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).
How to get there
If you arrive by air or train, come out of the front of the airport on the arrivals (street) level, and turn right past the taxi ranks and islands for bus stops. If you arrive by bus, alight and just keep walking in the same direction. Walk under the bridge that connects the airport to car parks and hotels. The road swings around to the left. At this point, you will see the Sheraton and a row of large, modern buildings disappearing down to the Hilton in the distance (Fig. 2A and B).
There is a zebra crossing trending towards the Sheraton side of the road. Take this and then the bicycle path to the right, keeping the World Trade Center to the left, and cross the road towards the big pond. Take the footpath right towards the hut – ‘Leger des Heils’ – which is underneath the viaduct supporting the exit road from the airport departures level. Boulders of sandstone are large and prominent, beneath this viaduct, so it is comfortable to examine them even in wet weather.
For further, general discussion of the features discussed below, you are referred to Tucker (2011) or a similar text on physical sedimentology. Without crossing any more roads, about 20 boulders can be examined (Figs. 2 and 3); crossing roads and going further under the viaduct, the number approaches about 50. Boulders not protected by the viaduct have algae growing on them, turning green.
The largest boulders are about two metres maximum dimension, angular and have been ‘dropped’ in various orientations, but are commonly on edge with vertical bedding (Fig. 2A and B).
Bedding varies from fine, on the scale of about 10mm or less (Figs. 2D and 3A), to massive up to 0.5+m in thickness (Fig. 2B). However, many of the more massive beds are found to consist of very numerous, well-cemented thin beds (Fig. 3G). The principal rock type is fine- to medium-grained sandstone, but the grains are not rounded as in a desert deposit. It is common for bedding planes to show abundant white mica in the sandstone (Fig. 2E). None of the beds are conglomeratic and quartz pebbles are not apparent. Thin beds (about 10 to 30mm thick) may spall off.
Channels are few and small. The best examples are seen in a boulder close to the hut (Fig. 2C). These are small, but each with an obvious fill of parallel-laminated beds, which therefore indicate the way-up of the sequence.
These provide a further way-up structure (Figs, 2C, and 3C and E). Some of the fine-scale beds fan out within a boulder, suggesting that part of a larger structure is seen, but only imperfectly.
Not fossils, but smooth-sided rip-up clasts that have eroded out and are of differing sizes (Fig. 3D, F and H). It is presumed that these were originally clasts of thinly bedded mudrock.
Reduced horizons and joint surfaces
These boulders are mainly an oxidised red in colour, a good indicator of terrestrial deposition. Careful examination shows some green rocks (= reduced). These mainly occur as green beds, mostly narrowly defined (about 10mm thick), but some are thicker (Figs. 2D, and 3A and C). These green beds may be either deposited in a reducing environment or be discoloured later. Green colouration along joints is undoubtedly a post-depositional feature (Fig. 2C).
Thin veins of quartz fill some joints and appear either cross-cutting beds or in plan-view (Fig. 3B).
Many blocks have obvious parallel boreholes (Figs. 2B and 3G) perpendicular to bedding. This suggests that these blocks were exposed on a flat-lying floor of a quarry and were drilled for blasting along vertical joints. This further suggests the probability that most of these boulders came from a limited part of the sedimentary sequence, perhaps one or a few beds.
The assignment of these rocks to the NRS is almost certainly correct. Other ‘red bed’ successions are well known from various parts of the geological column – the Devonian Old Red Sandstone immediately comes to mind – but the NRS is the thickest and most extensive in northern Europe. Supporting evidence from palaeontology is lacking, not surprisingly but unfortunate. Finding a tetrapod track, such as Chirotherium Kaup, would have been particularly exciting for this ichnologist (Tresise with Sarjeant, 1997).
The question of provenance is speculative, although Germany must be suspected as the probable source. Long-standing houses of merchants in Leiden were all built with Mississippian limestone of similar provenance by the evidence of their enclosed fossils (Donovan and Madern, in press; D.M. van Ruiten, research in progress). These probably represent canal boatloads of stone brought from Belgium(?). The NRS of Schiphol is probably analogous, albeit a late twentieth century accrual; and most likely represent several truckloads of sandstone from a single quarry brought to Schiphol to discourage motorists from turning this tempting covered space into an unofficial free car park.
What of the rocks themselves? For choice, I prefer fossiliferous sedimentary rocks of almost any lithology. In the absence of fossils, I enjoy sedimentary rocks with a range of interesting sedimentary structures and some of the most interesting are to be found in ‘red beds’. I admit to being lucky that these beautiful imported boulders are almost on my doorstep, but suggest that it is a rare airport of such size or importance that offers such a fine geological field trip within a few hundred metres of its front door.
Donovan, S.K. 2014. An unnatural bridge in an artificial limestone environment, the Netherlands. Cave & Karst Science, 41: 118-119.
Donovan, S.K. 2015a. Urban geology: Two granites. Deposits, 41: 8-9.
Donovan, S.K. 2015b. Urban geology: Boulders and the Dutch. Deposits, 42: 8-9.
Donovan, S.K. & Madern, P.A. (in press). Rostroconchs in Leiden. Swiss Journal of Palaeontology: 4 pp.
Hounslow, M.W. & Ruffell, A.H. 2006. Triassic: seasonal rivers, dusty deserts and saline lakes. In: Brenchley, P.J. & Rawson, P.F. (eds), The Geology of England and Wales (second edition). Geological Society, London, 295-324.
Roekel, A. van. 2007. Discover Fossils in Downtown Amsterdam. Second revised edition. Uitgeverij De Vuurberg, Amsterdam, 17 pp.
Staalduinen, C.J. van, Adrichem Boogaert, H.A. van, Bless, M.J.M., Doppert, J.W.Chr., Harsveldt, H.M., Montfrans, H.M. van, Oele, E., Wermuth, R.A. & Zagwijn, W.H. 1979. The geology of the Netherlands. Mededelingen Rijks Geologische Dienst, 31-2, 9-49.
Tresise, G. with Sarjeant, W.A.S. 1997. The Tracks of Triassic Vertebrates: Fossil Evidence from North-West England. The Stationery Office, London, xii+204 pp.
Tucker, M.E. 2011. Sedimentary Rocks in the Field: A Practical Guide. Fourth edition. Wiley-Blackwell, Chichester, ix+276 pp.