Stephen K Donovan (The Netherlands).
In a country with a limited resource of pre-Quaternary geology in outcrop, the Netherlands nevertheless has a wealth of rock types in building stones (Donovan, 2015a; Donovan and Madern, in press), street furniture (Donovan, 2015b) and artificial ‘outcrops’ (Donovan, 2014). Perhaps the commonest rock type seen in Dutch cities is limestone, particularly imported Mississippian (Lower Carboniferous) limestones (van Roekel, 2007; Donovan and Madern, in press), but also Upper Cretaceous limestones from the province of Limburg in the south of the country (van Staalduinen et al., 1979, p. 47). Less common are massive sandstones, both used as building stones and occurring as boulders (Donovan, 2015b) – most of these that I have seen are, presumably, Pennsylvanian (Upper Carboniferous). The area of outcrop of Carboniferous rocks in the Netherlands, again in the province of Limburg, is limited. Carboniferous rocks used for buildings or street furniture are assumed to come largely, probably entirely, from the more extensive outcrops that are quarried elsewhere.
One rock type that is not commonly encountered is red siliciclastic rocks such as siltstones, sandstones and conglomerates. This is despite the broad distribution of the Permo-Triassic New Red Sandstone (NRS) in northern Europe (Hounslow and Ruffell, 2006, fig. 13.2). In my pursuit of river-rounded boulders in the human environment of the Netherlands, I have only seen one NRS specimen of note – a coarse-grained sandstone with abundant gravel-sized fragments truncated by a scoured, erosive contact with an overlying conglomerate (Fig. 1). This is at the edge of a car park at the Leiden University Medical Centre, an unprepossessing resting place for such an attractive specimen.
Another exception to this trend, but obviously not one composed of river-rounded samples, is found at Amsterdam Schiphol Airport. This is one of the busiest airports in northern Europe, yet in this complex of terminals, runways, car parks, hotels and roads is an oasis of large red sandstone boulders containing a diversity of sedimentary geological features (Figs. 2 and 3).