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Coastal Morphology Group • Scripps Institution of Oceanography

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Fossil Coastlines

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Dropcap How do we study the history of the coast when sand beaches leave behind no fossil record?

Fortunately, Native Americans arrived on California's southern coast by 12,000 years ago. Early people made use of food resources from estuaries and open coast habitats, harvesting shellfish for food and disposing of the shell debris near their encampments.

These prehistoric refuse heaps are called "middens," and the deposits are dominated by the durable remains of marine mollusk shells. The content of these early "landfills" tells us about the presence or absence of nearby beaches.

Mollusks such as mussels and clams are highly sensitive indicators of coastal environments because they have very specific habitat requirements such as those found in estuaries, rocky intertidal, or sand beaches.

The shell middens, many still intact today, provide us with snapshots of what the coast looked like in the past. Radiocarbon dates on the shells tell us when different coastal environments existed. An 8,000 year old Pismo clam in a midden tells us that a sand beach existed nearby at that time.

The shell midden on San Miguel Island shown at right contains dense deposits of abalone shell and other rocky coast mollusks. The artifacts pictured below are abalone pry bars, gorges (the earliest fish hooks dated 8-10,000 years ago: bone sharpened on both ends was attached to a cord line and baited), and later circular fish hooks.

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The animated diagram below summarizes what we have learned from archaeological records around the region.
The timing of beach accretion is an important input to the modeling of coastal evolution. When beaches form, the platform and sea cliff are protected from wave attack. Equilibrium sand beaches that adjust to seasonal changes in wave direction and energy will slow coastal erosion. Visit the Modeling section to see how these paleocoastlines reconstructed from the archaeological record aid in modeling the evolution of the southern California coast.

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Coastal Morphology Group • Scripps Institution of Oceanography

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