Stephen K Donovan (The Netherlands)
The city of Amsterdam in the Netherlands is surrounded by a great defensive earthwork on its landward side, the Stelling van Amsterdam (= Defence Line of Amsterdam), along which are a series of forts and batteries (Figs. 1A-E and 2). This major structure was built between 1880 and 1914. The principle feature of this defensive system is a raised earthen embankment or dyke, still imposing today although breached or flattened in many places to make way for modern developments, most commonly roads. The embankment is often flanked by two canals, one on either side.
The defence plan was to flood the area outside the embankment, using associated canals, dams and locks, which were seen as an effective means of slowing an invading army. Weak areas were to be defended by one of 46 forts or batteries (Fig. 2). By the time an invasion came, in 1940, such a technological solution was left far behind and the defence plan proved of little use against Blitzkrieg tactics and the Luftwaffe.
So much for history. As I have said, much of the embankment and its fortifications are still standing, making an impressive feature in an otherwise flat landscape between towns. The embankment goes through Hoofddorp, the town where I live, which is close to Amsterdam Schiphol International Airport. Indeed, part of the embankment is within easy walking distance of my house. Parts are adapted as foot or cycle paths, but, in Hoofddorp, the embankment is cut, not just by roads, but also the main Amsterdam – Schiphol Airport – Leiden – Rotterdam railway line, between 24 and 25, the Batterij aan de Sloterweg (Battery on the Sloterweg; see Fig. 2). The railway was built in the late 1960s.
You may by now be asking what this all has to do with geology? The embankment between the railway line and the Battery on the Sloterweg, trending southeast away from Hoofddorp, was recently restored and landscaped (Fig. 1C-E). The slopes were scraped and replanted with grass, the battery spruced up (Fig. 1A and B) and the cycle path resurfaced. Scraping the surfaces clean exposed the underlying soil, rather obviously, but also pulled to the surface rare pebbles and cobbles of rock and builders’ rubble, which presumably constitute part of the core of the embankment. Over a period of some months, I have made a small collection of rock samples to explore its geodiversity (Fig. 3).
The bulk of the clasts from this site are not rocks at all, but broken chunks of man-made rubble, such as bricks, tiles and concrete. I have chosen not to illustrate any of these, but a plate almost as diverse as Fig. 3 would be possible. The presence of such a wealth of builders’ rubble is surely indicative of the source of much of the core of the embankment and it is probable that some of the rocks, particularly the limestones, had a similar origin.
Some clasts have an unexplained ‘local’ provenance. Cobbles of granite (Fig. 3A and B) are irregularly shaped and do not appear to have been involved in building. Rather, they are the size and shape of track ballast used at Hoofddorp railway station (Fig. 1F). How they came to be transported a few hundred metres to be found on the embankment close to the battery is more difficult to explain.
Fig. 3C illustrates part of a bed of Upper Palaeozoic (Mississippian?) limestone, which is about 40mm thick. This dark grey limestone weathers pale grey and lacks obvious macrofossils. Similar limestones are common facing and ornamental stones in the Netherlands (van Roekel, 2007; Donovan, 2014), but this specimen shows no obvious signs of having been worked, so it may have been discarded by a stonemason or builder. Another clast of similar limestone (not figured) has one obviously smooth, worked surface and is rich in small, indeterminate macrofossils, best seen on broken surfaces.
A well-cemented, medium grained sandstone is pale grey in colour, weathering to straw brown and a range of darker, colour-banded shades (Fig. 3D). The sandstone is ‘clean’, that is, dominantly quartz with just a few lithic grains of darker colours. Quartz grains are glassy and moderately rounded, without showing marked sphericity. The clast shows no indication of bedding. It is presumably derived from a massive bed of fluvial or marine sandstone and, again, may be a stonemason’s reject. Similar colour bands are a common feature of some Carboniferous sandstone (Simpson et al., 2014, fig. 10).
