Slate is formed from mud and silt, or very occasionally volcanic ash, which were deposited in layers usually referred to as bedding. In spite of the intense pressure and high temperature required to transform the original deposits into slate, it is often possible to see traces of the original bedding in the finished slate. These strata of bedding are readily recognised when the individual layers of mud and silt are not too homogeneous, but vary in composition or grain size. This heterogeneity can be seen on the surface of the slate as changes in colour due to compositional variation, or defraction of the cleavage due to changes in the grain size.
Bedding may difficult to identify in heterogeneous material where it is orientated parallel to the cleavage surface. Conversely it is most easily observed when cutting the cleavage surface at a moderate to high angle.