Book Reviews

Geology of the country around Snowdon (sheet memoir119 (England and Wales», M. F. Howells and M. Smith,1997, HMSO, London, x + 104 pp. ISBN a 11 884523 3(PB), £45.00.Snowdon 1:50 000 geological maps £9.95 each: solidedition ISBN 0751831476 (flat), a 7518 3148 4(folded); solid and drift edition ISBN a7518 28661 (flat),075182867 X (folded).

At last the long overdue geological 1:50 000 maps andaccompanying sheet memoir for Snowdon have beenpublished. They cover the area south from Snowdon andBetws-y-Coed to Porthmadog down on the coast andaround the slate town of Blaenau Ffestiniog to the east. Inthe southern part of the district Cambrian sediments formthe northern flank of the Harlech Dome, while themajority of the Snowdon area consists of a complexsequence of Ordovician sediments with related extrusivevolcanic rocks and intrusions.

The memoir starts with a short introduction, includingthe physical features of the district, brief details ofprevious research, and a short summary of the generalgeological history. The Cambrian sediments are dealt within a short chapter, which includes details of how the smalloutcrop in the far northwest of the map area relates tothose in the south, and a plate of fossils from theMawddach Group which forms most of the outcrop asonly the upper part of the Cambrian is seen.

The chapter on the Ordovician takes up over half thememoir and starts with an overview of the mainstratigraphical subdivisions and how they vary inthickness across the area. There are clear figures withdetails of the Tremadoc sequence which was originallyconsidered to form the top of the Cambrian in NorthWales, as there is a major unconformity before the Arenigtransgression. The chapter has many well presented platesof fossils, including an illustration of the differencebetween a deformed trilobite and its original form. Afterthe mid-Ordovician hiatus there are locally melangedeposits around Rhyd, northeast of Porthmadog, whichare now shown to be the product of gravity sliding andmass flow of partly lithified sediment and not tectonic inorigin as had been supposed. Large scale maps, sectionsand photographs are used to show details of this, alongwith a very good field sketch plan and section showing asmall raft of Garth Grit in the melange. The SnowdonVolcanic Group, which comprises acidic ash-flow tuffs,intrusive and extrusive rhyolites, basalts, hyaloclastitesand basic tuffs associated with marine sediments, is dealtwith at some length. It is shown that the Snowdon Calderawhich was about 10 km wide developed above basementfaults and the eruptive centres shifted temporally andspatially to result in a complex and laterally varyingsequence. The last section gives all too brief details of the

Proceedings of the Geologists' Association. 111, 190-192.

related dykes and larger intrusions, including the Tan-y­Grisiau granite which the associated gravity and magneticanomalies indicate is a much larger body at depth. Theresults of palaeomagnetic work by Piper, Nowell &Crimes (1995) showing that after deformation the plutonwas magnetized in late Ordovician times and by the end ofthe Silurian the area had been rotated anticlockwise by alarge amount have been omitted.

The structure, metamorphism and geophysical investi­gations of the area are briefly set out in chapter four withdetails of the folds, faults and cleavage. The otherwiseinformative gravity and magnetic cross-section ispresented without the density variations at 15-20 kmwhich are used to account for the regional gravitygradient, and so it is not clear how much of the modelledanomaly is due to the uppermost 8 km shown. The chapteron economic geology gives a brief outline of the slateindustry which peaked at the end of the nineteenthcentury. The mineralization of the district is discussed:copper along with minor amounts of lead and zinc weremined, and some of the mines with their histories arelisted. Details of the Quaternary deposits are brieflydescribed along with the glacial history of the area.

Overall the memoir is about the right length and wellpresented with numerous black and white photographs,which give the reader a good idea of the landscape of thedistrict and nature of the outcrops, as well as plates offossils and informative well-drafted figures. There are twoversions of the accompanying 1:50 000 map. Both have aseries of generalized vertical sections to show the complexdifferences in stratigraphy between locations, along withinsert 1:250 000 Bouguer gravity and Aeromagneticanomaly maps. The colour schemes used on the maps arewell thought out and printed so that as much of the basemap as possible can be clearly seen. The solid and driftedition shows the underfoot geology including theQuaternary deposits and areas of made ground: from thisit is possible to judge on what basis the solid boundarieshave been drawn. The solid edition only shows the pre­Quaternary geology so that it is much clearer to read andthus understand the structure of the area, but a pecked lineis not used to mark uncertain boundaries as is normallydone.

REFERENCE

PIPER, J. D. A., NOWELL, D. A. G. & CRIMES, T. P. 1995.Palaeomagnetism of the Tan-y-Grisiau granite, North Wales:evidence for a subvo1canic origin in Late Ordovician times.Geological Journal, 30, 39-47.

DAVID NOWELL2 Tudor Road,

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