During a 1977 biological study of the dunes, nearly thirty new life-forms were discovered and, many species were found that had never been recorded in California or North America. For instance, the endangered Howell's spineflower, Chorizanthe howellii, is found nowhere else in the world.  A wingless fly, Aptilotus sp., was discovered in one of the swales surrounded by dunes.  This genus of fly was known in Europe, but had never before been seen in North America.  The bog bean, Menyanthes trifoliata, is thought to be a relic from a cooler climate associated with the last glacial period, since it is found farther north or at high elevations.

Scientists believe that arthropods (insects and spiders) make up 90% of the animals found here.  At least 748 species of arthropod have been identified. Among the plant species, twelve are considered sensitive and three are listed as threatened or endangered.  In addition, these dunes may provide rare nesting and wintering habitat for the threatened Western Snowy Plover.

Native cultures inhabited this site with low impact for at least 4,000 years hunting elk, sea lion, sea otter, fish and shellfish.  Settlers, arriving in the late 1800s, destroyed the fragile plant cover, exposing the sand, through logging, cattle grazing and road building.  Recently, off-road vehicle driving (now prohibited) and other recreational activities have added to the damage. Onshore winds, blowing across the uncovered sand, accelerated dune migration inland, covering forever some of the unique habitats, and threatening to engulf  homes and roads to the east.

Recognizing the importance of protecting what remains of this unique feature, the Department Of Parks and Recreation designated the Ten Mile Dunes a State Preserve in 1995.  Your cooperation and assistance is needed to help protect these dunes.  Please avoid stepping on plants and walk on the beach whenever possible.

 

Twelve thousand years ago, if you were to stand on the bluffs above where the beach is today, you'd find the coastline three to five miles out to the west. The huge continental ice sheets had then locked up so much water that sea level was 300 feet lower than it is now.  Where the Ten Mile River presently empties into the ocean, you'd look down at the river incised into a gorge 250 to 300 feet deep, cutting through bedrock on its way to the coast.  Now, with most of the ice melted, sea level has risen, and where the river empties there's a buried valley, filled with sediments and topped with beach and dune sands.

This is a dynamic coast.  In the winter of 1997-98, the beachfront dunes were pummeled and in some places overrun by giant waves generated by the El Nino storms.  But all of this relatively short-term activity occurs within much longer-term cycles of movement of the earth's crust.

The rise and fall of sea level, occurring in cycles of several thousand years, are superimposed on much longer periods of coastal uplift responding to crustal tectonic forces.  The uplift isn't like a constantly moving escalator, but occurs periodically, about every 100,000 years, with plenty of quiet time in between for wave action and near-shore streams to form coastal terraces.  As you go inland from the coast and climb into the hills, you cross at least five of these terraces, each about a hundred thousand years older and about a hundred feet higher than its successor. The uplift is related to the nearby boundary between the Pacific and North American plates, just a few miles offshore, which itself moves in fits and starts along the San Andreas Fault.  The most recent episode occurred during the great San Andreas Earthquake of 1906, when the Pacific Plate moved instantly fifteen to twenty feet northwestward with respect to North America.

The rocks and soils we see along the beach bluffs near and south of Ward Avenue also reflect the coast's dynamism.  The bedrock, mostly hard, fractured, fine-grained sandstone, represents material scraped up from the sea floor as an eastward-migrating island arc, like Japan or the Philippines, collided with the western flank of ancient North America sixty to eighty million years ago.  Softer sandstone and pebbly conglomerate of the youngest coastal terrace directly cover the bedrock on the beach bluffs.  Yellowish and reddish layers in these deposits attest to their partial oxidation where iron-rich minerals turn rusty.  The gray zone above the colorful beds is a tight clay, little affected by air and water, thus not oxidized.

The dune sands on top of the terrace deposits come primarily from the Ten Mile River.  Comparing aerial photographs from the 1920s with recent aerial photos shows that three lobes of dunes have marched inland, engulfing the youngest terrace surface.  Like an army on the move, this rapid advance could not occur without an appropriate and timely supply, and the Ten Mile River has provided the sand in great abundance.  Enhanced erosion of the Ten Mile watershed, in response to the high tempo of logging and road building over the past eight decades, is the likely source of material that helped accelerate the dunes' movement.

From near Ward Avenue and southward the bedrock is well exposed, especially on Laguna Point below the seal-watching platform.  But from about a quarter mile north of Ward Avenue northward to the river, beach and dune sand completely cover the terrace sandstone and the bedrock.  I suspect that there is a downwarp that causes the bedrock and terrace deposits to disappear beneath the sand along this four mile stretch of beach.  They reappear suddenly on the north bank of the river, brought up by a fault that projects southeastward from the coast along the north bluffs of the Ten Mile valley.

You stand on the cutting edge of North America along this stretch of coast.
 

 

 

Ten Mile Coastal Trail Foundation

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