The western Inner Piedmont in upstate South Carolina consists largely of amphibolite-facies gneisses, schists, and amphibolites. Prominent regional units are Henderson augen Gneiss, Table Rock gneiss, and the Tallulah Falls and Poor Mountain Formations. Exposures and coring in Falls Park and along the Reedy River, Greenville, SC, reveal an augen gneiss that appears to be different than these regional gneiss units. The Falls Park augen gneiss (informal name) consists of microcline augen (up to 15 mm in size), set in a matrix (0.25 – 3 mm) of modal microcline (7 – 26 %), plagioclase (16 – 28 %), quartz (36 – 38 %), biotite (14 – 24 %), and muscovite (1-2 %). Accessory phases are myrmekite, garnet, ilmenite, magnetite, and pyrite. The augen gneiss exhibits a medium- to coarse-crystalline texture with gneissic banding wrapping around the augen. Geochemically the augen gneiss has a lower silica content (5-7 wt. % lower) and a higher iron oxide content (3-4 wt. % higher) than most other regional gneisses. Whole rock geochemistry of the augen gneiss suggest a granite to granodiorite protolith.
In the Falls Park exposures, the foliation is oriented generally N71°-75°E, 30◦ SE. A dominant, essentially vertical joint set strikes N46°-50°E. Other mesoscale and microscopic features include folded foliation, other systematic joints, mylonitic textures, strained ribbon quartz, and shear bands. Garnet and biotite appear to be in equilibrium in augen gneiss samples. Minor rocks associated with the augen gneiss are pyrite-muscovite schist and a garnet-biotite schist, with deformation features like those in the augen gneiss. Although it is not spatially contiguous with, and is likely different than, the Henderson or Table Rock augen gneiss units, the mineralogical and structural features of the Falls Park augen gneiss are consistent with the known regional deformational history.