Peel Boundary Fault

The Peel Boundary Fault (Peelrandbreuk) is a geological fault zone named after the Peel area. In the Netherlands, the fault runs roughly from Roermond to Oss and separates the higher-lying Peelhorst from the slowly sinking Roerdal Rift Valley (Roerdalslenk). This process is still ongoing; the average vertical movement along the fault is approximately five centimetres per thousand years. Earthquakes still occur along the fault, such as in Roermond in 1992 and near Uden in 1932.

Location of the Peel Boundary Fault in the Netherlands
Location of the Peel Boundary Fault in the Netherlands.

Where you can see the Peel Boundary Fault

The Peel Boundary Fault can be clearly seen in the landscape in some places. In the Saint Anna Forest near Uden, for example, you can observe a height difference of almost two metres. Here, you can also observe that the vegetation on the elevated part of the fault is different from that on the lower side. This is because the soil on the elevated side is much wetter. Soil experts would normally expect it to be the other way round; however, due to the tectonic movement of the soil, the density of the soil around the fault is much greater and practically impermeable to water. As a result, groundwater flowing from the higher Peelhorst towards the lower Roerdal Rift Valley is forced to the surface at the Peel Boundary Fault.

This groundwater naturally contains a lot of dissolved iron. As soon as the iron particles come into contact with oxygen, rusty deposits are formed in the soil. When this process is repeated in the same place over and over again, large, hardened iron concentrations may form, the so-called bog iron banks.

These high-lying, marshy areas with iron-rich seepage water are called wijstgronden, a rare phenomenon that is nowhere as clearly visible in Europe as near Uden. That is why the province of Noord-Brabant declared the special area ‘De Wijstgronden’ as the province’s first geological monument in 2004. At present, there is even the ambition to have the entire area around the Peel Boundary Fault recognised as UNESCO Geopark Peelhorst .

The Peel Boundary Fault with disturbed sediment layers in a trench dug near Bakel (photo: VU - Ronald van Balen)
The Peel Boundary Fault with disturbed sediment layers in a trench dug near Bakel. (Photo: VU – Ronald van Balen)

Walking and cycling around the Peel Boundary Fault

Various walking and cycling routes have been marked out in the area around the Peel Boundary Fault. The Fault Experience (‘Breuken Beleven’) route runs between Gemert and Bakel. This route takes you, amongst other places, past wijstgronden, bog iron banks, rust-coloured water streams, and steep edges at the Peel Boundary Fault. Nature Centre De Specht in Handel has marked out the ‘Fault Path (‘Breukenpad’) Route especially for children. While walking the route, children are playfully taught about the origins and workings of the Peel Boundary Fault. In 2019, the Foundation for Voluntary Landscape Management (SVLU) in Uden realised a wooden footbridge (“knuppelbrug”), which allows you to stroll across the fault and its surrounding wijstgronden. 

Routes

Water stream with dissolved iron near the wijstgronden (Photo: TNO - Pieter van der Klugt)
Water stream with dissolved iron near the wijstgronden. (Photo: TNO – Pieter van der Klugt)

Geology in brief

Plate tectonics causes stress in the Earth’s crust, which can crack and break. The Peel Boundary Fault is a so-called shear fault. Through stretching, the Earth’s crust here moves apart, causing the Roerdal Rift Valley to sink or subside slowly downwards along the fault line of the Peel Boundary Fault. The subsidence of the Roerdal Rift Valley has largely been kept up by filling it up with sediment, thereby keeping the height difference at the surface relatively small. 

Geological structure

At DINOloket.nl, the data portal of the Geological Survey of the Netherlands, you can make your own cross-section of the subsurface. At the surface, we find the Boxtel Formation with, amongst other things, covering sand from the last ice age. Underneath, we can observe various river deposits of the Maas (Beegden Formation), the Rhine (Sterksel Formation, Waalre Formation, and the Kiezeloöliet Formation), and local rivers (Stramproy Formation). Underneath, we find the marine deposits of the Oosterhout Formation and Breda Formation, which are from the time when the area was still covered by a shallow sea. The cross-section below shows the displacement along the Peel Boundary Fault and other faults. One can see that thicker sedimentary layers were able to form in the continuously sinking Roerdal Rift Valley than on the higher-lying horst. 

Cross-section across the Roerdal Rift Valley with the Peel Boundary Fault, from Bladel (left) to Gennep (right)
Cross-section across the Roerdal Rift Valley with the Peel Boundary Fault, from Bladel (left) to Gennep (right).