De ontwikkeling van het landschap ten oosten van Winterswijk

At the request of ‘Dienst Landelijk Gebied’ at Arnhem, Alterra b.v. carried out pedogenetic-geographical studies between 1995 and 1997, for the benefit of the Winterswijk-Oost reconstruction project (Kleijer & ten Cate, 1998). These studies took place in close co-operation with the ‘Geologisch Veldlaboratorium Winterswijk’. Rather than constituting a preconceived goal, the present paper is the result of an experiment in uniting geological and pedogenetic-geographical data. These data were previously acquired during a unique research project, whereby, among other things, of the clay encountered in each well it was determined whether it was of Cainozoic or Mesozoic age, or if it represented till (= ground moraine). Combining pedogenetic-geographical and geological knowledge and experience did not only result in a reliable model of the (sub)soil, it also yielded a graphic expression of the development of the Winterswijk landscape during the past 100,000 years. Although the Winterswijk-Oost reconstruction project covers but a small portion of the East Netherlands Plateau, it does constitute the most intricate part and as such it is representative of research that will broaden our knowledge. Both the extremely complex geological history and pedogenetic-geographical history of this region explain its unique character. The present paper attempts to summarise results of recent research projects which, in general, are dispersed in various short contributions and internal reports. In the discussion that follows, very few previous literature sources are consulted: as we have based our observations mainly on newly acquired facts, not speculations. Neither did we do much historical research – this we wish to leave to others. We did, however, consult the first measured map of the Netherlands, by Lieutenant-General Cornelis Rudolphus Theodorus Baron Krayenhoff (1758-1840), produced between 1801 and 1811, in connection with the course of present-day streams. In this respect, older unmeasured maps often present conflicting pictures. As a result of the work presented here, many previous assumptions on the development history of the study area (van den Brand et al., 1981) need to be corrected. Pedogenetic-geographical research and geological analysis have both demonstrated that many present-day brooks do not represent natural streams. This corroborates previous, albeit less well-founded, assumptions (van de Westeringh, 1984; van den Bosch, 1992-1993, part 4-92). In addition, a reliable picture of the developmental history of the present-day Winterswijk landscape has now been gained. By publishing the present report, we wish to record the results of our studies, aiming at a larger readership consisting of natural scientists, archaeologists, environmental protectionists, district water boards, educational services and the general public. The study area is part of the so-called East Netherlands Plateau; it is bordered in the east by the uplands in Germany (50-70 m + NAP = Normal Amsterdam Level) and in the west by a glacial meltwater gully and the valley of the Aaltense Slinge (Map 1). In places, this meltwater gully cuts to over 100 m – NAP and is filled with sediments of Late Pleistocene age. The depth of the boundary with underlying Cainozoic strata fluctuates throughout the study area. In the east, Mesozoic (Triassic-Cretaceous; Herngreen et al., 2000) deposits are encountered at shallow depths. The Cretaceous strata in particular may contain water in extensive zones and these account for seepage mainly at Kotten into the Boven Slinge, into the basal Cainozoic sands (Ratum Member) as well as into Quaternary sediments (van den Bosch, 1995, appendix 5). The depth at which Cainozoic strata are encountered sinks considerably towards the west, down to c. 100 m – NAP in the southwest of the study area. These deposits consist mainly of impermeable clays. The base of the Cainozoic sequence (Rupelian) consists of sand (Ratum Member); the base of the Breda Formation also comprises watersupplying glauconitic sands (Miste Bed), which appear in the westerly part of the study area. The latter deposit is no longer fed by ground water from the deeper subsoil. There are no comprehensive geo-hydrological studies of the systems on the East Netherlands Plateau. Just two more, less detailed, studies are available, both of which are based on the intricate geological structures in this area. The first is a research project around the pumping station Corle (Reitsma, 1981; Schoonderbeek, 1999); the second focuses on the sandpit ‘t Hilgelo, north of Winterswijk (Willemsen, 1998). The latter study also included a portion of the Winterswijk-Oost reconstruction project. We may conclude that the groundwater map of the Netherlands, sheet 41 East (Bloemendaal & Comelissen, 1985), is based on insufficiently detailed geological knowledge. Other, mainly small-scale studies have not been tested sufficiently against intricate geological models. Data presented here show that the relief of the Winterswijk landscape, down to relatively small details, has developed naturally on a complex geological base. To this day, the natural relief remains largely undamaged. The landscape surface, however, has largely been determined by human activities, which has had severe impacts on flora and fauna. Humans have previously reshaped the entire area; consider for instance the numerous ‘enkeerdgronden’ which locally reinforce the relief, and the brooks most of which should in fact be considered as constructed drainage systems. Over large areas, people cut sods, and as a result of cultivation and modern agricultural techniques the former podzol soils have largely disappeared. Despite all this, the original relief, however, has remained almost intact. Remaining original natural features such as the unique glacialtill landforms, the stream valleys and cover sand heights are now threatened by nature development schemes, in which soil is being excavated on a large scale, so as to remove the fertile topsoil or create wet sites. In this way not only the last remains of natural podzols are destroyed, but also the original relief is violated indiscriminately. Landscape reconstruction is much to be preferred, so that the natural development of the area may be safeguarded. The ‘Stichting Aardkundige Waarden’ should designate the area as landscape-forming geological monument (‘aardkundig monument’) and geological structure (‘aardkundige structuur’), in order to preserve present-day conditions. Nature design and nature development should then be implemented only within the natural concept of landscape development.