Most common sedimentary rock

Mudrocks, which include mudstone, shale and siltstone, are sedimentary rocks made up of particles smaller than 0.0625 mm. They account for around 65% of all sedimentary rocks on Earth, and possibly up to 80%. They consist mainly of clay minerals and most formed at the bottoms of oceans and lakes. Dead organic material, which mixed with the original sediments, is responsible for the occurrence of fossil fuels in rocks such as shale.

Low grade regional metamorphism turns shale into slate.