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!
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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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