Locally, where the crystalline rocks consist of marble, the dissolving action of slightly acidic ground water has created solution openings that yield large volumes of water. Although there are considerable differences in the mineralogy and texture of the rocks comprising the Piedmont and Blue Ridge aquifers, the overall hydraulic characteristics of the aquifers are similar. Locally, however, the occurrence and availability of ground water varies greatly because of the complex variability in rock type. Such variability makes it impractical to describe ground-water flow regionally. Accordingly, specific examples taken from local studies are used to illustrate different aspects of the hydrology of the crystalline rocks.
Rock type varies markedly from place to place.
For example, Blue Ridge and Piedmont rocks are divided into more than 90 units on the 1976 geologic map of Georgia. Locally, quartzite and marble are present. Most of these metamorphic rocks were originally sediments, but some of them were originally igneous or volcanic materials. Most of the rocks have undergone several periods of metamorphism. Locally, they contain mineralized zones, some of which are ore bearing.