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Eastern greywacke basement rocks

Greywacke Basement Map

Greywacke is New Zealand’s most common rock type. It is a hard, grey, “dirty” sandstone by one definition and greywacke is also used to collectively describe the sandstone, accompanying mudstone and other less common rock types.

These rocks originally accumulated as sediment in a submarine trench at the ancient plate boundary on the margins of the Gondwana super-continent between 300 and 100 million years ago. Through deep burial and tectonic forces these sediments lithified (solidified) into hard rock. Despite their hardness greywacke rocks can be highly fractured resulting in loose and unstable ground, particularly along fault zones

In the Southern Alps mountaineers sometimes refer to these shattered rocks as “weetbix”. Greywacke forms the older basement rocks of the eastern part of the South Island and the higher non-volcanic mountains of the North Island. The hills and coast of the Wellington region are made of greywacke.

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