This section is from the book "British Wild Flowers - In Their Natural Haunts Vol5-6", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
In Section VIII some remarks have been made as to the similar character of Bogs, Heaths, and Moors, and the natural passage of one of them into the others has been pointed out in Sections I - III and V-VIII. Heaths in the usual sense are lowland, and drier than moors, and are usually developed upon a sandy or gravelly soil. Even in the uplands a heath only merges into moorland vegetation where the surface forms hollows and thick peat accumulates. A moor requires no surface soil of a particular character, but it does demand a certain sort of water, acid in character.
The peat of a heath is also acid, but relatively dry. Occasionally there is a thick impervious layer of sand and gravel (moor pan), which is cemented together by mineral solutions derived from the peaty soil. In the case of a wet heath there is a deeper layer of peat, often associated with a bog, where there is a resemblance to the moor formation. On the other hand, heathland, as will be seen, often passes into grassland.