About 400 million years ago in this region, layers of sediment accumulated in ocean waters were subjected to intense pressure and heat during tectonic and mountain-building action, causing a metamorphism of minerals collectively known as shale and transforming them into slate, a fine-grained, foliated rock consisting of some of the thinnest sheets on earth. The natural face of this rock displays the grain, a lasting trace of the original sediment. The seam shows the parallel lines that allow this stone to be easily split. Slate is anti-bacterial, stain resistant, noncombustible, and unmatched in strength and durability. Quarries yield red, unfading green and dark green, fading green, unfading mottled purple, and unfading gray and black.