This wall is on a building at Swarthmore College, PA. The rock is a schist, generally a higher metamorphic grade than the rocks of the Vassalboro Formation. They show a lot of mica on the exposed surface. These surfaces also display small characteristic folds which give them their finely-rippled surface texture -- a texture which catches and reflects sunlight all through the day. Because schists are micaceous and tend to split into thin layers, they may be prone to mechanical weathering as water seeps in along the layers and freezes, causing them to split apart.