You find a layer of limestone with a specific fossil clam above a layer of sandstone in Europe. You find a layer of limestone with the same fossil clam below a layer of sandstone in North America. The layer of:

Fossil layers are fossils that formed in sedimentary rock. Sedimentary rock is rock that is formed in layers by the depositing and pressing of sediments on top of each other. Sediments are any loose material that gets broken away and carried: pieces of rocks, pebbles, sand, clay, silt, boulders, dead organisms, animals, plants, shells, insects . When sediments move and settle somewhere, they are being deposited. When, over a long time, layers and layers of sediments get deposited on top of each other, the weight of the top layers presses down on the bottom layers, forming them into rock called sedimentary rock. The oldest layers are on the bottom, and the youngest layers are on the top. Because sediments sometimes include once-living organisms, sedimentary rock often contains a lot of fossils. Fossils are once-living organisms that have been turned into rock, in which the shape or form of the organism can still be seen.

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