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Step 3. Water level rises, and eroded limestone is colonized by coral. (After Purdy, 1974)

However, this still did not explain the mechanism for Darwin's subsistence of the island. We now know, for example, that the Ice Ages, by locking up much of the earth's water into ice, brought about large changes in the level of the earth's oceans. At the peak of the last Ice Age, 18,000 years ago, the sea level stood almost 130 meters lower than it does today. In some cases it has probably been the rising of the oceans and not the sinking of the island that has created atolls.

Karstic saucers. Darwin's is not the only theory of reef formation, and there are some areas where geological evidence does not accord well with his speculations. A newer explanation has been offered, called the karstic saucer theory. "Karst" is the name given to the formations caused by the action of rainwater on exposed limestone—caves, sinkholes and underground channels. (The name comes from the type region around the Dinaric Alps, near the Adriatic coast of Yugoslavia.)