Geology
of Anglesey: A journey through time
Part
1 Part 2 Part
3
Part 4
Part
1: Setting the scene:
some troublesome rocks....
Geologists have been debating the problems of Anglesey's geology since
the early 19th Century, when the first attempts to describe the
island's rocks were made. Nearly a century later, the island was
mapped, privately, on a 6 inches to the mile scale, by Edward Greenly;
on seeing his superb maps, the Geological Survey decided to publish a
one-inch map as well as a 2-volume Memoir, these being printed in 1919.
The root of almost all of the controversy since Greenly's time has been
the interpretation of the old, pre-Arenig rocks on the island. Thought
at one time to be entirely Precambrian, it is now known that they
straddle the period between late Precambrian and late Cambrian times,
between about 700 and 500 million years ago.
These old rocks, consisting mostly of turbidites, debris-flow deposits,
metabasites, granites and gneisses, with metamorphic grades varying
from blueschist to greenschist to amphibolite facies, clearly represent
parts of an ancient destructive plate margin. The problem lies in the
interpretation of how each unit is related to the others, for they are
typically separated by major structural breaks, steep zones of shearing
and mylonite development, indicating that they have arrived in their
current juxtaposition by tectonic movements. The trouble is, we don't
know how far they have travelled to get where they are now.
Let's now get familiar with the individual rock units, because their
names will crop up frequently in this and the following pages. We'll
worry about their relationships later!
South
Stack Group: this group consists of bedded alternating sandstones
and siltsones, including some massive quartzites. These are clearly
turbidites - clastic sedimentary rocks deposited by turbulent
underwater currents laden with debris in a marine sedimentary basin.
New
Harbour Group: these, too, are mostly turbidites, but their
grainsize is typically much finer, with abundant, greenish silty or
muddy layers (pelites) alternating with thin sandy layers (psammites).
The sediment is chiefly the reworked product of island-arc volcanoes.
The group also includes some bedded cherts, tuffs and spilites -
undersea lavas of ocean-ridge affinity - metamorphosed into mafic
greenschists. Less competent (rigid) than the South Stack Group, the
strata usually show spectacular, intense folding.
Gwna
Group: again,
this unit consists of greenish silty and sandy layers, but it features
an extraordinary and spectacular mélange. A mélange is
the product of a major undersea debris-slide of catastrophic
proportions, and this one, cropping out not only on Anglesey, but also
on northern Llyn from Nefyn to Bardsey Island, lives up to the name.
Within that green silty matrix there are clasts, ranging from pebble
size up to rafts over a kilometre across, of a variety of rocks, all
jumbled together. The diverse clasts include stromatolitic limestone,
basaltic pillow-lavas, bedded cherts, red mudstones, white quartzite
and - rarely - granite.
Skerries
Group:
this is a weakly deformed, bedded succession of pebbly sandstones,
conglomerates and basalts, the pebbles being derived from a
sub-volcanic granitic to felsic volcanic source. The presence of
abundant granitic and volcanic clasts suggests probable derivation from
an island arc, the location of which is yet to be identified.
Coedana
Gneisses and Granite: cropping out in central Anglesey is a
muscovite-granite, intruded into obviously high-grade acid and basic
gneisses. Both appear to be related to ancient island-arc magmatism,
and as such may represent the exhumed roots of an arc complex
(sometimes referred to as the Older Arc).
Gabbros
and serpentinites: apparently restricted in their occurrence to the New
Harbour Group, these basic to ultrabasic intrusive rocks are highly
altered in places (with conversion of olivine to serpentine). Locally,
they have been quarried for ornamental serpentine, and at one locality,
for chromite
Blueschists: forming a linear belt
in the south-eastern part of Anglesey, these rocks include metabasites
containing a metamorphic mineral assemblage that shows that they have
been subjected to unusually high pressures, such as those found deep
within a subduction zone.
So, now we know what Precambrian rocks we have on Anglesey, we can
continue on to the next part: how geologists have attempted to
interpret them over the years!
Go to Part 2
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Above: quartzites of the Cambrian (and formerly thought to be
Precambrian) South Stack Group, exposed at South Stack on Holy Island.
Photo: Stewart Campbell.
Above: intensely folded rather pelitic strata of the New Harbour Group,
at Rhoscolyn. Photo: Bill Fitches.
Above: red, jaspery chert in between spilitic lava "pillows", from the
Gwna Group on Llanddwyn Island. Photo: Brian Windley.
Above: the Coedana Granite in situ.
The coarsely crystalline nature of the rock is evident. Photo: Jana
Horak.
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