Dike - a crack in the rock that became filled with magma, then cooled. They formed when Stone Mountain had cooled partially from the outside and some cracks reached down to the still - liquid magma. You can determine the relative age sequence of dikes by studying how they cross-cut one another.(picture)
Exfoliation - peeling that occurs on the surface of a massive body of rock. When Stone Mountain formed deep underground, it was under a great deal of pressure. Later, erosion removed the many miles of rock that had covered the mountain, causing relief of the pressure. As the rock "loosens up" due to the relief of pressure, it peels, or exfoliates. Exfoliation first appears as small depressions in the rock, and enlarges to form natural steps. (picture)
Rock - an aggregate of minerals that has a consistent composition
throughout an area large enough to depict on a geologic map. Rocks fall
into three classes:
Metamorphic - formed by recrystallization in the solid state (without melting) due to heat and pressure. Most common group in Atlanta area.
Sedimentary - formed from sediments laid down by water or wind,
then compacted and cemented underground. Not found in Atlanta area; nearest
localities are Cartersville and Macon.
Gneiss - (pronounced "nice") - a metamorphic rock characterized by bands or layers of light and dark minerals. Most common rock type in the Atlanta area, seen at Peachtree Center MARTA station, Clairmont Road at I-85, etc.