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Conglomerate is characterized by rounded pebbles cemented together. The pebbles can be most any kind of rock. The material that cements them together is often calcium carbonate but there are other minerals that can act as cement. The pebbles of conglomerate have been rounded by the action of moving water. This could be from a river, stream or from waves on a long ago beach.
Because they have been transported by water the pebbles may be from a wide variety of rock types. The size of these rock fragments is over 2 mm in diameter. They are often sorted by size.
After the pebbles have been deposited they are compacted by the sediments that pile up on top of them. Over very long periods of time the pebbles become cemented together by other minerals. Silica, calcite, and iron oxides are the most common cementing minerals.
The rounded rock particles in the conglomerate are easily visible to the naked eye.Sedimentary Rock Examples