This post is part of the GeoConnect³d blog.
In the southeasternmost part of the Netherlands, a region known as South Limburg, the presence of karst phenomena has been known for a long time. Traces have come to light in many places through the extraction of chalk. More specifically, these features often concern solution pipes: vertical shafts formed by the dissolution of limestone by percolating rainwater. The widespread and free availability of LIDAR data for the entire Netherlands now enables a systematic inventory of dolines formed as a result of solution pipes.
A quick refresher. Dolines are natural depressions that occur at ground level in karst landscapes. They occur mainly where the subsoil consists of limestone or evaporites, both of which are soluble in water. When rainwater percolates through the soil, it takes dissolved organic acids with it on its way down.
The acidic water dissolves the rock, forming crevices, cavities and caves. The two main ways in which dolines are subsequently formed are dissolution and collapse. With dissolution, the overburden gradually sinks into a crevice or solution pipe, creating a depression at ground level. Solution pipes are funnel-shaped; they can be several metres wide at the top and reach depths of tens of metres. Collapse is often more sudden: the limestone ceiling of a cave collapses, causing the rock above it and the overburden to fall, creating a depression in the landscape.
As mentioned, karst phenomena, and especially dissolution pipes, are common in South Limburg. The limestone from the Late Cretaceous is found close to the surface here. Research of the prehistoric flint mines east of Maastricht (Rijckholt) gave a rare insight in the spatial distribution and density of solution pipes. A map of the excavations shows a large density of ‘geological solution pipes’ with a diameter of up to about three metres. At a depth of 5-10 metres below ground level, 8-10 solution pipes per 100 m2 can be seen. Many solution pipes have also been found in limestone quarries, such as those in the former ENCI quarry south of Maastricht. The solution pipes are filled with sediment from overlying strata. In South Limburg, this is sediment of Paleogene or Neogene age. In the areas where the Meuse used to flow, it is often sand and gravel from the fluvial terraces of the river Meuse and loess.
So, solution pipes are common, but are dolines also widespread? No, they are not. This can be explained by the overburden that masks the subsidence in the underlying rock. To find dolines, we therefore need to look for areas with a thin overburden. This is the case in the area in the far southeast corner of South Limburg. There the overburden is at its thinnest and there are various plateaus. According to the Digital Geological Model (DGM) of TNO, Geological Survey of the Netherlands, limestone of the Gulpen Formation can be found close to the land surface. Ideal conditions for dolines.
Inspection of the area with the aid of LIDAR data (Actueel Hoogtebestand Nederland, AHN) reveals something reminiscent of a crater landscape. A quick-and-dirty mapping reveals 569 doline-like depressions in the area (Figure 1). As expected, the circular depressions are mainly located on the plateaus, with a density of a few dolines per 1000 m2. Most have a smooth edge, while some are more sharply defined. This may have to do with age and process of formation (solution, collapse), but a striking correlation with land use is present too. Dolines in agricultural lands are smooth edged, while those in forested areas generally have sharp edges.
During a field inspection, it is easy to locate the depressions with the LIDAR data at hand. Most are dry, a few contain water. The diameter varies between a few metres and about ten metres. The depth is usually no more than one or two metres. One of the largest depressions on a forested plateau, the Vijlenerbos, has steep sides and a depth of about four metres (Figure 2). Most of the depressions are probably dolines. Some may be mis-interpretations, as they could actually be man-made drinking pools for cattle or a small quarries. A major bombardment in World War II is unlikely.
The exact age of solution pipes and dolines in South Limburg is unknown; no research has been published on them. The dissolution of the limestone probably started tens of millions of years ago, when the rock was uplifted above sea level. Observations of solution pipes in the ENCI quarry show that the fill consists of thick layers of loess, which indicates that there was already a depression during the Pleistocene when the loess was deposited.
The process of doline formation is still active. Proof was recently provided by a hiker. Alongside a footpath south of the town of Epen, he found a hole in the ground with a diameter of about one metre (Figure 3). The hole had appeared in the middle of a meadow where cows graze. A potentially hazardous situation. During the construction of the A2 motorway tunnel in Maastricht, there were many problems due to solution pipes filled with gravel of the river Meuse. The highly permeable infill of the pipes shed a lot of groundwater. In Germany, a high-voltage transmission tower recently collapsed because one of its legs was resting on a solution pipe. Investigations are underway to assess the stability of such towers in South Limburg.
It seems that solution pipes occur in a large part of South Limburg. Because the overburden is often too thick, dolines do not form everywhere, making it difficult to map the solution pipes. Only in the far southeast of South Limburg dolines are easy to identify thanks to LIDAR data. Despite their invisibility, karst phenomena are a real problem, for which extra attention on a regional scale is desirable.
TNO, Geological Survey of the Netherlands
Deeben, J., De Grooth, M.E.T., De Kort, J.W., Lauwerier, R.C.G.M., Ter Schegget, M.E., 2011. Archeologische onderzoek in de omgeving van het prehistorische vuursteenmijnveld te Rijckholt – St. Geertruid – De resultaten van 2008 en 2009. Rapportage Archeologische Monumentenzorg 202. Rijksdienst voor het Cultureel Erfgoed, Amersfoort, 305 pp.
Rademakers, P.C.M., 1998. Geologische orgelpijpen. Grondboor & Hamer 3, p. 71-78
Rademakers, P.C.M., 1998. De prehistorische vuursteenmijnen van Ryckholt-St. Geertruid. NGV, Afdeling Limburg, Maastricht, 334 pp.
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