GEOLOGICAL JOURNAL

Geol. J. 44: 623–624 (2009)

BOOK REVIEW

ANGLESEY GEOLOGY—A FIELD GUIDE;DAEAREG YNYS MON—ARWEINLYFR MAESby Jack Treagus. Seabury Salmon & Associates,Ludlow, 2008, funded by GeoMon Anglesey Geopark.No. of pages: 168. Price £11 including postage andpacking (cheque to AGP), available from Dr. M. Wood,College, Llansadwrn, Menai Bridge, Anglesey LL595SN. ISBN 0-9546966-2-X (softback springbound).

Anglesey Geology is the third general field guide to the islandto appear. The first was a very brief account of 12 pagespublished by Greenly in 1921, designed to be used inconjunction with his memoir and map. The second, of31 pages, by Bates & Davies, was published in 1981 in theGeologists’ Association series of field guides. In contrast tothe earlier guides, Anglesey Geology, although containingfewer localities than the GA guide, runs to 168 pages (butas the text is bilingual, English and Welsh, probably about100 pages). It is beautifully illustrated, with photographs incolour and maps largely in black and white. It is aimed at theenthusiastic amateur, as well as school and college parties,with jargon kept to a minimum.

The introduction forms a useful summary of presentknowledge of the geological history of Anglesey andincludes a simplified geological map. Itineraries are givenfor 14 areas. Some, such as Rhoscolyn, may take a full day,but most can be visited in two to three hours. Reflecting thedominance of the Mona Complex, and the author’s interestin tectonics, 11 are wholly or partly on these. Each itineraryincludes directions (from the Britannia Bridge on the A55), alarge-scale sketch map of the location, colour photographsand in some cases cross sections. All the localities appear tobe easy to find. The text includes clear descriptions of therock types and structures (aided by the photographs),together with interpretations—either by the author or fromcited works.

The Rhoscolyn location is the most detailed described inthe guide (it is also the subject of a recent guide by Wood).The Rhoscolyn Anticline here is claimed by the author to bethe most accessible and well-exposed major fold in theBritish Isles, and has been the subject of at least fiveinterpretations in recent years. The author’s interpretationis that it is a first phase upright anticline, as illustrated inFigure 1 (this could have been redrawn and simplified, withthe location of the Coastguard station added).

Although most of the locations have featured in previousguides, the Hen Borth area on the north coast is new. It isintriguing in that it shows a tectonic style and fold

orientation not observed anywhere else in Anglesey—ofnortheast facing recumbent folds (with axial planarcleavage).

The Porth Wen and Porth y Corwgl locations highlight adebate on the tectonic relationships between the MonaComplex and the Ordovician rocks, which was instigated byBarber & Max in 1979, and which could profitably beexplored by field parties. On the north coast they claimedthat both units share a common deformation. Figures 2–3 ofthe Porth Wen location show deformed pebbles in theOrdovician conglomerates. At Porth y Corwgl, theOrdovician conglomerates contain clasts of the adjacentAmlwch Schists, which again are claimed to share acommon deformation with the sediments. In contrast, Ihave observed clasts in which the schistosity is truncated bythe surface. I have also observed southwards younging in theOrdovician—contrary to the northwards youngingsuggested in the guide.

The Lligwy Bay location is confined to the Old RedSandstone rocks of the north side: unfortunately omitted arethe Carboniferous conglomerates, sandstones and lime-stones of the other side, with the spectacular collapsestructure of the Lligwy Bay Disturbance. The ORSstructures here are intriguing and include an overturnedanticline with a cleavage, and thrusting—described as asignificant new element in the tectonic evolution ofAnglesey (one first recognised by Greenly as a third[post-Caledonian] event). In addition to the structures, thereare descriptions and illustrations of the rocks, including aspectacular exposure of a mud-cracked surface.

The Carboniferous is described only at Red Wharf Bay(location N). Here there are sandstones and shales betweenthe limestones, with sandstone-infilled pipes at the base ofeach of the two sandstones. These horizons mark periods ofemergence of the carbonate platform, when karstic erosionmay have formed the pipes, later infilled by fluviatile sands.Some pipes are cut by others, suggesting at least two phasesof pipe formation. Is it possible that some of the pipes mayhave been formed, not by dissolution, but as potholes?

Post-Palaeozoic deposits and events are briefly described,where they occur at locations of other interest. At Trwyn yParc the Gwna Limestone has pipes, rather similar to theCarboniferous ones, infilled with Miocene clays. A drumlinis well exposed at Hen Borth, one of a swarm which coversmuch of the north of the island. It may be noted that at TrwynDwlban (location N, north of locality 4), where boulder clayoverlies the limestones, there are good glacial striae on thelimestone surfaces, including miniature crag and tailstructures formed by cherts.

Copyright # 2009 John Wiley & Sons, Ltd.

624 book review

There are only some very minor errors in the text. In thedescription of location F, the village should be Llanfachraeth(correctly stated as some 8 km from Valley) and notLlanfaethlu. There are a few minor errors in grid references,though given the quality of the directions given, and themaps, these should not pose any problem. In the glossary,melange is described as the result of tectonic processes, incontrast to sedimentary processes.

Overall, this is an extremely impressive guide, which canbe unhesitatingly recommended: the author is to becongratulated, both for the content and the presentation. Itshould be indispensible for anyone visiting the island. Thetext is clear, and sufficiently detailed to engender interest in

Copyright # 2009 John Wiley & Sons, Ltd.

both the rocks and their interpretation. The illustrations areexcellent and well repay repeated study.

DENIS BATES

Aberystwyth University, Institute of Geography and EarthSciences Aberystwyth University, Penglais, Aberystwyth,

SY23 3DB, UK

Published online 1 June 2009 in Wiley InterScience(www.interscience.wiley.com)

DOI: 10.1002/gj.1166

Geol. J. 44: 623–624 (2009)

DOI: 10.1002/gj