Thanks for asking!
The rocky appearance of the Central California coastline is not the
result of glaciation, meteor strike(s), or any singular event.
The Central California coastline began to take shape relatively
recently in geologic time, about 28 million years ago, with the
formation of the Coast Range mountains. Some of the individual rock
formations that contribute to the structure of the coastline began
forming early as 200 million years ago, in places far distant from
their current placement.
The formations and gigantic rocks that you find impressive are
structures known as coastal bluffs. The California Coastal
Commission's California Coastal Resource Guide describes these
geographic and geologic features:
"The precipitous cliffs, steep-walled bluffs, and rocky headlands that
characterize much of California's coastline are evidence of the
ongoing geologic processes that shaped the western margin of the North
American continent. Unlike the Atlantic and Gulf coasts of North
America, whose gently sloping seashores are the result of gradual
submergence of the continent's edges, the sheer walls and elevated
terraces of the California coast were created by abrupt faulting and
uplift. Bluffs and sea cliffs are a testament to the erosive power of
waves, winter rainstorms, and wind, while headlands remain where
coastal rock has withstood weathering by these elements.
Coastal bluffs are actually the seaward edges of marine terraces,
shaped by ocean waves and currents, and uplifted from the ocean floor.
Characteristic of the California coast from Mendocino County to San
Diego, coastal bluffs are less evident along the Northern California
coast where the coastal mountains plunge abruptly into the ocean.
Rocky headlands are more prevalent along the Northern and Central
California coast but may occur anywhere erosion-resistant rocks are
found along the shore.
Coastal bluffs are composed mainly of sedimentary rocks such as
sandstones and shales that are particularly prone to erosion. Grains
of quartz, feldspar, and mica compressed into layers of sandstone
crumble easily; when wet, shales and siltstones disintegrate, and
clays and mudstones, soften and liquefy. Lying on top of the
sedimentary deposits of many bluffs is alluvial soil, loosely
consolidated sand and gravel deposited by ancient rivers and streams.
Examples of sedimentary coastal bluffs are the sandstone bluffs of
Santa Cruz, the alluvial cliffs at La Jolla, and the shale cliffs of
Point Loma in San Diego County.
Rocky headlands are composed of igneous rocks--granites and
basalts--that are resistant to wave erosion. Granitic formations
include the Point Reyes Headlands in Marin County. Morro Rock in San
Luis Obispo County, Point Dume in Los Angeles County, and Point Sur in
Monterey County are outcroppings of basaltic lava.
Sea caves, sea stacks, and arches, are created by erosion of less
resistant components of coastal landforms. Sea caves are formed by
wave erosion where fractures occur in the bluff face. Sea stacks and
arches, numerous along the wave-battered Mendocino coast, mark the
last stand of more resistant rocks. Erosion of the sandstone cliffs at
Natural Bridges State Beach in Santa Cruz created a number of arches;
today only one remains, and eventually it too will collapse into the
surf.
Landslides and cliff retreat are part of the natural process of
coastal erosion along the California shore. Waves that undercut bluffs
often initiate landslides. During winter storms heavy surf drags sand
off-shore, denuding many beaches and exposing the cliff base to direct
wave attack. Most cliff retreat occurs at this time; powerful breakers
crash into the cliffs, splintering the softer rocks into fragments
that fall into the retreating surf. Incessant winter rains beating
down on coastal bluffs slowly penetrate rock fractures, lubricating
the joints between the rock layers. Fractured shales, sandstones, and
siltstones are most likely to slip and cause landslides, especially at
locations where the land slants toward the beach. A coastal landslide
of mud and rock can scrape a clean path, sweeping away roads and
structures as it plunges seaward. Areas well known for landslides are
Devil's Slide in San Mateo County and Pacific Palisades in Los Angeles
County."
