Brownstone is a red to brown variety of sandstone, whose unusual color is caused in some instances by the presence of red iron oxide which acts as a cement, binding the sand grains together. During the Triassic period, vast thicknesses of brownstone were deposited in the Connecticut River Valley of the northeast, and they were used to build the many brownstone houses and buildings in the area between Baltimore and Boston. Is sandstone a durable building material? Discuss
Source: The Free Dictionary