[ROCK1] Geologists have separated all rocks into one of three categories. This is somewhat artificial because certain rocks are transitional between groups. In fact, there are rocks, such as coal, which are classified differently by different groups of geologists. But generally, this three-fold separation has proven very useful for understanding the genesis of rocks.

Amongst its many traits ("boring", "too hard", etc.), geology is a very visual science. As a result, geologists have developed standard symbols to represent rocks on maps and in diagrams. Becoming familiar with these symbols will help you to quickly identify rocks in illustrations in lecture and while you study your textbook. The symbol generally used for igneous rock -- randomly arranged dash or hatch marks -- represents the randomly oriented mineral crystals typical of these rocks. Each type of sedimentary rock (e.g. limestone, shale, sandstone) has its own standard symbol (as well as abbreviations  - i.e. LS, SH, SS) -- which we will explore later. Finally, metamorphic rock symbols depict the deformation of mineral grains or rock structure by heat and pressure, and so consist of wavy (deformed) lines.

You should look in your textbook to find an example of a rock from each group, preferrably one with which you are already familiar.