Westerwald (natural region)
As well as being a low mountain range, the Westerwald is also a natural region in the system of natural regional division of Germany. Within that it is a major unit group with the number "32". According to this system the major unit group of the Westerwald belongs to the basement plate ("Grundgebirgsschollenland"), which describes the type of mountain-building process by which it was formed. The major unit group extends across the states of Hesse, North Rhine-Westphalia and Rhineland-Palatinate. It is roughly bounded by the valleys of the Lahn (east and south), Rhine and  Sieg (via Heller), whereby the hills immediately south of the Heller and Sieg are not considered to be part of it. Natural region divisions. Below the major unit group are the major units which, in turn, are divided into sub-units, part-units and base-units. The Westerwald in a narrower sense is generally considered to be the major unit groups 322 to 324.