimentary rock from the ancient sea bottom. It is composed of layers of silica formed from the skeletons of microscopic, one-celled ocean animals called radiolarians.

The last rock is Greywacke rock. A sedimentary rock made up of ancient, cemented grains of sand, susceptible to erosion by rain and wind. Greywacke sandstone is the most common rock at Lands End.

Before we leave here, touch the window in front of you. Our windows are state-of-the-art, specially designed to deflect heat in summer and hold it in winter. Beyond them is one of the world's best views: Seal Rock towards the southern end, cypresses to the north, and the Pacific Ocean reaching to the western horizon.

Now for the next stop on the tour, turn left and walk to the far end of the farthest slanted panel.Take another step, reach out, and touch a vertical wood wall.Go around it to the waist-high counter.Take just two steps and stop.Then turn so the counter is at your back. Walk forward five or six steps and stop at another waist-high counter. This is a merchandise counter.