Westmorland Slate

January 1, 201312:00 pmMay 3, 2017 2:48 pm
A close up of a polished slab of green Westmorland Green Slate from Honister. (Courtesy Mikenorton via Wikimedia Commons CC-by-SA 3.0)

A close up of a polished slab of green Westmorland Green Slate from Honister. (Courtesy Mikenorton via Wikimedia Commons CC-by-SA 3.0)

Also known as Honnister Slate, this is variety of slate from the Borrowdale Volcanic Group of Cumbria. Unusually it is formed from a tuff (volcanic ash deposit) – erupted during the Caradoc Age of the Ordovician (458 to 448 million years ago) – which has been subjected to high pressure and heat, metamorphosing it to a slate. Large amounts of the mineral chlorite give the slates a green-grey or green colour.

The green-grey slates are from the lower seam quarried near the Honnister Pass (and now marketed as Honnister Slate) while the green slates are from the upper seam near Ambleside. Relict igneous crystals give individual slates a much bumpier surface than a typical mudstone slate (e.g. Welsh Slate) and in some cases roof pitches may be steeper then their smooth slate equivalent as a result.

Many quarries formerly worked the slates, initially as independent companies but increasingly merging into larger conglomerates; first the Lakeland Green Slate Ltd then incorporated in the Burlington Slate Co. in 1975. Today the main workings are quarries at Elterwater (NY 324 048) in the upper seam and the Honnister Slate Mine (NY 222 135) in the lower.

An example of its use is for the Jacob Fountain inside Malvern Theatres. Anonther example of use is said to be the Chancel roof of St Peter’s Church, Bromyard.

For more information see this article on the British Slate Forum ->

Written by Elliot Carter - Modified by Ella Young

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    Also known as Honnister Slate, this is variety of slate from the Borrowdale Volcanic Group of Cumbria. Unusually it is formed from a tuff (volcanic ash deposit) – erupted during the Caradoc Age of the Ordovician (458 to 448 million years ago) – which has been subjected to high pressure and heat, metamorphosing it to […]