B OOK R EVIEWS I'l l

Groundwater - our hidden asset, co mpiled by R. A.Downing on beh alf of the UK Groundwater Forum, 1998,British Geological Survey, Nottingham, 61 pp . ISBN0852723040 (PB), £6 .50.

This booklet has been produced on behalf of the UKGroundwater Forum, an ass ociation of org anization sincluding the Environment Agency, the Water Service sand Water Companies Associations, the Scottish andNorthern Ireland Forum for Environmental Research, theBritish Geological Survey and the Foundation for WaterRe search. It is designed to be a layperson's gu ide togroundwater and is sui table for students of sciencesubjects at Key Stage 4 and above. It is published as partof the British Geolo gical Survey 'Ea rthwise' series.

Groundwater is written at two levels. A general accountof key issues is printed in black typeface and can be readseparately as a continuous text in its own right. Thegeneral text is supplemented by more detailed rev iew s ofimportant issues and case studies which are printed in bluetypeface .

The booklet can be broadly divided into three groups oftopics. Following a brief introduction which sets thescene, the first third of the booklet covers a number of keyis sue s in order to define and explain relevant termin ology,examine groundwater exploitation, describe the character­istic s of aquifers and their distribution throughout the UKand consider charac teristics of groundwater flow andgeochemistry. Groundwater resources and groundwaterde velopment form the sec ond major section of thebooklet, whilst en vironmental issues surroundinggroundwater exploitation form the third part . This lattersection includes well illus trated ca se study material onselected topics such as low river flow s and wetlands, seawater intrusion and aquifer vulnerability and risk fromrural and urban pollution.

Whilst not written for the groundwater specialist , awealth of topi cal and up-to-date information is packedbetween the covers of this booklet. It is wri tten in a clearand concise style and intro duces, defines and explainsrelevant terminology, Case study mate rial is well chosento illustrate key issues and explore environmentalproblems and the ir possible solutions. The photographs,maps, line and block diagrams are rep roduced in fIillcolour and are of extremely high quality.

At £6 .50 this booklet is excellent value for money. Thesteering commi ttee and Dr Dick Do wnin g (the compiler)should be congratul ated on fulfilling the key obj ect ive ofthe publication which was ' to demystify the subject' .

IAN FO STERCoventry University

Geology of Rum and the adjacent islands. (memoir for the1:50000 Geological Sheet 60 (Scotland» , C. H. Em eleus,1997 , HMSO, London, xii + 170 pp . ISBN 011 8845 17 9(PB), £35 .00 .

This geological memoir and the accompanyi ng map

( 1994) were produced as a result of a contract awarded tothe University of Durham. It des cribes the Small Isles ofthe Inner Hebrides , which comprise the islands of Rum ,Eigg, Muck, Ca nna and Sand ay bet ween Skye andArd namurcha n off the western coast of Sco tland . As usualthe inside of the front cover has an overall summary of thegeo logica l succe ss ion and eve nts in the area. The outcro pshave a large age range : Precambrian Lewisian gneisses(Arc hacea n) and Torr idonian sedi ments (Proterozo ic):thin remnants of Mesozoic dep ositional epi sodes; earlyTert iary (Paleocene) igneous intru sions and associatedlavas of the Rum central complex; and finally scatteredQuatern ary glacial and raised beach deposits.

The introduction sets out how the varied geology of theislands results in contrastin g sce nery. along with a clearoverall summary set in a regional contex t and finall y abrief hist ory of more than two hundred years of geo log icalre search on these islands. But it docs not include a simpli­fied topograph ic and bath ymetric map of the area.

Th e small outcrops of often thermally metamorph osedLewisian gneiss mostly within the mai n ring fault o f theRum central complex are dealt with in a short secondchapter. The predominantly flu vial clas tic Protero zoicsed iments on Rum were deposited about 850 Ma ago andall belong to the Torridon Grou p, the uppe rmostsubdivision of the Torridonian. They are well preserved,and detailed enviro nmental in terpretations for eac hformation within a re vised stratigra phy for the group arepresented along with detailed stra tigra phic logs and clearphoto graphs.

