Greenbrier is underlain by a Precambrian metamorphic rock formation of the Ocoee Supergroup known as Roaring Fork Sandstone. This rock type, created from ancient ocean sediments 750 million years ago, dominates the mid-level elevations in the north-central Smokies. As the elevation increases to the south, a thrust fault known as the Greenbrier Fault separates the Roaring Fork sandstone from Thunderhead sandstone and Anakeesta Formation, both of which are common along the Smokies crest. To the north, as the land flattens out (near the park boundary), another thrust fault, the Gatlinburg Fault, separates the Roaring Fork formation from a layer of siltstone, which underlies the foothills around Emert's Cove.