Particularly near the battery, chunks of coal (Fig. 3E-G) are locally common. It is reasonable to assume that these formed a (lost) part of the fuel supply for the troops at the battery.
Moderately well-rounded pebbles and small cobbles of vein quartz are the most common lithic clasts (Fig. 3H); one of the figured specimens (Fig. 3I) preserves part of the mineralised rock wall with a distinct patchy red coloration (jasper?). These clasts are not spherical, but are invariably well rounded, indicating water-borne transport (fluvial?). Like most of the specimens discussed below, they are probably derived from a local river gravel (Vossestein, 2014).
Other rounded pebbles preserve narrow quartz veins within them (Fig. 3M and O). Both of these specimens appear to be well-lithified sandstones; the bedding in Fig. 3 may be parallel to the plane of the page, whereas the vein dips gently towards the bottom. The ‘line’ apparent in the centre of this vein shows clearly where crystals grew from either side and met in the middle.
A further siliceous clast (Fig. 3J) is a translucent brown to dark grey colour and is a moderately well-rounded pebble of chert. Again, this was likely transported as part of a river bedload. Most probably, it was derived from silicified Lower Carboniferous (Mississippian) limestones (Donovan et al., in press) or flint from the chalk (Upper Cretaceous). No fossils are apparent.
The most delicate pebble is a dark grey, weakly fissile mudrock (Fig. 3K). Again, this is unfossiliferous and its provenance indeterminate.
Fig. 3L illustrates a well-cemented, impure, medium-grained sandstone. The clast is moderately well-rounded and, again, is interpreted as being removed from a river or river gravel deposit. Bedding is faintly apparent, oriented more or less left-to-right, but irregular, although it does not appear to be cross-bedded.
A hard, black, fine-grained and flat pebble about 13mm thick, with obvious quartz veining, is basaltic (Fig. 3N). The grains are too small to be apparent with a hand lens, but, of all the clasts, it is the most parallel-sided, suggesting it represents a thin, chilled lava flow. It has been freshly broken in the lower left corner as illustrated.
Different in colour from other rounded clasts is a red sandstone pebble (Permo-Triassic?), the colour enhanced for photography by making it wet (Fig. 3P). This specimen hid its secrets better than most so, after photography, I broke it across the centre, left-to-right, with a hammer. Under a hand lens quartz grains are readily apparent, as well as lithic grains of mafic crystals.
From the above, it is apparent that there are two main sources of clasts in this structure.
- Builders’ rubble, mainly man-made, such as concrete, bricks and tiles, but also including limestone and sandstone (Fig. 3C and D). Both are probably Carboniferous and, in one example, preserve undoubted evidence of the stonemason’s art.
- Rounded pebbles and cobbles from river bedload and gravels. These clasts would have been derived mainly (probably entirely) from the drainage of rivers outside the Netherlands.
- Minor components include displaced railway ballast and coal from the battery’s fuel supply.
It was not anticipated that this collecting exercise, undertaken for personal interest, would yield such data worthy of interpretation. Yet, the clasts divided themselves into distinct groups that have provided convincing evidence regarding provenance.
Donovan, S.K. 2014. Urban geology: A sunny Sunday in Hoofddorp. Deposits, 38: 8-10.
Donovan, S.K., Jagt, J.W.M. & Deckers, M.J.M. (in press). Reworked crinoidal cherts and screwstones (Mississippian, Tournaisian/Visean) in the bedload of the River Maas, south-east Netherlands. Swiss Journal of Palaeontology.
Roekel, A. van 2007. Discover fossils in downtown Amsterdam: Ancient ocean life in Amsterdam’s alleys. Uigeverij De Vuurberg, Amsterdam, 17 pp.
Simpson, M. & Broadhurst, F., revised by Strother, P. del & Rhodes, J. 2014. A Building Stones Guide to Central Manchester. Third edition. Manchester Geological Association, Manchester, vi+67 pp.
Vossestein J. 2014. The Dutch and their Delta: Living below sea level. XPat Scriptum Publishers, Schiedam, 295 pp.