Torlesse Supergroup These are tough sedimentary rocks mostly made of layers of sandstones and siltstones. The 'greywacke suite' may also include basaltic and spilitic lava, tuff, jasper and chert. Limestone is rare. There are local variations in both origin and composition, induration, and metamorphism. The rocks of the Torless Supergroup are marine sediments of detritic origin, deposited on the western margin of the New Zealand landmass.
Ages range from permian to lower cretaceous.
These are the metamorphosed equivalents of the Torlesse Supergroup, with gradations in the degree of metamorphism. Five sub-zones have been defined based on mineral associations of chlorite, biotite and garnet. Schistosity may have a strong influence on landforms. The platey structure of the rock encourages disintegration, and these rocks are more prone to weathering and erosion than greywackes. Schistsity also favours the formation of landslides, often slow and involving whole mountain sides.