Nick Petford* claims another first for the 1815 William Smith map – could it also represent the first speculative record of pluton shape?
Geoscientist Online 20 August 2007
The William Smith Map that hangs on the main staircase of Burlington House achieved notoriety when it was described by Simon Winchester in the introduction to his bestselling book "The Map that Changed the World" (2001). This book not only drew public attention to William Smith but also to this particular version of the map, which the description (not explicitly) tends to lend special status as the "original" map. This certainly has been the experience of staff at the Society who have been involved in showing the map to the visitors who flocked to see it after publication.
Recent restoration of the Society's display copy revealed two important new facts. First, the 1838 watermark on Thomas Edmonds's paper (on which the base map was printed prior to hand colouring) confirmed something that had always been believed by connoisseurs of Smith's "1815" maps, namely that this was a very late product indeed. Smith died in 1839, and it may well be that he never even saw this particular copy (Dr Tom Sharpe, Nation al Museum of Wales, pers. comm.).
The restoration of the map between 2006 and 2007 necessitated its removal from a large old-fashioned wooden case of uncertain age on the upper part of the East Staircase, Burlington House, where it was described by Winchester as hanging behind blue curtains (placed there in the latter half of the 20th Century at the behest of Dr John Fuller to discourage further fading).
What this revealed was that the map had indeed hung in that position since the Society took possession of its new apartments in Burlington House in 1874 – the plastered wall behind wearing still its original builder's finish. This also suggested that the plate glass that protected it was indeed a Victorian shop window, made using moulding and grinding techniques long before the invention of the Pilkington Float Glass Process.
The depiction of temporal sequences of strata on maps to allow a vertical picture of the sub-surface geology was pioneered by Lavoisier, several decades before Smith’s publication (Gould, 2000). But among the many "firsts" with which Smith's map is credited (not always accurately) is the inclusion of a cross-section with exaggerated vertical scale, showing Smith's conception of the deep structure of southern Britain along a line stretching from Cardigan Bay in the West to the mouth of the Thames in the East. Smith thought of the western and northern massifs of Britain (Cornwall, Wales, the Lake District, Southern Uplands) as essentially the same, and referred to their rocks in his stratigraphical table as "Killas and Slate of Cornwall, Wales, Westmorland and Scotland". These were the folded, slaty basement against which the mostly more fossiliferous stratified rocks of Britain leant, with their south-easterly dip, like "slices of bread and butter".
The slates, he noted, also enclosed granites, which he indicated at surface in, for example, Dartmoor, and the Southern Uplands – in fact, in all places except mainland Wales from where he knew of no reports of surface occurrence. He did mark granite at surface on the island of Anglesey, but none occurred at surface along his line of section.
From this Smith seems to have surmised that the topography of Wales was too high for erosion to have reached the granites he nevertheless believed to be present at depth. He therefore indicated them on his section at some distance below the surface in what may be the first recorded example of a granite "pluton".
The picture above shows the cross section in detail. Students of granite emplacement geometry will be that Smith chose to depict their geometry as sheets.
Granite Sheets: the original Alpha geometry?
The sheeted geometry of Smith’s granites (see picture). has a distinctly contemporary feel. Debate has raged since the early 20th century on the origin of granite and the shape and volume of granitic rocks at depth (Pitcher, 1993). The historical development of early ideas leading to the correlation of granitic pluton shape with salt and volcanic domes, and ultimately diapiric structures, are reviewed in Petford (1996) and Clemens et al (1997). While there is still uncertainty and debate concerning the three dimensional form of granitic rocks, it is fair to say that a consensus is emerging that supports a broadly tabular shape as the most favoured geometry. It is of significant historical interest that this is the shape chosen instinctively by Smith, who was unburdened by the need to sketch his plutons as inverted teardrops. The erroneous link between granite plutons and salt diapirs would not be made for a further century (Young, 2003).
Further reading
- Clemens, J.D., Mawer. C.K. & Petford, N (1997). Ascent Mechanisms of granitic magmas: causes and consequences. In: Holness, M (ed). Deformation-Enhanced Fluid Transport in the Crust and Mantle. Chapman & Hall, 145-172.
- Gould, S.J. (2000), The Lying Stones of Marrakech, Johnathan Cape, London, 372 pp.
- Petford N., 1996. Granitoid ascent: dykes or diapirs? Trans. Roy. Soc. Edinburgh 87, 105-114.
- Pitcher, W.S. (1993), The Nature and Origin of Granite: London, Blackie Academic & Professional, 321 pp.
- Winchester, S (2001) The Map That Changed The World, Viking Penguin, 256 pp.
- Young, D.A. (2003), Mind over Magma: The Story of Igneous Petrology, Princeton University Press, 704 pp.