This is a slightly expanded version of my letter “W” contribution to the current weekly Alphabetical Series in the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils”. In the 2021 version of this Series I posted “W is for “Ward Beach Zoophycos”.
W is for Wacke
This stone is a wacke – in fact, it is a Gemstone Beach greywacke (pronounced “grey-wacky”) photographed for me by Chrissy for my blog series on stones from that beach – it’s the first stone in this Post. Greywacke is the most common stone on NZ beaches, providing what often looks to be large areas of boring grey. I hadn’t collected any greywacke specimens myself as these usually bland grey sandstones are not very interesting to a tumble polisher. However, they are very interesting for other reasons. In 2003, Heather Nicholson submitted a doctoral thesis at the University of Auckland, at the age of 72!, with the title “The New Zealand Greywackes: A Study of Changing Geological Concepts to 1985”. She chose as the title for a talk to the Nelson Rock and Mineral Club, “Greywacke: The World’s Most Boring Rock… though it isn’t really because it’s got some fascinating stories to tell”.
Wackes are poorly sorted sandstones, the tiny grains of sand in them being angular, not rounded (Mindat). These grains are mainly quartz, feldspar, pyroxene and mica. Textbooks often refer to greywacke as consisting of such grains “chaotically” embedded in silt and clay. Greywacke is the bedrock of not only New Zealand but also the whole Zealandia continent, as explained so well in the excellent YouTube video “Greywacke Bedrock – The Backbone of New Zealand”.
Some New Zealand greywacke is over 300 million years old. Rock material was eroded from the ancient super-continent Gondwana to be deposited by rivers in the sea along its eastern coast, forming the basis for Zealandia and New Zealand. Undersea avalanches and strong turbidity currents meant a mix of poorly sorted grain sizes can be found in greywacke (Wikipedia). This means greywacke is a “turbidite” (see photo below, left), forming via deposition from sediment laden currents that move very rapidly downslope through water.
There are interesting links between Germany, Scotland and New Zealand in relation to the concept of greywacke. “Greywacke” is a word of German origin, from “grauwacke”, meaning grey sandstone. Te Ara The Encyclopedia of New Zealand notes that “grauwacke” was first used in the 18th century to describe rocks in Germany’s Harz Mountains. Ernest Dieffenbach, a German scientist who travelled widely in New Zealand between 1839 and 1841, was the first person to apply the name to local NZ rocks. English geologists regarded greywacke as an “uncouth foreign term”, but it was adopted in Scotland. Archibald Geikie’s text-book of geology, published in 1903, gave descriptions of greywacke, and helped persuade New Zealanders that it was an appropriate term for their most widespread rock.
In the 1960s some geologists argued that the term greywacke was vague and imprecise. A subcommittee of the NZ Geological Society recommended that it be dropped, but this was widely ignored. The term greywacke is possibly used more widely in New Zealand than anywhere else in the world.
The next Post in this Series is “X is for Xcellent Xample of an Xtrusive Igneous Stone on Xmas Eve”. An Index for this Alphabetical Series is here.
W is for Washe…https://washekoda.blog/2025/12/28/washe-koda-iktomi-sha-kon-a-hay-cougar-friend-trickster-from-the-the-land-of-smokesmokey-mountains-tennessee/