Sedimentary rocks are formed by the deposition of sediment. Sedimentary rocks are formed from parts of sand, shells, and pebbles. Together, all these parts are called sediment. Gradually, the sediment accumulates in layers and over a long period of time hardens into rock. Sometimes, sedimentary rock is fairly soft and may break apart or crumble easily. You can often see sand, pebbles, or stones in the rock, and it is usually the only type that contains fossils. Sedimentary rocks are formed when rivers bring sediment the ocean. Sedimentary rocks containing fossils from each time period. As sea levels change and the seafloor is pushed upward, sedimentary rocks are exposed. Erosion by rivers revels strata older strata contain older fossils.The reason the sedimentary rock might have fossils and crystals in them is because, when they are formed underground there is compaction and cementation thats when pressure holds the sediment tightly together to form the sedimentary rocks.(strata=layers)
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