A Late-Glacial Lake Deposit near Waskemeer (Prov. of Friesland)

The numerous bowl-shaped, sometimes fairly deep depressions which may be wholly or partly filled with organic sediments constitute a characteristic feature in the ground moraine landscape of the northern Netherlands. As for the origin of these depressions, some of them were supposed to be kettle-holes (cf. De Waard 1947), while others were regarded as the product of wind erosion during the last part of the Late-glacial (Veenenbos 1952). As it was during the Riss (Penultimate) glaciation that a part of the Netherlands was covered by the ice cap, it was difficult to understand why interglacial (Eemian) peat or gyttja is never found in the depressions considered to be kettleholes, but only organic deposits from the Late-glacial and later. In place of the kettle-hole theory another hypothesis for the origin of this last type of depression was brought forward by Maarleveld and Van den Toorn (1955). According to these authors, some of the depressions in the northern Netherlands are to be interpreted as the remains of pingos. Today the formation of pingos takes place in the tundra area of Alaska, Greenland and Siberia, i.e. in regions with a permanently frozen subsoil. Pingos are round to oval mounds with a core óf ice and water. Although they are mostly much smaller, they can reach a diameter of more than 1000 m and a height of 100 m. The formation of these hills must be ascribed to the forces which come into play during the re-freezing of the water-logged topsoil thawed in summer. When, during the summers, the ice in the core melts off, the pingo collapses, and eventually a hollow is left in the soil. The depression is often surrounded by a ridge, the solifluctcd material of the pingo skin (for literature see Maarleveld and Van den Toorn 1955). Such a ridge has in fact been observed around some hollows in the ground moraine landscape of the northern Netherlands.