Peat

Peat is a deep organic layer which results from the partial decomposition of plant material. It is most common, in this area, on the Pennine moorlands where cold and waterlogged conditions slow bacterial activity and allow the build up of this surface layer. The peat acts like a large sponge retaining much rainfall which is then released slowly via tubes and gulleys into the mountain streams which feed the rivers.

The peat is acidic. There is little lime in the underlying rocks to neutralise the water and so streams tend to be acidic. In addition the dark peat stains the water which passes through it.