The rocks of Kā-Tiritiri-o-te-Moana Southern Alps show different stages of compression in the mountain building process, starting with the meta-sedimentary rocks of the Torlesse Supergroup. These rocks are old (ranging from around 150 - 350 million years old) and stressed, but still hanging on to their origins as marine layers formed down the continental slope by turbidity currents off the coast of Gondwana, and not quite metamorphosed yet.
The Torlesse Supergroup dominates the mountain ranges and foothills east of the Main Divide, so much so, that our braided rivers forming the Canterbury Plains are characterised by grey "greywacke" sandstone cobbles, most used as aggregates for our roads.
Less well known is that the Torlesse Supergroup is not just made of sandstone but also contains many layers of mudstone (argillite), red coloured chert and even some conglomerates.
