Networks of unusually large fossil periglacial polygons, Campine area, northern Belgium

A series of polygon networks has been discovered on the most recent LiDAR (Light Detection and Ranging) DEM (Digital Elevation Model) of Flanders (Belgium) available in a resolution of 1 m². They are located in the sandy Campine area (northern Belgium) and resemble thermal contraction crack polygon networks from present-day permafrost regions. Different network types were observed, ranging from orthogonal to hexagonal and various combinations of these. The inter-polygon troughs are typically several decimeters deep and up to several meters wide. The average polygon size is ca. 3000 m², which is equivalent to a diameter of ca. 60 m if the polygon shape is approximated with a perfect circle, or a side of ca. 55 m length if it were to be approximated by a perfect square. The average size is (much) larger than any of the studied present-day analogues, and also larger than fossil networks in the western part of Flanders, Poland and France. In contrast to the Campine polygons presented here, the fossil analogues in these countries were detected using satellite imagery and orthophotos, which may partially explain the observed size differences. The morphometric analysis of the Campine networks shows relationships between polygon type and local geomorphological position as orthogonal networks seem to have a preference to develop near shallow valley slopes. In addition, GPR (Ground Penetrating Radar) radargrams were acquired across polygon boundaries to investigate subsoil disturbances related to the former position of ice wedges or sand wedges. However, the evidence is not unequivocal due to the low dielectric contrast between the host and wedge material. It is not clear yet whether smaller 2nd and 3rd order cracks did develop but without leaving a topographical imprint. The observed polygon networks in the Campine area are interpreted as first-order networks that developed during a time span of several thousands of years, up to 10 kyr at most, in a former Late Weichselian permafrost climate.