A low-lying polygon locality on the Laholm plain, Swedish west coast

Abstract

Abstract A type of crop marks observed in aerial photographs from the coastal plain of southwestern Sweden was discussed in some earlier papers and was found to display relict patterns of ice-wedge polygons. The polygons occur in different levels of the plain which was raised above the sea in Late-glacial time. In order to reach a more exact dating of the frost Assuring process which was the origin of the pattern, it is important to find the lowest situated polygons which are the youngest ones and, accordingly, those faintest developed. As the polygon net has no topographic representation in the surface, and the reproduction of the pattern is depending on both vegetation and meteorologic parameters, the search for low-lying polygon sites had to be extended over some years. The so far lowest found locality (9.04 m a.s.l.) is related to earlier observed low polygon sites and to dated levels of a shoreline diagram. A test pit was made for studying the material and its structural details corresponding to the faint lines. The ice-wedge cast (sand-filling in a clay matrix) that appeared in the section showed indications of erosion during, or after the fossilization. The original polygon surface was obviously influenced by the Postglacial transgression and the pattern is buried by a cover of sand — a fact which explains the very faint reproduction in the present-day ground surface. The morphological evolution of the polygon pattern from the formation in the Younger Dryas period to the erosion and burying during the Post-glacial transgression is outlined, and the possibilities of finding still lower-lying polygon sites are discussed.