The shale from which slate originate were deposited previously on clay beds. Subsequent earth movements tilted these beds of shale, at first horizontal, and the intense metamorphism that converted these into slates folded and contracted them. Slate, then , belongs to the metamorphic group of rocks and can be defined as a fine grain rock derived from clays and shale and possessing a cleavage that permits it to be split into thin sheets. INTERIOR use only slates are designates with an "I" or "interior" on specification sheets. EXTERIOR use is labels as either "E" or "exterior" on specification sheets.
