Metamorphism reflects the mineralogical changes imposed on sedimentary or igneous rocks as a result of increases in temperature or pressure. These can derive from a number of factors, such as the effects of the intrusion of a body of hot magma at a high level in the earth's crust. This is a process known as contact metamorphism, the effects of which are dominated by increases in temperature but the changes occur only locally. Wider, regional effects of metamorphism are experienced where major crustal processes, linked to plate tectonics, cause increases in both temperature and pressure. The mineralogical changes are most evident where major plate collisions have led to mountain building episodes and deep crustal burial, and the rocks become totally transformed both mineralogically and texturally. In contrast, metamorphic changes are more difficult to detect in areas of more modest tectonic processes, such as across much of Wales, where the mineralogical changes are more subtle and the original rock textures are largely preserved. Indeed, until relatively recently Wales was described as being 'un-metamorphosed'. However, research led by the Museum has not only identified that the Lower Palaeozoic sequences in Wales have experienced 'low grade' metamorphism, but has also established Wales a type region for metamorphism of a kind that is linked not to plate collision but to crustal extension. This arises as heat flow from lower crustal levels is induced as extension occurs and isotherms (levels of equal temperature against depth) are brought closer to the earth's surface, effectively heating the rock sequence from below.