Greywacke

Key facts and history

Greywackes are very frequent among Silurian and Cambrian rocks but less common in Mesozoic or Cenozoic strata.^[2] Greywackes are abundant in Wales, southern Scotland, the Longford-Down Massif in Ireland, the Lake District of England, and compose most of the Southern Alps of New Zealand.^[3] The grains in greywacke are often angular and not well-rounded, and the rock is frequently hardened by recrystallization, including the addition of silica between grains.^[2] While some greywacke deposits show cleavage, this feature is less developed in them than it is in slates.^[2]

In the Ecca Group of South Africa, researchers have identified both feldspathic and lithic varieties of greywacke.^[4] In Britain, greywackes were among the first rocks studied by geologists after the Geological Society was founded in 1807, and their distribution across the country helped reveal the sequence of geological strata.^[5]

What the research sources say

The Ancient Egyptian Materials account states that greywacke is a term referring to an immature sandstone, generally indurated, dark grey, and consisting of poorly sorted angular to subangular sand-sized grains.^[1] The account explains that the metamorphism of greywacke increases the cohesiveness of its mineral grains and rock hardness, making it less susceptible to fracture during carving.^[1] It also reports that the metagreywacke of Wadi Hammamat was called bhn (bekhen) by the ancient Egyptians, and some stone artifacts were inscribed with this name.^[1] The source adds that the origin of the modern word "basalt" is from these words for greywacke.^[1]