Book reviews

Geology of the country around Hitchin (sheet memoir 221(England and Wales» , P. M. Hopson, D. T. Aldiss and A.Smith, 1996, HMSO, London, xi + 153 pp. ISBN a II8845 18 7 (PB), £55.00.

The geo logica l memoir for the Hitchin area is part of theincreasing output of the British Geological Survey. Newmemoirs are appearing much more rapidly than they used toafter the publication of the geological map for an area, oftenwithin a year or two.

Inside the front cover there is a useful summary of thegeological succession and the thicknesses of the units in theHitchin District. The first chapter describes the geographi­cal setting, and summarizes the history of research alongwith a brief geological history of the area . The solid forma­tions, when exposed at the surface, range from the LowerCretaceous Woburn Sands and Gault in Bedfordshire to thenorthwest, though the Upper Cretaceo us Chalk sequencewhich underlies northern Hertfordshire to the small Tertiaryoutliers in the southeast of the map. On top of these solidformations large parts of the area are covered by muchyounger Quaternary drift deposits.

The next chapter is on the concealed formations beneaththe district with the boreho le and geophysica l evidence forthe nature of the Jurassic and Palaeozoic formations with, atdepth , the Precambrian rocks of the Midlands Microcraton.There is a useful table of the physical propert ies of the mainrock units along with plots of the geology and level of thePre-De vonian and Permian surfaces.

The chapter on the Lower Cretaceous formations adds tothe information on the subject in the 1994 memoir for theadjacent Leighton Buzzard sheet, with some useful tablesand figures - surprising, considering the poor and limitedarea of their outcrop.

A large amount of information is contained in the chapteron the Chalk. The old lower, middle and upper subdivisionshave been kept, because the resistant Melbourn rock at thebase of the Middle Chalk and the Chalk Rock at the base ofthe Upper Chalk form positive topographic features that canbe traced during field mapping . There is a good discussionof the Cambridge Greensand that is now considered to formthe base of the Lower Chalk, along with all the other unitsof the Chalk sequence. After general descriptions there arevery detailed notes and diagrams for many locations in thedistrict.

The short chapter on the Tertiary beds includes the resultsof a borehole drilled to help with the surveying of theseout lying deposits and their relationship to those in the restof the London Basin and beyond.

Proceedings of the Geologists ' Association . 109, 159-1 60.

The chapter on the Quaternary is very extensive and isdivided into Pre-Anglian , Anglian and Post-An gliansections. The Letchworth Gra vel is described for the firsttime, and it is suggested that it was deposited by a Pre­Anglian stream drainin g southward through the Hitchin Gapinto the Thames Basin. The glaciotectonic Chalk rafts of theBarkway and Therfield area south of Royston are describedin terms of the Anglian ice impin ging on the Chalk escarp­ment , and this section complements the recent paper byHopson (1995). The sequence of events in the glacialhistory of the Hitchin Gap is set out in a series of cleardiagrams along with descriptions of the material filling thechannels.

The chapter on structure starts with geophysical andsatellite lineament s which seem to reveal very little aboutthe surface geology but suggest possibil ities about thenature of the basement rocks below the area. In the Chalk anorthwest-trend ing fault along Lilley Bottom inter sectingthe Chalk escarpment at Barton-Ie-Clay is suggested,though it could be a monoclinal roll, and it has not beenmarked on the published 1:50 000 geological map. At theend of the chapter there is a good detailed map show ing thefaultin g of the Melbourn Rock at the base of the Midd leChalk ju st to the west of Hitchin.

The economic geology chapter has an interesting sectionon the exploitation of the Cambridge Greensand for calciumphosphate, with a note that John Lawes made a fortune fromthis and as a result founded Rothamsted Experiment alStation . There is a table of all the known landfill sites in thearea and what they are filled with , along with a note on thegeneration of landfill gas at Arlesey by rotting householdwaste. The hydrology of the area is discussed in somedetai l, along with the problems of aquifer pollution.

Overall, the Hitchin memoir contains a surprising amountof well set out information, much of which was collected inthe recent survey of the area, but at £55 .00 it is far tooexpensive. It is a great pity that excellent memoirs such asthis, as a result of the government's forcin g near-marketeconomics on the BGS, have to be priced at such a highlevel, becau se they will have a much more limited publiccirculation than they deserve .

REFERENCEHOPSON, P. M. 1995. Chalk raft s in Anglian till in north

Hertfordshire, Proceedings of the Geologists ' Association, 106.151- 8.

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