Marine Terraces

We all live on a giant staircase in coastal Southern California. The land has been rising for over a million years as it moves northwest along the San Andreas fault system. The ocean has meanwhile risen and fallen as the amount of water frozen in our polar regions during ice ages changes. The motion is not steady for either the ocean or land. Our stairs are marine terraces cut by the ocean each time the relative levels of land and sea remain still for few thousand years. Each step slopes gently out to sea.

You can see a terrace being cut now during any winter storm. The cobbles on the beach are thrown against the cliffs by each breaker. The bottom few feet of cliff are eaten away by the impacts. When the resulting overhang is large enough, the whole cliff falls and the debris is broken and washed out to sea by the waves. The cliffs recede so fast that even we, during our short lives, can see the process.

Cobble layer at base of Lindavista Formation
Cobble layer at base of
Lindavista Formation
Cobble layer
Cobble layer

Geologists found sixteen terraces in San Diego by detailed mapping and elevation measurements of terrace shoreline sand abrasion platforms. In Torrey Pines Reserve, they found the following terraces: Nestor, Guy Fleming, Parry Grove, and Clairemont. The steps are not obvious here because they have been covered with mud washed down by streams, cut by canyons, and faulted. The old abrasion platforms where the ocean cut steps can be recognized by layers of the cobbles that were the ocean’s tools. The cobbles are now at the base of the sediments that buried the steps. The easiest cobble layer to find is the one at the base of the Lindavista formation covering the Clairemont terrace. In the Bay Point, layer of cobbles mark the base of each terrace.