# Killas

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{{Short description|Rock formation}}
{{about|Cornish mining|the plural|killa (disambiguation)}}
{{Use dmy dates|date=April 2022}}

[[File:Millook cliffs enh.jpg|thumb|right|upright=1.2|The recumbent [chevron fold](/source/Chevron_(geology))s in the cliffs at [Millook](/source/Millook), considered to be killas, in Cornwall<ref name="Camm_2011">{{cite book |title=The Geology and Landscape of Cornwall and the Isles of Scilly |publisher=Alison Hodge |last=Camm |first=S. |year=2011 |publication-place=Penzance |pages=11–14 |isbn=978-0-906720-83-7}}</ref>]]
'''Killas''' is a [Cornish](/source/Cornish_language) [mining](/source/mining) term for [metamorphic rock](/source/metamorphic_rock) [strata](/source/stratum) of [sedimentary](/source/sedimentary) origin which was altered regionally by the [Variscan orogeny](/source/Variscan_orogeny) and then locally by heat from the intruded [granites](/source/granites) in the English counties of [Devon](/source/Devon) and [Cornwall](/source/Cornwall). The term is used in both counties.<ref>{{cite web|url=https://sites.google.com/view/cornwallgeoconservationgroup/cornish-geology|title=Killas|last=Cornwall Geoconservation Group|author-link=Cornwall Wildlife Trust|year=2023|work=Cornish Geology|accessdate=4 January 2023}}</ref>

== Etymology ==
"Killas" first appeared in print in 1674 as "kellus".<ref name="WEHD">{{cite web |url=https://wehd.com/51/Killas.html |title=Killas |work=World English Historical Dictionary |access-date=26 February 2026}}</ref> As a Cornish [place name](/source/Toponymy) element, ''kellys'' is found in [Porkellis](/source/Porkellis) where it means 'lost' or 'hidden' (a [passive-participle](/source/Passive_participle)), but its [etymology](/source/etymology) as [Cornish dialect](/source/Cornish_dialect) is unknown.<ref>{{Cite book |last=Padel |first=O. J. |title=Cornish place name elements |date=1985 |publisher=Engl. Place Name Soc |isbn=978-0-904889-11-6 |series=English Place Name Society |location=Nottingham |pages=48}}</ref><ref>{{Cite book |last=Padel |first=O. J. |title=A popular dictionary of Cornish place-names |date=1988 |publisher=Hodge |isbn=978-0-906720-15-8 |location=Penzance}}</ref> The reconstructed proto-Celtic verb ''*kel-o-'' from the PIE ''*<sub>4</sub>k̑el-'' also means 'to hide'.<ref>{{Cite book |last=Matasović |first=Ranko |title=Etymological dictionary of proto-Celtic |date=2009 |publisher=Brill |isbn=978-90-04-17336-1 |series=Leiden Indo-European etymological dictionary series |location=Leiden |pages=199}}</ref><ref>{{Cite web |title=Indo-European Lexicon: Pokorny Master PIE Etyma |url=https://lrc.la.utexas.edu/lex/master |access-date=2026-02-25 |website=The Linguistics Research Center, The University of Texas at Austin}}</ref>

==Petrogenesis==
The deposition of the killas strata occurred during the [Devonian](/source/Devonian) and [Carboniferous](/source/Carboniferous) geological periods. The [sediment](/source/sediment)s are not evenly spread over the county, with the Carboniferous beds only found in the north of Cornwall.

The depositional environments of the killas were very varied, as is revealed by the fossil content and the sedimentary sequences. The fossils indicate changes from anaerobic, deep ocean-basin environments to shallow sea environments. The deformed [brachiopod](/source/brachiopod) fossil ''[Cyrtospirifer verneuili](/source/Cyrtospirifer_verneuili)'',<ref name="CSM">{{cite web|url=http://projects.exeter.ac.uk/geomincentre/geology.htm|title=Geology|last=Camborne School of Mines|author-link=Camborne School of Mines|date=15 January 2007|work=CSM Virtual Museum|publisher=University of Exeter|accessdate=25 February 2012}}</ref> known to quarrymen as the [Delabole Butterfly](/source/Delabole_Butterfly), was found in the upper Devonian beds of North Cornwall.

Shortly after the deposition of the sediments, the Variscan orogeny caused low grade regional metamorphism of the sediment followed by the intrusion of the [Cornubian batholith](/source/Cornubian_batholith) and the subsequent [contact metamorphism](/source/contact_metamorphism) created the metamorphic rocks seen today. The orogeny put intense pressure on the sediments causing them to be [folded](/source/fold_(geology)) and [faulted](/source/geologic_fault). This is most apparent in the cliffs at [Millook](/source/Millook) Haven on the north Cornwall coast, where the cliffs display an impressive series of recumbent angular folds.

==Economic uses==
Killas underlies two thirds of Cornwall and on and around [Dartmoor](/source/Dartmoor) in Devon, and, as a result of [hydrothermal mineralization](/source/hydrothermal_mineralization) from the granites, contain the majority of the mineral [lode](/source/lode)s or [veins](/source/vein_(geology)) which at one time provided up to half of the world's [tin](/source/tin) and [copper](/source/copper), and economic prosperity to Cornwall (and to a lesser extent Devon). Of lesser economic importance, the stone itself is an important building stone, while locally, in the Delabole to Tintagel region, there are good quality roofing slates with the largest single source in the county being [Delabole](/source/Delabole) slate quarry, which has provided a high quality stone for at least six centuries.

== See also ==
{{Portal|Cornwall}}
* [Cornish Killas](/source/Cornish_Killas) - [national character area](/source/national_character_area) 152

==References==
{{Reflist}}

==External links==
* [https://sites.google.com/view/cornwallgeoconservationgroup/cornish-geology Killas page of the Cornwall Geoconservation Group, the geological advisory group of Cornwall Wildlife Trust]

Category:Geology of Cornwall
Category:Geology of England
Category:Metamorphic rocks
Category:Mining in Cornwall

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Adapted from the Wikipedia article [Killas](https://en.wikipedia.org/wiki/Killas) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Killas?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.