Salisbury Falls

Salisbury Falls consists of a roughly four-metre high waterfall which cascades down into a small pool before flowing some 30 metres into the Aorere River (the site of the main swimming hole). Observant visitors will notice the plethora of perfectly shaped skimming stones lying in and around the waterfall. This collection of thin, flat, shiny grey pebbles is a product of the geology of the area around the falls. These pebbles are made of a rock called phyllite.

Phyllite is a low-grade metamorphic rock formed from mudstone, which is itself formed when millions of tiny tiny grains of sediment accumulate together and are buried under even more sediment. Over time, the weight of the sediment accumulating on top of the tiny mud grains squishes the mud, forcing out water and other things – this is called lithification, and is what transforms mud into the harder rock that geologists call mudstone. For many mudstones this is the end of the road, but some mudstones go through even more burial, or are forced deeper underground by plate tectonics. These mudstones are squashed further, and heated – temperature increases with increasing depth below the earth’s surface. This squishing and heating leads to a process which geologists call metamorphism.

Metamorphism is when a rock’s chemical and physical characteristics change as a result of heat and/or pressure. The large rocks which form the steep sides of the swimming hole are a type of metamorphosed mudstone called phyllite, which is very very old – somewhere between 443 and 490 million years old.

You may have noticed that, along with the flat pebbles common at Salisbury Falls, a number of the large rock faces which surround the swimming hole are very smooth and planar. The large smooth faces and the small smooth pebbles are linked, and are both caused by the metamorphism. These rocks contain minerals called micas (biotite and muscovite are common examples) which grow at a flat orientation and are very thin – much like a piece of paper. The phyllite rocks have layers upon layers of mica minerals in them which, when stacked together, form a much harder rock, like a ream of paper. When these rocks break, it tends to fracture along the lines of the mica ‘paper’ rather than through it. This causes the flat shiny ‘cleavage planes’ that you observe in the rock.

Phyllite is the dominant rock at Salisbury Falls, but a number of other rock types are found here – mostly due to the powerful water washing them down from upstream. These include different varieties of granite, limestones, sandstone and others. How many can you collect and identify?