Grove Hermans

In the wall of the abandoned gravel pit Hermans, to the northeast of As, important geological phenomena are clearly visible. The gravel pit is very important for the study of deposits and the formation of the Kempen Plateau (the main terrace of the Maas) during the Quaternary (since 2.5 million years ago). Physical geography Location The now abandoned pit is located at an elevation of 85-90 meters +TAW, at a natural steep edge in the northern edge of a dry side valley of the Bosbeek. Behind the gravel pit, there is a pine plantation, to the northeast and northwest of which are some plots of mixed forest. The immense pit wall, with a height of 15-20 meters and a length of approximately 700 meters, exposes a thick package of red gravels of Quaternary age, resting on Tertiary sands (Member of Genk). Geology The locally vertical pit wall does not form a uniform whole, but consists of a succession of gravel, sand, and clay layers. These layers are limited in size and usually form deposits in small, shallow channels. Laterally, different layers alternate in the pit wall. Vertically, however, there is always a clear succession. Consequently, the deposit can be divided into different packages. Each package consists of a layer of coarse gravel at the base, which becomes finer upwards, transitioning into a sand layer, and in some cases even into a clay layer. At the top of the clay layer, a new package begins with a gravel layer again at the base. It can be inferred that the gravel package was not deposited as a homogeneous whole. Sedimentation occurred in numerous small, shallow beds, sometimes less than 20 meters wide, which deposited gravel in the same horizontal plane, sand in a different channel, and clay elsewhere. This flow system was sometimes interrupted by a significant deposit of coarse material. Afterward, this cycle began again. The gravel package was formed by a so-called wild river with numerous branches that deposited various types of material depending on location and time. This fluvio-periglacial deposition was possible because mass movements on the slopes brought large sediment packages to the valley floors, greatly increasing the river loads and subsequently suppressing vertical erosion. In various places in the profile, large gravel blocks are frequently found, sometimes more than 1 cubic meter. The only possible explanation for the presence of these blocks is that they were transported on ice floes and deposited when they became too small to bear such heavy loads. This suggests that the deposition occurred during a cold period. At the top of the gravel package, the horizontal layering of the original deposit has disappeared. The gravel surface is undulating over a height of about 2 meters and consists of a regular alternation of depressions and mounds. The mounds were formed by gravels that were pushed upwards, with their long axis oriented vertically, while the depressions were filled with loamy cover sands carried by the wind. It is also notable that the undersides of the depressions are concave to flat and almost always occur at the same depth. The sand bags are, as it were, bounded at the bottom by a horizontal plane that runs through the pit. Upon excavation of the upper surface, it appears that the visible gravel domes connect to each other to form polygons. These gravel polygons surround a central core of fine material. The whole forms a polygonal ground, which is now only formed in Arctic areas. There are differing opinions regarding the formation process of polygonal soils. This process is best explained by a theory of convection currents. At the end of the Arctic summer, the thawed topsoil or mollisol freezes again from the surface and expands downward. The mollisol is characterized by material with a high moisture content, in which plastic deformations can occur. Eventually, in the mollisol - between the superficially frozen layer and the permafrost or permanently underlying layers - convection currents arise due to internal pressure, causing the lower layers of the still unfrozen portion to move upwards. The gravels are pushed up and gradually form mounds. The depressions between the pushed-up mounds are filled with cover sands. The base of the sand bags corresponds to the top of the then-existing permafrost. Here, the long axis of the stones is positioned perpendicularly (least resistance). This polygonal ground corresponds to a significant cold period following the gravel deposition. Within the gravel package itself, cryoturbations also occur at varying depths. In the western part of the pit (about 100 meters from the end, approximately 15 meters from the top), a clay layer together with the surrounding gravels has been significantly deformed by frost processing, while the overlying sediments are horizontally layered again. These cryoturbations occurred during the gravel deposition itself (syngenetic) and therefore prove that the deposition occurred during a cold period. The main terrace of the Maas The fact that the main terrace of the Kempen, to which this pit belongs, is a Maas deposit is evident from the morphological element that it is located on both sides of the Maas valley and can also be traced downstream from the Maas. The gravels in the eastern part of this main terrace form a petrografically homogeneous population that continues north of the steep edge Bree-Grote Brogel. The gravel package forms the substrate of the entire Kempen Plateau, which is built up from continuous fluvial sand and gravel deposits. To the north, the thickness of the gravel package can vary. The gravel package is only locally covered with cover or dune sands in which the current podzol soil has developed. In various places, these so-called coarse gravels come to the surface or are covered by a thin layer of sand. The deposits of the Kempen Plateau mainly originate from the Maas, but the sand facies also contain Rhine deposits. The Hermans pit is situated in the gravel facies, the youngest deposit found on the Kempen Plateau. The deposit is well-drained and contains no nourishing components. The water table is located deep against the eastern plateau edge. Further west, the water table is located higher. The plateau gravels are characterized by a dominant reddish-brown color that intensifies towards the surface. Locally, this type of gravel is referred to as "forest gravel" to distinguish it from the pure, gray "Maas gravel." The reddish-brown color due to iron enrichment in the form of removable pellets around the gravel clay is a secondary feature, a phenomenon that occurred after the deposition of the gravel package. Locally, black manganese enrichments can also occur. Weatherable minerals have disappeared from the deposit. Among other things, the reddish-brown color of the gravels, the black manganese enrichments, and the absence of weatherable minerals indicate strong weathering after the gravel deposits. This fossil soil was formed analogously to the existing podzol soils in a warm climate and is described as the "As soil." It developed after the gravel deposition, but before the deposition of the yellow cover sands and the formation of the polygonal soil. This fossil soil was most likely formed during one or more warm periods of the Middle Pleistocene (850,000 to 130,000 years ago), including very likely the Holstein interglacial (370,000 to 130,000 years ago). Both the mineralogical and petrografical study of the deposits show that they largely originate from the still rising Ardennes. The angular flints come from the chalk cliffs exposed at Visé and Maastricht. Sometimes weathered granite can also be found among the fine gravels, brought from the Vosges and along the Moselle (which was then still a tributary of the Maas). The most common gravels are: quartz, especially vein quartz; numerous quartzites, mainly revinien quartzite (large blocks characterized by pyrite prints and often with secondary vein quartz); sandstones, including red Devonian sandstone and fine spirifer sandstone; conglomerates (with large blocks of red conglomerate from Burnot), flint or firestone, usually very angular. The rounded flint stones originate from the base gravels of the Tertiary layers.

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