Ordovician (map symbol O)

Ordovician rocks are found in the western South Island and southern Fiordland.

Limestone and marble are characteristic of the Ordovician in northwest Nelson, with the marble being particularly well exposed on Takaka Hill. Hard sandstone, argillite, black shale, and white quartzite bands occur mainly to the west of the limestone. Many of these lithologies are metamorphosed to some degree. Alternating hard sandstone and argillite extend down the South Island West Coast as far as Milford Sound. Rocks at Preservation Inlet in southern Fiordland have Ordovician fossils.

Paleogeographic conditions
The sandstone and argillite were deposited in a fairly deep trough lying offshore from Gondwanaland, while the limestone was deposited on the Cambrian volcanic chain which had become extinct.

The Ordovician rocks in northwest Nelson are complexly deformed and some were also involved in the major thrusting which affected Cambrian rocks. The Ordovician rocks further south are not as deformed, but tend to be steeply dipping and in places tightly folded.