Your specimen is probably slate. Slate forms when shale,
mudstone and volcanic ash are altered by low grade metamorphism. The pressures
and temperatures are low so the only soft rocks like these are changed. The
minerals in these parent rocks realign themselves into layers which allows slate
to be split into sheets very easily.
Slate is a waterproof rock and has been used for many years to tile roofs. It is
not found in the Potteries area but can be seen in many parts of Wales where
slate mining was an important industry for many years.