File:Gray slate (Martinsburg Formation, Ordovician; near Bangor, Slate Belt, Pennsylvania, USA) 1.jpg

English: Gray slate from the Ordovician of Pennsylvania, USA.

Slate is a foliated metamorphic rock. It forms by low-grade metamorphism of shales and non-fissile mudrocks. Slates typically weather into relatively thin, platy pieces and superficially resemble fissile shales. Slate's foliation is often not conformable to the original bedding. Sometimes, the original bedding (and fossils) can be discerned on foliation planes of slate. The color of slate is generally blackish, grayish, greenish-gray, or brick-red

Distinguishing slate from shale can be tricky, but slates are harder and heavier than shales. The broken edges of shales and slates also have different geometries - slate has a more consistently stepped broken edges, while shale often has blocky broken edges. When tapped together, two pieces of shale make a relatively soft “clicking” sound. Two pieces of slate tapped together make a louder “ringing” sound, or resonance.

Historically, slate was often used as roofing material, due to its impermeability (it keeps out the rain). Slate roofs do weather eventually, but the physical degradation rate of slate slabs is relatively slow.

Slates sometimes contain fossils, but they are usually only recoverable if the metamorphic foliation and the original shale bedding are congruent, or if the original bedding planes are readily split. This doesn’t happen at many localities.

The gray slate seen here is from eastern Pennsylvania's Slate Belt, a famous slate quarrying district. Slate was extracted to make roofing tiles.

Stratigraphy: Martinsburg Formation, Ordovician

Locality: unrecorded / undisclosed site near the town of Bangor (possibly a quarry), Slate Belt, Pennsylvania, USA