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Wave-Cut Platform, California

Erosional Landform Pictures

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The flat surface on this northern California headland is a wave-cut platform (or marine terrace) that now lies above the sea. Another wave-cut platform lies under the surf. (more below)
Land planed flat by surf
Photo (c) 2008 Andrew Alden, licensed to About.com (fair use policy)
The Pacific shore in this photo is a place of wave erosion. The surf chews at the cliffs and washes their pieces offshore in the form of sand and pebbles. Slowly the sea eats into the land, but its erosion cannot extend in the downward direction beyond the base of the surf zone. Thus the waves carve out a fairly level surface offshore, the wave-cut platform, divided into two zones: the wave-cut bench at the foot of the wave-cut cliff and the abrasion platform farther from shore. The bedrock knobs that survive on the platform are called chimneys. (See more of these features in the wave-cut platform gallery.)

Platforms may be seen at very low tides but are otherwise invisible unless they somehow end up above sea level. In the short term, geologically speaking, rising and falling sea level due to successive ice ages is responsible. But more slowly than these sea-level fluctuations, the land may rise and fall, too. The dance between land and sea may create a whole series of exposed (and submerged) wave-cut platforms. In this part of California, as many as five platforms are recognized and mapped on land alone.

In this view we have an active wave-cut platform in the middle (underwater) and an older, fossil platform in the foreground and background. Some places have as many as a dozen of these, stepping up the coastal mountainsides, where they are commonly called marine terraces. Careful users of geologic terms reserve "terrace" for a platform that is built up by deposition rather than carved by erosion. The strandline features around lakes are examples of terraces. But in fact, the thin veneer of ancient beach sand overlying the raised wave-cut platform is a deposit that qualifies as a terrace. So either term may be used loosely here, but in the literature our ideas and language must be more precise.

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