Obviously, there are many interactions between life and geology, from the very large (think drilling for oil) to the very small (think bacteria which literally digest rocks -- remember the lithoautotrophic organisms).

Here, though, I want to mention carbonate rocks.

Carbonate rocks are made up of, essentially, sea shells. The chemical composition is CaCO3 (you can check the valences on the periodic table), and the Ca is why we call these rocks carbonates.

Beach (left) along coast of Antigua, Caribbean Sea. Sediment (right) is composed of shelly debris.
From http://www.science.ubc.ca/~geol202/sed/carb/carbover.html

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