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| teacher guide
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Evidence
from the Past
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2
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Rocks
are products of environmental processes in the world around us.
Observing an eruption of Mount St. Helen, a flood on the
Mississippi River, or the movement of beach sands in the Bahamas
allows us to understand how rocks containing similar features
are formed. This is known as the principle of uniformitarianism.
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Limestone
is formed from the compacted shells of marine organisms or
precipitates like dust as the water evaporates. Its presence
indicates an area once covered by seas. |
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Sand
and gravel collect along seaside beaches and stream valleys to
be cemented by waterborne material. These sandstones may
preserve evidence of currents, wave actions, and the footsteps
of ancient animals. |
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Coal
forms when large quantities of plant material compress and decay
without oxygen, a condition common to swamps and marshes. |
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Shale
forms when muddy bottoms of quiet lagoons, swamps or rivers dry
out, compress, and harden through time |
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Layers
of gypsum form when calcium and sulfur concentration increase as
sea water evaporates |
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