


Evidence from the Past

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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.



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.


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.



Coal forms when large quantities of plant material compress and decay without oxygen, a condition common to swamps and marshes.



Shale forms when muddy bottoms of quiet lagoons, swamps or rivers dry out, compress, and harden through time

Layers of gypsum form when calcium and sulfur concentration increase as sea water evaporates

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