Central Coast, California

Channa NheanGeology 3

Professor LawlerDecember 4, 2011

Geological History of Pismo Beach began 180 million years ago

At the time Pismo was located further South,

more toward the equator.

The coastline was more Inland near the SierraNevada Mountains

This was an area of continental collision, where the Pacific Plate was going down Under the North American plate, a process known as subduction.

Bluffs at Pismo Beach, California

Sierra Nevada Mountains

The Sierra Nevada Mountains were formed when the coastline was further inland than it is today.

The piling up of oceanic crust material helped shaped the Sierra Nevada Mountains in the process

of subduction over 180 million years ago.

Morro Rock is an ancient volcanic mountain located in Morro Bay, the original home of the Chumash Indians. Morro Rock is approximately 20 million years old.

about 20,000 years ago, the sea level was 300 feet lower than it is today and Morro Rock stood ondry land. However, after the last glacial retreat, the sea rose once more surrounding the rock withseawater. Sections of Morro rock were blasted off to create the breakwater and causeway for Morro

Bay but that practice ceased in 1969 due to Morro Rock’s historical geological past and it has become the home of the endangered Peregrine Falcon.

Morro Rock, Morro Bay, California

Pismo Clams

Pismo clams have been found in archeological sites dating back to the beginning of the

Holocene era 10,000 years ago. The oldest Pismo clam shell ever found was

dated to be more than 25,000 years old. The Pismo Clam still exists today though the numbers

are dwindling.

Rocks!

Pardon my expression, I was caught off guard =)

This rock is found on, or rather makesup a cliff. It is chalky, white, angularand gritty when crushed because it is easily chipped or broken off.

Because it is found near the ocean,it may possibly be an evaporite but looks like chalk or a diatomite.

May possibly be ancient volcanic ash,under sufficient pressure and temperatures, it petrifies into a rock-like substance and erodedovertime is exposed above sea level(Foster, 1998).

It is definitely a sedimentary rock.

Pismo

Found right at the ocean’s edge, this rock is seems to be

composedOf a variety of other rocks and minerals. It is hard, rough and

Angular in form with grays, browns, pinks. It appears to be

a Sedimentary rock, possibly a

breccia due to the clasts being angular rather than round and is

a conglomerate.

One of the “Nine Sisters” of Morro Bay. This is a series of ancient extinct volcanoes. Created at mid ocean ridges by moving tectonic plates the hot magma cools in the ocean and (Farndon, 2006) over years of erosion has been uplifted and exposed. It is composed of granite, a form of igneous rock.

Morro Bay

Law of Superposition

Older Rocks

Younger Rocks

References

Fardon, John. (2006). The Complete Guide to Rocks & Minerals. London, England. Hermes House.