LOCATION: The outcrops are along a railroad spur to the PPG Chester Plant, just north of SC-9 and just west of S-12-74, east of Chester. Park along S-12-74 and walk west to the railroad cuts. (Fig. 5, Chester Quadrangle)
DESCRIPTION: The rocks exposed in the banks exposed of the railroad cut are mainly dark-colored, medium grained metagabbro that is the most extensive rock type in the York-Chester mafic complex. Nearly everywhere the metagabbro is deeply weathered and forms low, flat, thickly vegetated topography. This is the best accessible exposure found in the region so far. The metagabbro is generally massive, but locally it is strongly deformed and has a tectonic foliation. There are shear zones of chlorite-rich schist, retrograded from the metagabbro. The metagabbro has some inclusions of mafic and possibly ultramafic rocks, and it is cut by several mafic dikes and a number of pegmatite and granite dikes. In thin sections the massive rocks have relict igneous hypautomorphic-granular texture, strongly modified by replacement of original minerals by fine-grained, greenschist facies minerals such as actinolite, chlorite, epidote and albite. The texture, chemical composition, and relict crystals of hornblende, pyroxene, and calcic plagioclase indicate that the rocks was originally hornblende gabbro or gabbro-norite. Regional metamorphism here was under greenschist facies conditions, in contrast with regional amphibolite facies conditions in most of the Charlotte belt.
Near the southern end of the railroad cuts and about 80 meters north of SC-9, there are several low exposures of residual boulders of unmetamorphosed olivine gabbro. The gabbro is very different from rocks farther north in the outcrop, although there are no clear differences in soil and saprolite in between that would define a contact. The gabbro is similar to rocks that make up the Chester gabbro pluton, about 1.4 km to the west along SC-9. The gabbro is medium-grained and composed of plagioclase, olivine, augite, hypersthene, hornblende, and opaque minerals, with small amounts of biotite and spinel. This occurrence of gabbro is interpreted to be a dike or a small plug related to the Chester pluton, but it could be part of a larger body underlying the covered area south of the outcrop. These outcrops illustrate the difficulties of mapping gabbro versus metagabbro.