The restricted occ urrences of Mesozoic sediments onthe islands are dealt with at length in the nex t chapter.Th ey give important insights into the much thickeroffshore successions. There are small outliers of Triassicsandstones on Rum whi ch are more clos ely related tooutcrops down the pal aeoslope to the north on Raa saythan those on Mull and Ardna murchan to the south and areprobably a simi lar age to the Mercia mudsto ne. Scatte redoutcro ps of thermally metamo rphosed Lower Jur assicmarine deposits are preserved by the down-faulting of themain ring faul t in eastern Rum and a clearly drawn col ourI:10000 geo logical ske tch map is presented of thiscomplex area. The Middle and Upp er Jurassic formationsof Eigg and Mu ck are give n loc al Sco ttish names. a tabl eshowi ng their broad relation ship with the local offshoresuccession and stratigraphy wo uld have been useful.However, the acco unt contains a lot of useful detail, withfigures showing the corre lation between different sec tion sand outcrop maps of the mos t important locations. Thethree thin deposits of Upper Cretaceous age found trappedbeneath the lower Terti ary basalts of Eigg are desc ribed indetail. They are important in und erstanding the period s oferosion in the Cretaceous and in the interval befo re theeruption of Palaeogene lavas.

The fifth chapter starts with a brief introduct ion to thePalaeogen e Rum central volcano (25 x 20 km exposedcore) and its related intrus ions and lavas which arede scribed in detail in the next five chap ters, whichbetween them comprise half the memoir. Thi s is justified,as these rocks cov er large part s of the islands and have had

192 BOOK REVIEWS

a significant bearing on our general understanding of theorigins of igneous layering and emplacement of magmas.This introduction could have been usefully extended, by apage or two, with some general background concepts ofigneous geology and geochemistry. The rest of thischapter deals with the early mainly acidic history of theRum volcano. Minor intrusions are dealt with next and aresubdivided by type and form. The minerals in only one ofthe excellent colour photomicrographs is labelled in thesechapters and this, or small insert line drawings (withlabelling) in the space next to them, would have been veryhelpful.

The third chapter on the Palaeogene deals with themafic layered suite of peridotitic and gabbroic rocks of theRum central complex. The small-scale layers in theseintrusions were deposited as crystals settled out of aconvecting magma chamber and sometimes showstructures which are analogous to those in sediments.After general accounts of the Eastern, Western and Centralintrusions there are sections focusing on different aspectsof these rocks with many tables, figures, maps andinteresting photographs.

The account of the lava formation and associated sedi­ments of the islands is divided into the mainly basalticearly flows of Eigg and Muck, then those of Canna andSanday followed by the later Eocene acidic pitchstones ofthe Sgurr of Eigg. The final igneous chapter is a shortdiscussion on the extensional tectonic framework andpatterns of magma generation that resulted in the differentrock types associated with the early Tertiary Rum centralvolcano.

Little is known about the pre-Mesozoic structuralhistory of the Small Isles and not much more abouttectonic movements before the inception of volcanism inthe Paleocene which controlled the development of laterfaulting and minor folding. The main faults and theirhistories are described along with a series of clearstructural maps. Two end models that fit the very highpositive Bouguer gravity anomaly over Rum are presentedin the geophysics chapter to demonstrate that the real highdensity intrusion beneath Rum is probably somewhere inbetween these two extremes. These models are a good

example of the ambiguity of gravity interpretation. Alongwith the gravity map that for once shows the distributionof the observations, a simplified aeromagnetic map withoutline geology is presented and discussed. Brief detailsabout the radiometric age determinations for the Tertiaryigneous rocks are given, showing that they are betweenabout 63 and 52 Ma old and were reversely magnetizedapart from a few dykes.

The late Quaternary features of the islands arediscussed in the penultimate chapter. Rock benches, ofprobable marine origin, around the coasts developedbefore the late Devensian (about 26 to 13 ka) when theSmall Isles were covered by widespread ice sheetsoriginating from the mainland. During this periodperiglacial features formed and the massive weight of icedepressed the land so that a series of beaches up to 30 mabove sea-level developed. As the climate warmed, globalsea-levels rose rapidly, when ice sheets melted andretreated, to about 6 m above present sea-level before theice-free land continued to rise slowly to its present level.

Finally, chromite deposits are discussed, and in light ofRum being a National Nature Reserve their exploitationfrom the layered suite is considered very unlikely, whileoffshore concentrations of these eroded minerals may, inthe future, be considered economic.

For anyone interested in the geology of the InnerHebrides, central volcanoes and their associated intrusionsthis memoir will be a key reference and is good value formoney compared to others in the series. However, as aresult of transferring too much of the costs of thenecessary writing up of results from the mapping 'of anarea on to the price of producing these excellent memoirsthey will soon be discontinued, as no doubt uneconomic,after more than a century of publication. For the generalreader there is still a lot of information in the text, as thetechnical details can either be skipped or decoded withtextbooks.

DAVID NOWELL2 Tudor Road,

New Barnet,Herts.