GEOLOGIC HISTORY SHAPED RACING AT ROAD ATLANTA
Geologic History and Geologic Setting
Georgia encompasses four physio- graphic regions (Figure 3), with Road Atlanta located in the Piedmont Region which is characterized by metamorphic rocks and complex geologic structures as shown on Figure 4 (Georgia Geologic Survey, 1977). The geologic history of
Road Atlanta is complex and the land has been shaped by Plate Tectonics destroying, and then creating, an ocean basin and subsequent mountain build- ing events from 200 to 300 million years ago (University of Georgia, 2015).
The U.S. Geologic Survey (2003) reported that Road Atlanta is situ- ated in the Midland Subsection of the Southern Piedmont Province (Figure 3),
specifically the Winder Slope District. This province is characterized by gently rolling topography with a 300-ft eleva- tion change (from approximately 1,000 to 700 ft) at its southern edge, with dome-shaped granite mountains and deeply-cut narrow stream valleys. The Piedmont Province is located at the southern edge of the Brevard Fault Zone (Figure 4) which is a northeast trending strike-slip fault. Faulting likely began during the Late Silurian and continued through the Permian (University of Georgia, 2015). Dextral faulting in a rel- atively straight trend has been reported with low to moderate dipping (USGS, 2003). Interestingly, many authors have suggested this fault zone was caused by a single thrust fault that dips 40º to 50º to the southeast based on seismic reflec- tion data (USGS, 2003); however, the interpretation of this data has been and remains the subject of debate.
Figure 3 - Physiographic Regions
Road Atlanta is perched upon Piedmont crystalline bedrock within the Brevard Belt (Figure 4). The Brevard Belt is approximately two miles wide and is also referred to as the Brevard Fault Zone. The Brevard Belt is comprised of the Brevard Series which includes mica schist containing biotite and muscovite, and smaller percentages of graphite schist, gneiss, and amphibolite (Georgia Geologic Survey, 1997, and USGS, 2003). The mica schist of the Brevard Series has been mapped as Cambrian while other geologists have suggested these rocks are pre-Cambrian (USGS, 2003). Thus, it is difficult to establish stra- tigraphy of these crystalline rocks due in part to complex geologic structures and extreme metamorphism within the Brevard Metamorphic Belt (Georgia State Division of Conservation, 1952). It is the extreme metamorphic process that altered the sedimentary-origin shales into the mica schist crystalline rock. The mica schist observed in the general area of Road Atlanta is gray to black and its lenticular structure of schistosity, with a characteristic secondary cleav- age observed within the Brevard Fault Zone, gives this mica schist its scaly appearance (Georgia State Division of Conservation, 1952).
Figure 4 - Geologic Map
Piedmont soil is typically the red clay that Georgia is famous for and is abun- dant at Road Atlanta. The soil is colored red by iron oxides that are derived from weathering of micaceous, mafic-mineral and feldspar-rich metamorphic rocks. Due to the elevation changes created by weathering and erosion, many soil units
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