Geologic Terms relevant to Stone Mountain

Physical Features

Joint - a crack in the rock. Many Stone Mountain joints were formed due to shrinkage as the magma cooled many miles beneath the surface. Joints provide a pathway for ground water and collect soil that permits plants to become established. (picture)

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)

Earth Materials

Mineral - a naturally occurring inorganic substance with a specific internal (crystalline) arrangement of atoms and a chemical composition that varies only within specific limits. Examples found on Stone Mountain (both in the rock as a whole and in dikes and veins) include quartz, feldspar, mica, and tourmaline.

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:
 

Igneous - formed by crystallization from a magma. Found at Stone and Panola Mountains.

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.
 

Two rock types that are common in our area are:
 
Granite - A specific type of igneous rock formed by the cooling of magma deep underground. It is composed of a specific proportion of minerals such as quartz, feldspar, mica, and amphibole. Technically, Stone Mountain is composed of a closely related rock called "quartz monzonite," but the phrase "Stone Mountain Granite" is used by many, including geologists.

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.