A fine-grained red, reddish-brown, and gray sandstone from Seneca Creek, Maryland; it contains coarse- to fine-grained angular quartz as well as some feldspar and mica fragments. It is fairly easy to carve but darkens and hardens when exposed to air, making it one of the most durable building stones. Seneca sandstone was extremely popular in Washington, D.C. during the 'brownstone era' of about 1840 to 1880; the original Smithsonian Institution building, for example, is made of Seneca sandstone.
