Urban geology: An inselberg in Rotterdam
Stephen K Donovan (The Netherlands)
The Low Netherlands, much of which is below sea level, is a broad area of the country that (very approximately) parallels the coast and is kept ‘dry’ by major works of civil engineering (IDG, 1985, pp. 6-7). Geologically, it is a flat expanse of Holocene deposits; most of the author’s experience is in the coastal plain (de Gans, 2007), where I both live and work.
There is no significantly older geological deposit or feature anywhere in this region – no coastal cliffs, mountains or quarries to tempt the attention of the wandering Earth scientist. So, it is commonly the ex situ that demands the geologist’s attention rather than the in situ. For example, I have commented previously on such diverse topics as the use of imported limestone to make a false natural bridge (Donovan, 2014), various aspects of building stones (for example, Donovan, 2015, 2019) and gabions mimicking sedimentary bedding, at least from a distance (Donovan, 2018).
Of these examples, the natural bridge is the most exotic; although such a bridge might be expected in karstified limestone landscapes almost anywhere, my own experiences of them are limited to the Antilles (Miller and Donovan, 1999; Donovan et al., 2014). In this article, I describe a further man-made structure mimicking an even more exotic geomorphological phenomenon, most closely associated in the minds of Earth scientists with Africa. It is a structure that I have, until now, only known from textbooks – I refer to a mock inselberg in Rotterdam.
Blijdorp Diergaarde (= zoological gardens) in Rotterdam has a geographically-themed layout, different parts exhibiting faunas of the major continents (apart from Antarctica) and the oceans. On my latest visit, I was intrigued by internal rearrangements that divide the zoo into its geographical components more strictly than before. And it was while I strolled through the Africa region that I happened on a large open aviary for vultures and other birds of prey.
This aviary is a walk-through structure and it was probably the first time since I left my teaching job in Jamaica, over 20 years ago, that I have been standing under a low-flying vulture. The public are fenced off from a grassed surface with a pond, small trees and nesting boxes. Yet, for the geologist, pride of place must go to the structure at the north end of the aviary, an isolated hill, a mock granite inselberg (Fig. 1).
Recognition of this inselberg had me scrambling for my favourite references in geomorphology (Dury, 1959; Whittow, 1984; Small, 1989). Blocky inselbergs are, indeed, typical of tropical savannahs in Africa, but are also found in Australia, the southwest USA, Spain and elsewhere. They are called kopje in South Africa, from a Dutch word meaning head, perhaps describing the shape; inselberg is German and means ‘little mountain’. Whittow (1984, p. 273) succinctly defines an inselberg as “… a prominent steep-sided hill of [resistant] sold rock [commonly granite], rising abruptly from a plain of low relief”.
These low domes are exposed by the erosion and stripping away of the surrounding residual, superficial ‘soils’, that is, the regolith. The regolith is typically a thin layer, which can be removed when the major storms of the rainy season lead to flooding and turbulent flow. The inselbergs themselves suffer exfoliation by dilation, that is, “… pressure release within a rock mass by the removal of overlying layers by denudation” (Whittow, 1984, p. 146). This leads to the formation of planar, approximately parallel joint planes (Small, 1989, fig. 3.2).
So, is the mock inselberg in Rotterdam a good representation? Taking into account the limitations of area (a vulture aviary versus the African savannah), the problem of transporting such large, rounded masses of granite (not a rock type found in situ in the Netherlands) and the very different climatic regime, I was impressed. This is not some man-made replica of rock, but real granite in naturally rounded boulders. That is, the granite is well-rounded, not blocky as might be expected if it was quarried. Parallel joint planes are not developed and never will be, but this is a trivial whinge about a welcome contribution to the aviary’s scenery. It was a surprise and a pleasure to discover the Blijdorp inselberg – enjoy it yourself the next time you are at the zoo.
References
Donovan, S.K. 2014. An unnatural bridge in an artificial limestone environment, the Netherlands. Cave & Karst Science, 41: 118-119.
Donovan, S.K. 2015. Urban geology: Two granites. Deposits, 41: 8-9.
Donovan, S.K. 2018. Urban geology: gabions in the Dutch townscape. Deposits, 53: 14-16.
Donovan, S.K. 2019 (in press). Urban geology: Mississippian in the Mainstreet. Geology Today.
Donovan, S.K., Harper, D.A.T., Jackson, T.A. & Portell, R.W. 2014. A note on a coastal natural bridge in Antigua, West Indies. Cave & Karst Science, 40 (for 2013): 105-108.
Dury, G.H. 1959. The Face of the Earth. Penguin, Harmondsworth.
IDG [Information and Documentation Centre for the Geography of the Netherlands] 1985. Compact Geography of the Netherlands. Ministry of Foreign Affairs, The Hague.
Gans, W. de 2007. Quaternary. In: Wong, Th.E., Batjes, D.A.J. & Jager, J. de (eds), Geology of the Netherlands, pp. 173-195. Royal Netherlands Academy of Arts and Sciences, Amsterdam.
Miller, D.J. & Donovan, S.K. 1999. Geomorphology of the Natural Bridge at Riversdale, parish of St. Catherine, Jamaica. Caribbean Journal of Science, 35: 112-122.
Small, R.J. 1989. Geomorphology and Hydrology. Longman, London.
Whittow, J. 1984. The Penguin Dictionary of Physical Geography. Penguin, Harmondsworth.