Over time, these grow large enough to weaken the rock. Modern use of deicing salt has exacerbated this problem. Brownstone's popularity waned because of this susceptibility to weathering. Unlike granite and marble, brownstone has never regained its popularity as a building stone in Boston. But it's better known in such areas as Beacon Hill, Back Bay, and Cambridge as a roofing material.
Slate formed from sediment that fell into deep water rich in organic matter is black.
A small amount of iron added to the mix, however, drastically alters the color of the sediment. In an oxygen-rich environment, the iron turns the slate red, just like a nail rusting in the rain. Slate has always been a popular roofing material, because it helps slow the spread of fire. Limestone Roughly 330 million years ago, a quiet tropical sea spread across much of what would become the Midwest. The warm, clear, shallow sea held a diverse range of swimming, crawling, and bottom-dwelling invertebrates.