Urban geology: A sunny Sunday in Hoofddorp
Stephen K Donovan (The Netherlands)
The last weekend in September 2013 was sunny after more than two weeks of grey skies, rain and even some fog. Saturday was spent as planned, moving bookcases ahead of Karen’s insatiable paintbrush, the walls changing from lime green to white as she progressed. Sunday morning was spent putting some books back onto bookcases, but I had to get out in the afternoon. It might be six months or more before I could venture out again in only a T-shirt, shorts, training shoes and floppy hat. I had my son, Pelham, as field assistant, but where to go? The answer was obvious to me – this was the day to consummate a project that I’d had in contemplation for some years.
The Netherlands is not renowned for its pre-Pleistocene geology. There is the type Maastrichtian (uppermost Cretaceous) in the south, some fine Triassic near the German border in the east and odd spots of poorly exposed Tertiaries. Where I live, in Hoofddorp (near Amsterdam Schiphol Airport), we live below sea level on the bed of a drained lake; but what Hoofddorp lacks in surface exposure, it makes up for in building and ornamental stones. To the south and east of the town is a business park in the Beukenhorst district, with a fine range of architectural styles and building materials, both man-made and natural stone.
One road, Siriusdreef, in this part of town has intrigued me for years. Street art is widespread in the Netherlands and this example uses large blocks of massive limestone to form bench-like structures that are perhaps meant to mimic rock exposures (Figs. 1 and 2). The pavement is only continuous on one side of the road, including on a bridge over a small canal. To dissuade motorists from parking on the pavement on what is only a narrow road, boulders of grey, crystalline limestone occur every 4m or so along the curb. The boulders, all no more than about one metre or so in maximum dimension, and the large blocks are presumably Mississippian (Lower Carboniferous) and have been imported from elsewhere – there is no Carboniferous limestone quarry in the Netherlands. However, the large, bench-like structures at the side of the road away from the kerb are even more fascinating.
As already stated, I assume that these structures are part of the Dutch street art tradition. They are in lines of two to four, most commonly three, large blocks of Carboniferous limestone, more or less oriented east-west with one exception, adjacent to a canal, which is more east-south-east to west-north-west. Various features suggest to me that they all came from the same limestone bed, including similarity of lithology and bed thickness.
These are unlikely to be intended as benches for the weary traveller, because there are also concrete seats on the street and the limestone blocks would not be clean to sit on. There are seven of these structures, which are outcrop-like, although obviously not placed here just for the amusement of geologists. I have given them the names of ‘pseudo-dykes’ – they certainly appear dyke-like at first glance – and numbered them PD1 to PD7 from northeast to southwest (Fig. 1). And they show a number of interesting features on the small (Fig. 3) and large scale (Fig. 4).
Features of the limestones
Bedding in the limestone is massive and only one block had part of the bed peeling off along a less well-lithified horizon. Internal bedding in some blocks is cryptic, but is demonstrated by the presence of planar nodules of dark grey to black chert, formed more or less in discrete horizons (Fig. 3E and G). Joints and calcite veins formed perpendicular to bedding surfaces are also indicative of orientation; some are still enclosed within the limestone bed (Fig. 3F) and others are well exposed (Fig. 4A and C). Rare joint or fault surfaces have slickenside-like structures (Fig. 3B) – my son spotted this feature before I did.
The limestones have local sparse accumulations of shelly fossils, mainly brachiopods, corals or crinoids. All would benefit from cleaning and a bleach bath, but, of course, this site is strictly no hammering and no collecting. The brachiopods are commonly small and even the largest specimen (Fig. 3C) is less than photogenic, much of the valve having either spalled off or is concealed by limestone. The corals are generally more attractive and include both solitary Rugosa (Fig. 3D), seen in transverse section, and tabulate colonies close to Michelinia (Fig. 3A).
Man-made ‘way-up’ structures
I interpret these blocks as coming from a quarry, where the limestone occurred as more or less horizontal beds. As already deduced, the blocks were most likely derived from a single bed; it is of interest to try to reconstruct its original field relationships. It seems that the blocks were broken up with explosives and the drill-holes for these are readily apparent (Figs. 3H, and 4A and B). These have an open upper end and terminate within the limestone with a base that is readily apparent (Fig. 3H). These make the drill-holes a suite of man-made ‘way-up’ structures, indicating the former orientation of the blocks at the time when they were quarried.
Therefore, the block on the left in Fig. 4A, with drill holes vertically down from the bedding surface, is deduced to be in the same orientation that it had in the quarry, but consider the contrasting information in Fig. 4B. The block on the left agrees with Fig. 4A, that is, it is in its quarried orientation. In contrast, the block on the right, with drill-holes up from the pavement level, is obviously inverted. Moreover, the arrangement of the drill-holes and the calcite vein (right of the middle drill-hole), and the topography of the exposed surfaces, shows the blocks in Fig. 4B were formerly adjacent within the bed.
My special thanks go to my son, Pelham, for his help, companionship and observations in the field.
References
Maley, T.S. 2005. Field Geology Illustrated. Second edition. Mineral Land Publications, Boise, Idaho: xiv+703 pp.