Definition and characteristics of very-fine grained sedimentary rocks : clay, mudstone, shale and slate

Clay, shale, mudstone, siltstone and slate are all very fine-grained sedimentary rocks. Their
character, including hardness, can vary markedly depending on their geological age and the
extent to which they have been buried and altered by tectonic events. In general, therefore, slates
and harder mudstone and shales tend to occur in geologically older rocks (pre–Permian
> 280 million years). Whilst clays and mudstone are valued for their plasticity, which allows
them to be shaped, slates are very hard and durable and economically valued because of their
ability to be split along thin parallel cleavage planes. However, the terms slate and shale may be
applied to rocks that lack plasticity and a sufficiently well developed slaty cleavage to make of
them of commercial interest. These materials may be suitable for use for low-grade aggregate
purposes.