On our wish list for a sample.
The following is excerpted from: http://www.mgs.md.gov/geology
Seneca sandstone is a reddish to purplish brown, fine-grained, arkosic sandstone or "Brownstone" was quarried extensively along the Potomac River in Montgomery and Frederick Counties. This Triassic stone was initially used for the "Potowmack Canal" built around 1774 and again for the C & O Canal in the early 1830's. It became a popular building stone during the period between 1840 and 1889. Many buildings in Washington, D.C., and in Baltimore were constructed of brownstone. In 1847, it was used to build the Smithsonian Institution Building in Washington, D.C. The stone is readily carved and chiseled when first quarried but later hardens on exposure to air. There is a tendency of the rock to spall if laid "on edge," therefore, careful placement was required.