Pressure and folding has transformed shale into slate, altering the original character of the deposits in various regions worldwide. [1]
Shale alternates with coal, stone, and conglomerate in coal-bearing strata, which vary in thickness from forty to fifty feet in central and eastern United States, but can reach 18,000 feet in some parts of North America and Europe. [2]
Shale depositions in early Carboniferous age show similarities between Europe and North America, indicating an arid climate with erosion from violent cloudbursts. [3]
Shale and slate of dark colors in transition rock deposits testify to the existence of plant life ancestors during the Carboniferous age, with some containing organic carbon. [4] Limestone changed into shale over 300 million years ago during a period of land submergence. [5]