Urban geology: two granites

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).

Figure 1.jpg
Fig. 1. Outline map of the Beukenhorst area of Hoofddorp, the Netherlands, showing the route to Siriusdreef; other roads are largely omitted for clarity (modified after Donovan, 2014, fig. 1). The railway (trellised-line) is one stop from Amsterdam Schiphol Airport to the east; similarly, the elevated bus lane of the Zuid Tangent (routes #300 and #310) come from Schiphol. Key: + = railway or bus station; B = Beukenhorst Zuid Tangent station; H = Hoofddorp railway and Zuid Tangent bus station; * =building with faulted granite slab in cladding.

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.

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