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Voids that are considerably larger than interstices and therefore termed "vugs" are present in ancient grainstone, in modern beachrock, and in loose carbonate beach sand. They are as large as 10 grains in diameter and are roughly spherical or lens shaped. The roof of the vug resembles a crude keystone arch. Similar vugs can be made in the laboratory by alternately draining and flooding loose carbonate sand in the manner characterizing the swash zone. During the flooding stage, bubbles of trapped air lift grains into the form of a keystone arch, which is stable after the bubble is gone. Keystone vugs in ancient rocks probably will prove to be useful indicators of beach deposition.
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