Ceres.ca.gov - Coastal Resources Guide
California Coastal Bluffs
http://ceres.ca.gov/ceres/calweb/coastal/bluffs.html
Additional resource links within the California Coastal Resources
Guide that pertain to the geologic history of the Central California
coast:
California's Coastal Geography
http://ceres.ca.gov/ceres/calweb/coastal/geography.html
California's Coastal Mountains
http://ceres.ca.gov/ceres/calweb/coastal/mountains.html
Additional resource links:
CSU Sonoma
A Brief Geologic History of California
http://www.sonoma.edu/users/f/freidel/california/390geology.htm
Peninsula Geologic Society
PGS Big Sur Field Guide
(12.8Mb in Adobe PDF format)
http://www.diggles.com/pgs/2000/PGS00-05b.html
To download, scroll 3/4 of the way down the page and follow download
directions next to the text link that specifies the PGS Big Sur Field
Guide. A photo album of 24 photographs is also available to accompany
the Field Guide information.
Google search terms used:
geologic history california coastline
Should you have any questions about the answer provided, please, feel
free to ask.
= larre-ga = |
Request for Answer Clarification by
apteryx-ga
on
14 Aug 2002 19:24 PDT
Thank you. This is very good.
And yet I feel that the specific focus of my question is not fully
answered. I was not asking about the rocky coastal formations in
general but specifically about those large, free-standing, single
rocks that stand up alone just offshore. You can see examples in
photos #1 and 20 in the PGS Big Sur Field Guide to which you supplied
a link. Some certainly appear to have broken off and rolled down from
those bluffs. Some might well be what's left after softer surrounding
material has been worn away by water and wind, hard as it is to think
of them that way. But the *same kind* (by appearance to an untrained
eye, at least) of scattered free-standing boulders are seen away from
the shoreline, on solid land, well up from the beaches; for example,
out in the midst of fields seen along Route 46 from Cambria to Paso
Robles. Are they explained by the same process that accounts for the
seashore boulders? Or are they not the same kind at all? They look
like part of the same pattern--as if a gigantic airborne rock had
smashed to pieces, some chunks scattering into the sea just offshore
and others striking the earth farther inland.
So I wanted to know about the pattern or distribution of those big
separate boulders, considered together, and not the cliffs or bluffs
along the coast.
|
Clarification of Answer by
larre-ga
on
14 Aug 2002 20:39 PDT
The scattered rocks are part of the same, continuing marine terrace
uplift process. "Offshore rocks and sea stacks are of more recent
origin than California's islands. North of Point Conception, where
most offshore rocks are found, storm waves generated in the North
Pacific buffet the shore and whittle away the coastal cliffs, leaving
isolated stands of the most resistant [granitic] rock. Fewer rocks lie
offshore of the Southern California coast, where the buffering effect
of the Channel Islands and the Southern California Blight-- an
indentation and southeasterly shift in the coastline south of Point
Conception--reduces the impact of storm waves on coastal cliffs."
Ceres.ca.gov - Coastal Resources Guide
California's Offshore Islands and Rocks
http://ceres.ca.gov/ceres/calweb/coastal/islands.html
Remember that the marine terraces now above the highway along that
stretch were once underwater and have been uplifted by geologic
processes. They exhibit the same characteristics of the shore itself,
including isolated boulders broken off from the cliffs, now displayed
50 - 1,000 feet above sea level. Additionally, traditional weathering
processes cause continued faulting within these exposed rock
formations. As faulting occurs, and tectonic forces jolt the area,
large pieces break off and roll downward toward the shore.
I have found no scientific supporting information about any
recognizable patter of California Coastal Rocks. Numerous geologic
maps are available which show the extent and layering of the various
rock formations. This CACoast data set for mapping, which includes the
Big Sur area, is an example of the extensive survey and mapping data
available for this stretch of coastline.
http://geo-nsdi.er.usgs.gov/metadata/open-file/01-179/basemaps/cacoast.faq.html
Many additional geologic maps are available from the United States
Geological Survey website at:
http://www.usgs.gov
=l=
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