Arthurs Point, Shotover River

You are deep in the heart of an extensive layered rock formation called the Otago Schist. Although these greyschists started off life as Triassic mudstones on the sea floor near Gondwana, the layers are not seabed layers (as they are for sedimentary rocks). The layers are mm-cm thick sheets of white quartz and grey mica and resulted from recrystallised mudstone. Heating and squeezing at about 350 deg C and 30 km depth in a Jurassic subduction zone 'sweated' out the quartz. The streaky lines (lineations) of quartz on the flat surfaces are stretch marks that record the plate motion direction at this time.

The in place rocks here are mainly grey schist. The pebbles on the river beach are much more varied.

Look closely at the rusty stained part of the outcrop. Here you'll find a thin sandwich of greenschist containing scattered cubes of pyrite (fools gold). The pyrite has rusted and stained part of the outcrop a brown colour.

Boulders and cobbles on the beach consist of many different rock types, including minor coloured schist lithologies. Here is a checklist (at least 18 different rock types, there may be more):

OTAGO SCHIST: psammitic greyschist, pelitic greyschist, chlorite greenschist, epidote greenschist, albite-porphyroblastic greenschist, magnetite-porphyroblastic greenschist, stilpnomelane greenschist, piemontite schist, spessartine quartzite (metachert), hematite quartzite (metachert), vein quartz. OTHER: cataclasite, pseudotachylite (both from Moonlight Fault Zone), lamprophyre dike (Miocene), limestone, conglomerate and quartzose sandstone (fossiliferous, all from Oligocene Bobs Cove beds), red and green speckled Caples Terrane greywacke Caples Terrane (glacially transported).