Shale is characterized by breaks along thin laminae or parallel layering or bedding less than one centimeter in thickness, called fissility. |
During compaction of siliciclastic muds the flaky clay particles are realigned to be parallel to each other and impart a fissility to the rock, which is commonly called shale. |
With increased pressure over time, the platey clay minerals may become aligned, with the appearance of fissility or parallel layering. |
Although often mistaken as a shale, siltstone lacks the fissility and laminations which are typical of shale. |