Urban geology: Two granites
Stephen K Donovan (The Netherlands)
Building stones may tell us something or nothing about the geology of the local area. As Ted Nield (2014) recently highlighted in his book, Underlands, stones used in Britain today are rarely local. Once upon a time, local stone would have been derived from a nearby quarry. Now, stones are commonly imported from overseas.
If that is the case in the British Isles, then pity the poor geologist in the Netherlands, where genuine exposures of rock only occur in the south, in the province of Limburg, and mainly consist of Upper Cretaceous chalks and limestones. In consequence, ornamental and facing stones on buildings are almost invariably imported.
I mainly have eyes for the imported Upper Palaeozoic limestones, probably mainly Carboniferous, but potentially including some from the Devonian. These rocks are common (van Roekel, 2007), but I also pay attention when I spy a beautiful granite, in the broadest sense, which are common on the fronts of banks and used even more extensive to clad offices. This article is about two such granites (out of many) cladding buildings in Noord Holland, which have particularly caught my eye. For a general mineralogical reference, I recommend Deer et al (1966).
A faulted granite in Hoofddorp
This site is close to the limestone street art in Siriusdreef (described by Donovan, 2014; and see also Fig. 1) and is an easy walk from Hoofddorp railway station or bus stops on the #300 express bus route. The building in question (there are others with similar cladding nearby) is numbered 49-67 (Fig. 2A) and the slab that particularly attracted my attention is near to the door marked ‘67’, to the right of the slab bearing the number. At the time of writing, this building is standing empty. The cladding is pale pink granite with a contrasting decorative stone of deep pink granite for window and door surrounds, and the like. It is the pale pink granite that is of interest, specifically the slab to the right of the ‘67’ sign. This is beautifully polished and is distinctly faulted (Fig. 2B-D).
The common large phenocrysts are lath-shaped, pale pink, potassium feldspars (=K-feldspars) up to 50 to 60mm in maximum dimension. These elongate phenocrysts show a broad trend to the right on this slab. Some smaller white feldspar crystals have interstitial mafic grains (Fig. 2E). Zoning of feldspars is uncommon (Fig. 2F), but apparent where there are pale rims to crystals. Quartz grains are smaller, about 10 to 15mm at most. Rarer mafic crystals (amphiboles?) show brown deterioration, mainly around the rims. Interstitial mafic grains are widespread.
The unusual feature of this slab is a zone of minor, sub-parallel faults towards the top and left, sloping between about 1 and 7 o’clock (Fig. 2B). Fault planes are thin. The most prominent and longest fault is towards the right; this is infilled in places by a quartz vein 1mm to 2mm thick. Many grains are truncated by this fault (Fig. 2C and D). The two less prominent faults are towards the upper left corner of the slab; the smaller of these dies out in the slab.
The pale facing stones of this building first interested me for a trivial reason, because of their similarity to our kitchen top at home. This, I know, came from Italy, so, I suggest (tentatively) a provenance without any suggestion as to age. The illustrated slab is of some interest as experience has taught me that building and facing stones are commonly chosen to exclude such features as faults and joints. Until now, I have figured a fault in a block in a stonewall (Donovan and Jackson, 2000, fig. 3F), but this was an exception. However, the slab in Fig. 2 appears solid and was obviously robust enough to survive quarrying, cutting, transport and construction, without any obvious adverse effects. Long may it stand and entertain visiting geologists.
Shap granite in Amsterdam?
Perhaps “… one of the best known and most distinctive granites to be found anywhere in Britain” (Smith, 2010, p. 54) is the Shap granite of the Lake District, a Devonian intrusion (see, among many others, Firman, 1978, pp. 156-158; Mosley, 1990, pp. 9, 28, 41-42; Skipsey, 1992; Prosser, 2006; the colour photographs in Smith, 2010, pp. 54-55 are particularly recommended). The rock is distinctive in having large rectangular crystals of orthoclase (a K-feldspar), with common Carlsbad twinning parallel to the long axis of crystals. These phenocrysts occur in intimate association with finer-grained orthoclase, “… glassy quartz, smaller white feldspars and black biotite mica form a finer groundmass” (Smith, 2010, p. 54) with muscovite, and minor hornblende and other minerals (Firman, 1978, p. 157; Prosser, 2006, p. 117).
In the centre of Amsterdam, I have discovered a facing stone on a group of shops that has beautiful twinned orthoclase crystals (Fig. 3). I do not include a map because this site is easily found. The shops are opposite the Stedelijk Museum in the Museum Quarter, in Van Baerlestraat; this is on a number of bus and tram routes, perhaps most usefully trams #2 and #5, which terminate at Amsterdam Centraal railway station.
The granite faces numbers #58 (residential; door behind men on right, Fig. 3B), #58H (ABN-AMRO bank; Fig. 3B), #60 (Gaastra; Fig. 3A) and #62 (left in Fig. 3A). The pink coloration provided by the orthoclase in phenocrysts and groundmass is obvious. Up close, the twinned orthoclase phenocrysts are readily apparent (Fig. 3C-E). Xenoliths are rounded to rhomboidal, up to cobble-sized, essentially black and, again, reminiscent of those found in the Shap granite (Smith, 2010, fig. 82).
Acknowledgement
Many thanks to Leo Kriegsman (Naturalis, Leiden) for his comments on Fig. 3.
References
Deer, WA, Howie, R.A. and Zussman, J. 1966. An Introduction to the Rock Forming Minerals. Longman, London, xi+528 pp.
Donovan, SK. 2014. Urban geology: A sunny Sunday in Hoofddorp. Deposits, 38: 8-10.
Donovan, SK and Jackson, TA. 2000. Field guide to the geology of the University of the West Indies campus, Mona. Caribbean Journal of Earth Science, 34: 17-24.
Firman, RJ. 1978. Intrusions. In Moseley, F. (ed.), The Geology of the Lake District. Yorkshire Geological Society, Occasional Publication, 3: 146-163.
Moseley, F. (compiler). 1990. The Lake District. Second edition. Geologists’ Association Guide, 2: 213 pp.
Nield, Ted. 2014. Underlands: A Journey through Britain’s Lost Landscape. Granta Books, London, 288 pp.
Prosser, R. 2006. Geology Explained in the Lake District. Fineleaf Editions, Ross-on-Wye, 176 pp.
Roekel, A. van. 2007. Discover Fossils in Downtown Amsterdam: Ancient Ocean Life in Amsterdam’s Alleys. Second revised edition. Uitgeverij De Vuurberg, Amsterdam, 18 pp.
Skipsey, E. 1992. The Shap Granite. In Dodd, M. (ed.), Lakeland Rocks and Landscape: A Field Guide. Ellenbank Press (for the Cumberland Geological Society), Maryport, x+150 pp.
Smith, A. 2010. Lakeland Rocks: An Introductory Guide. The Landscapes of Cumbria, 4. Rigg Side Publications, Keswick, 84 pp.