South
Stack to Rhoscolyn, NW Anglesey
Grid Reference: SH 201 823 to SH 272 751
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Over 15km of coastal exposures of deformed and metamorphosed clastic
sedimentary rocks , originally thought to be late Precambrian, but now
thought to be mid- to late-Cambrian, make up this classic area,
featuring some of the most spectacular scenery on Anglesey. The rocks,
belonging to the Monian Supergroup, fall into two major divisions - the
South Stack Group and the New Harbour Group. The former is subdivided
into three units, the South Stack Formation, the Holyhead Formation and
the Rhoscolyn Formation. The strata exposed at South Stack were
considered for many years to be the oldest rocks on Anglesey. Recent
research has literally overturned this notion, and recent radiometric
dating of detrital zircons has given the South Stack Formation its
revised Cambrian, as opposed to late Neoproterozoic, age.
Lithotypes, varying from quartzites to pelites, have been intensely
folded, faulted and intruded by dykes, providing an outside laboratory
for tectonic studies for all ages. Despite the rather intense and complex structures and
a metamorphic overprint, examples of sedimentary structures, including
graded bedding, sole marks, coarse channel-fills and pre-lithification
structures produced by dewatering may still be recognised.
The site features the famous Rhoscolyn Anticline, a major fold of
uncertain age, with a steep to overturned SE limb carrying small-scale
S-bend folds, the hinge area featuring M-folds and the NW limb
displaying Z-folds. Another feature at Rhoscolyn is the memorial plaque
to Dennis Wood, situated on the crest of the anticline by the HM
Coastguard lookout. Dennis was a major player in the field of
geological research on Anglesey, and inspired generations of students
in his teaching.
Just inland, there are bodies of basic to ultrabasic rocks including
ophicalcite, serpentinite and gabbro. Their relationship with the
surrounding sedimentary strata (are they a dismembered ophiolite
sequence or are they just straightforward intrusions?) has - like many
other aspects of the geology in this fascinating area - been hotly
debated over many years! The former is the currently accepted model.
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REFERENCES
BARBER, A.J. & MAX, M.D. 1979. A new look at the Mona Complex
(Anglesey, North Wales). Journal of the Geological Society, London,
136, 407-432.
COLLINS, A.S. & BUCHAN, C. 2004. Provenance and age constraints of
the South Stack Group, Anglesey, UK: U-Pb SIMS detrital zircon data.
Journal of the Geological Society, 161, 743-746.
GIBBONS, W., TIETZSCH-TYLER, D., HORÁK, J.M. & MURPHY, F.C.
1994. Precambrian rocks in Anglesey, southwest Llyn and southeast
Ireland. In: GIBBONS, W. & HARRIS, A.L. (eds) A revised correlation
of Precambrian rocks in the British Isles. Geological Society, London,
Special Reports, 22, 75-83.
GREENLY, E. 1919. The geology of Anglesey. Memoir of the Geological
Survey of the U.K. [2 vols].
MALTMAN, A.J. 1977. Serpentinites and related rocks of Anglesey.
Geological Journal, 12, 113-128.
SHACKLETON, R.M. 1975. Precambrian rocks of North Wales. In: HARRIS,
A.L., SHACKLETON, R.M., WATSON, L, DOWNIE, C., HARLAND, W.B. &
MOORBATH, S. (eds) A correlation of Precambrian rocks in the British
Isles. Geological Society, London Special Reports, 6, 76-82.
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Above: the coast
leading to the South Stack lighthouse. Photo: Stewart Campbell
Above: a geological
field-party traversing the Rhoscolyn Anticline. Photo: Bill Fitches.
Above:
Soft-sediment dewatering lobe deformed by tectonic fold and cleavage.
Photo: Bill Fitches
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