The buildings shown above are the two oldest surviving buildings in the town of Rockmart, in northwest Georgia (U.S.A.). The upper is the old town store, and the lower is the Presbyterian Church.|
Both structures are built of slate, a fine-grained metamorphic rock with a well-developed cleavage or tendency to split along flat planes. The result is a platey stone, so that many of the stones visible in these buildings give an elongate aspect. One piece at the upper right of the image below is turned perpendicular to the others, so that a light gray cleavage plane can be seen, rather than the dark gray broken surfaces visible across the rest of the image.
The slate buildings are appropriate for Rockmart, because the town is surrounded by outcrops and quarries of the Rockmart Slate. Slate has been quarried near Rockmart since 1850, and it is still quarried by the
Rockmart Slate Corporation. Rockmart slate has been used widely as a buidling and landscaping stone, and it has been used as far away as London, England. It is also used as a light-weight aggregate in concrete where weight is critical, and so was for example used in the Golden Gate Bridge. The area's production of slate gave Rockmart its name when the town was chartered in 1872.|
The Rockmart Slate was deposited as a marine mud on the seafloor in the Ordovician, about 460 million years. It subsequently lithified into a shale, a sedimentary rock, and later was metamorphosed to form the slate we know today. James F. Renner's M.S. thesis work at the University of Georgia showed that the Rockmart Slate consists mostly of muscovite and clinochlore, a chlorite mineral. Comparison of this mineral assemblage with those in overlying rocks led Renner to conclude that the Rockmart Slate was metamorphosed before the overlying Mississippian Frog Mountain Sandstone was deposited, which would essentially require metamorphism of the slate in the Silurian, surprisingly soon after its deposition.
Today natural outcrops of the Rockmart Slate can be seen along one of Rockmart's new attractions, the Silver Comet Trail. The pictures below show an outcrop of the slate south of Rockmart along the trail.
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