Contact between Stone Mountain Granite and Promised Land Formation in Saprolite

At this writing (January, 1998) contact between Stone Mountain granite and the Promised Land formation is clearly visible in saprolite (chemically decayed or "rotten" rock) adjacent to U.S. Highway 78 east of Atlanta. Where U.S. 78 crosses the Yellow River, beside a driveway to the Yellow River Game Ranch, excavation exposes contact between these two dramatically different geologic formations. Stone Mountain granite is light-colored igneous rock; the Promised Land formation comprises layered felsic and mafic tuffs. Where contact is exposed in the east wall of this excavation, Stone Mountain granite has decomposed to nearly white sandy saprolite, while Promised Land tuffs have weathered to buff-gray and orange-brown bands. Translucent gray quartz layers and dikes remain relatively intact, while feldspar stripes in granite have weathered to pinkish-white kaolinite.

Intertongued Stone Mountain Granite (light - above)
and Promised Land formation (dark - below)

Dark areas on right are erosional cavities

Quartz dike shows as a dark vertical stripe in Stone Mountain Granite saprolite
(left of trowel)