Naturally occurring radioactivematerial (NORM) is a common characteristic of black shales like the Marcellus Shale. This is because radioactive elements preferentially adhered to the clay and organic material that sank to the bottom of the Devonian shallow sea and formed the Marcellus Shale. Because black shales tend to have more organic material (and thus more gas) and clay than other rock layers, the amount of NORM in these layers is higher as well