All rocks belong to one of three broad categories, based upon their mode of formation.

> Igneous rocks are those that crystallize from a molten state.

> Sedimentary rocks are those made from the weathered residue of older rocks, eroded, transported, and eventually deposited in a different location.

> Metamorphic rocks are those whose composition and/or texture has been altered by elevated temperatures and pressures.

Any one of these rock types can serve as the starting point for forming any other type. For example, an igneous rock, a metamorphic rock, a sedimentary rock, or a region with all three types might be weathered to produce a new sediment. The concept of changing these rock types into each other, perhaps over and over again, is called the "rock cycle" and can be diagramed in the form of a flow chart.

The heat and pressure of metamorphism almost always destroy fossils, and of course melting does as well. Some low temperature/pressure metamorphic rocks might retain fossils, and a volcanic ash or flow might trap and preserve fossils (think of Pompeii, Italy), but fossils are typically found only in sedimentary rocks. However, not all sedimentary rocks contain fossils.

The "rocks" of southwestern Georgia are primarily sedimentary, though most are not well lithified, and appear to be more like loose sediment. The physiography of the state and the characteristic rock types associated with each are shown on a simple diagram on the next screen.