Urban geology: the strange tale of a windowsill

Leiden, in the Dutch province of Zuid-Holland, is a city with a fine selection of fossiliferous building stones, mainly Mississippian (Visean, Lower Carboniferous) limestones. which preserve an array of fossils, such as rugose and tabulate corals, brachiopods, bryozoans, molluscs, and crinoids. However, this fine diversity of body fossils is not supplemented by a similar composition of trace fossils. Despite having examined these rocks over many years, when leading student fieldtrips and in collusion with co-workers, SKD has found no evidence of burrows, nor any borings in bioclasts, which can be locally common at some localities where Mississippian strata are exposed (for example, Donovan and Tenny, 2015).

Fig. 1. Map of the centre of Leiden (modified after Van Ruiten & Donovan, 2018, fig. 1); Leiden Centraal
railway station is north of the north-west corner of this map and less than 15 minutes to walk to the North End
Pub. The emphasised streets (both with a central canal) are coral localities; the Rapenburg is the shorter street,
running north-south towards the left of the figure. Key: + = Hortus Botanicus; * = North End Pub. Original
redrawn from Google Maps using Photoshop by DM van Ruiten.

It is therefore of note to recognise an uncommon rock type among the building stones of Leiden that is dominated by burrows and lacks body fossils. This article highlights this distinctive building stone that has engrossed SKD for some years. The street Rapenburg (Fig. 1) in Leiden is a favourite route for building stone tours of the city. Although the dominant building materials are bricks, there are ample rocks to make a visit worthwhile. When SKD has led groups of students from the University of Leiden on geological excursions down the Rapenburg, the start is commonly at the North End Pub (Fig. 1). This is presumed to be a former bank building and it is sumptuously faced with polished, imported larvikite. Elsewhere in the Rapenburg, the common rock used for facing and ornamental stones at pavement level is grey, Lower Carboniferous (Mississippian) limestone. As there is no outcrop of Upper Palaeozoic limestones in the Netherlands (compare De Mulder et al., 2003), it also represents an import, long before the current international traffic in rocks (Nield, 2014). Available evidence indicates that these limestones were imported from southern Belgium (southern Ardennes, near the cities of Dinant and Namur) at the beginning in the sixteenth century (Van Ruiten and Donovan, 2018). The age of these limestones has been established using the combined evidence provided by their lithology and palaeontology (Donovan, 2016; Donovan and Madern, 2016; Van Ruiten and Donovan, 2018).

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