Many sedimentary rocks consist of particles formed from the erosion of older rocks and then cemented together again. Particles carried a long distance before they are finally deposited can become very well rounded or broken down into smaller particles. Sandstones are made of particles 0.1-2 mm across, while conglomerates, or pebble beds, have particles larger than 2 mm. Subsequent erosion of sandstones and conglomerates may release these rock particles again, which may then be incorporated into a new sandstone or conglomerate. Pebbles of particularly hard rocks, such as the quartzite pebbles in this conglomerate at Cushendun, may be reworked several times over the course of hundreds of millions of years; indeed many of the modern pebbles on the beach at Cushendun are directly out of the conglomerate, which is about 400 million years old.
Fossils may be common in marine sandstones
but often are rare in sandstones deposited on land or in rivers.
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