Geology of Georgia, Part: Home

I live in Georgia (you've probably already guessed that by now). You can tell a lot about the geology of the state in which I live just by looking at the lay of the land.

So to be more specific, I live on the southeastern side of Spring Valley Creek, which is a northeast-flowing tributary to Tanyard Creek, which is a north-flowing tributary to Peachtree Creek, which is a west-flowing tributary to the Chattahoochee River, which flows from Lake Lanier southwestward and then southward toward Florida, where it's joined by the Flint River and is called the Apalachicola River by Florida Man. The Apalachicola discharges into the Gulf of Mexico. If I peed in Spring Valley Creek (which surprisingly is not something I've ever done, despite living here for 18 years), my urine would flow to the northeast, then north, then west, then southwest, and then south before entering the Gulf somewhere west of Tallahassee.  It would flow almost in a circle around me, leaving only a narrow path southwest of me high and dry.

These right-angle turns in the waterways are part of what is known as rectangular drainage.  Good examples of rectangular drainage such as this are characteristic of what's been called the Gainesville Ridges District of the Southern Piedmont Physiographic Province.  The Piedmont (from the French words for "foothills") It is the non-mountainous, rolling terrain between the flat Coastal Plain to the south and the Blue Ridge and Valley and Ridge mountains of the southernmost Appalachians to the north.   

The Southern Piedmont has been divided into Upland and Midland Georgia Subsections.  The Gainesville Ridges District divides the Midland Georgia Subsection from the rest of the Upland Georgia Subsection.  The Gainesville Ridges consist of a series of northwest-trending, low, linear, parallel ridges separated by narrow valleys and appears on maps as a diagonal gash cutting across the northern half of the state from east to west. The ridges are generally composed of quartzite and gneiss, while the valleys are underlain by slate-like phyllonite and schist.   The courses of the Chattahoochee River and its tributaries are strongly controlled by the ridges resulting in the characteristic rectangular drainage. The southern boundary of the District follows a ridge that is continuous throughout most of its extent and is the drainage divide between southwest-flowing streams like the Chattahoochee and those that flow to the south, like the Flint River.

Soils in Atlanta, including my little corner adjoining Spring Valley Creek, are largely urban land.  "Urban land" refers to zones in high population areas and a largely built-up environment. Soils in urban land can be significantly changed by human-transported materials, human-altered materials, or minimally altered or intact “native” soils. Soils in urban areas exhibit a wide variety of conditions and properties and may have impervious surfaces, such as buildings and pavement.

Near Spring Valley Creek itself, the urban land is built up on soil of the Rion complex.  These well-drained soils form from weathered granite and gneiss on the backslopes, side slopes, and shoulders of hills.  Further uphill onto my property and into my backyard, the urban land is built up on soil of the Cecil complex.  These well-drained soils form from igneous and metamorphic rock on the shoulders and summits of hills.

The specific metamorphic bedrock that formed the Rion and Cecil soils in this part of the city is known as the Lithonia Gneiss. The formation was first described in detail by T. L. Watson in a 1902 report on the granites and gneisses of Georgia. He named it the "Lithonia contorted granite gneiss" due to its structure and chemical composition. G. W. Crickmay revised the name in 1939 to "Granite gneiss, Lithonia type," apparently because of the non-contorted appearance of the gneiss in areas other than the type locality in the town of Lithonia, Georgia. The rock was called "Lithonia gneiss" by Leo Herrmann in 1954 on the basis of its physical and chemical characteristics. 

The Lithonia Gneiss is a complex of metamorphosed granites and granitic gneisses of Devonian age (355 to 418 million years ago).  The most common rock type in the complex is a light gray to grayish-white, medium-grained, poorly foliated metagranite that is cut by numerous pegmatite and aplite dikes and sills of several generations. Dikes of different generations cross-cut older dikes.  Where deeply weathered, the Lithonia Gneiss forms a light whitish-yellow sandy soil. The gneiss is extensively quarried for crushed-stone aggregate and curbstone.  

The Lithonia Gneiss represents an intrusive granitic rock body that cut through the existing crystalline bedrock during the Devonian Period.  The sills and dikes present in the rock represent later intrusions of younger rock.  Following placement of the granite, subsequent metamorphic pressures have folded and faulted the rock into a metamorphic gneiss and into its present distribution in the Georgia Piedmont. Rock of the Lithonia Gneiss was probably a source for the magma that later formed Stone Mountain.