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| © Larry
Fellows
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This is a view of White Sands National Monument
gypsum sand dunes with ripples. Under proper conditions the
animal tracks in these dunes may become fossilized.
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In certain conditions, and over a very long period of time, sediment
becomes compacted and cemented into sedimentary rock. Fossils are more
common in some kinds of sedimentary rocks than others. There are many
factors that can contribute to the likelihood of an organism being preserved
as a fossil. Fossils are most common in limestones. That is because most
limestones consist partly or mostly of the shells of organisms. Sometimes,
however, the shells are worn so much that they look like sediment grains
rather than "real" fossils. Fossils are also common in shales,
which form from muds. Excellent imprint fossils can be formed in fine-grained
sediments like muds. Only some shales contain fossils, however, because
many areas of muddy ocean floor had conditions that were not suitable
for animal life. In this case, only swimming or drifting organisms that
die and fall into the mud have a chance to become fossilized. Although
this does happen, it is a very rare occurrence. Some sandstones contain
fossils as well. Most sandstones do not contain fossils, for various reasons.
Water currents in the environment might have been too strong for animals
to survive. Also, sands are very porous, so water seeping through the
sand might have dissolved the shells away long before the sand was buried
and changed into sandstone.
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