Ribbed moraines formed during the retreat of the Scandinavian ice sheet from eastern Himmerland, NE Jylland, Denmark

The glacial geology of Himmerland in the north-eastern part of Jylland, south of Limfjorden (Fig. 1) has never received any special attention. However, the occurrence of parallel ridges south of Torup was mentioned by Milthers (1948) who interpreted them as marginal moraines. The ridges were recently studied during mapping of eastern Himmerland. Systematic geological mapping of the area north and south of Mariager Fjord started in 2009 and was completed in 2013 (map sheet 1316 III; Pedersen et al. 2013). This was followed by the map sheet to the north (1316 IV). During the recent mapping the extent of the terrain with parallel ridges was determined (Fig. 2); the western boundary is found in Rold Skov (Pedersen & Jakobsen 2005) and the eastern boundary follows an ancient coastline in eastern Himmerland. The most impressive parallel ridges occur in a forested area east of Madum Sø where the top level of the ridge crests reaches an elevation of 95 m a.s.l. However, the majority of the crests are at 60–70 m a.s.l. and most of the ridges are c. 10–15 m high. The sediments in the ridges are dominated by coarse-grained sand and gravel, and accumulations of erratic boulders are found on the surface of the ridges. After completion of the mapping of the area, we decided to make a detailed investigation of the ridges using groundpenetrating radar (GPR) to map the internal structures of the ridges. We also studied sections in a small gravel pit at the northern boundary of Siem Skov (Figs 3, 4). The aim of this paper is to describe the terrain with parallel ridges, which we interpret as ribbed moraines. We discuss the geological setting of the ridges in relation to the data acquired during the systematic geological mapping and the accompanying GPR survey. A glaciodynamic model for the formation of the ridges is proposed related to the recession of the ice that had its source area in south central Sweden.