DUURSWOUDERHEIDE: PINGO RUINS

In Drenthe and Friesland there are many floats. A float is a natural or dug pool without supply and drainage. In or near settlements, these were often fire-fighting water ponds and drinking or washing places for the livestock. Natural floats can be created from pingo ruins or from wind-blown depressions.
Pingo ruins originated as remnants of ice lenses from the Ice Age. A pingo is a spherical mound with a core of pure ice that develops in an area of permafrost (permanently frozen subsoil) when pressurized groundwater enters the permafrost through a crack below and freezes there. The frozen groundwater forms an ice lens that pushes the ground above it upward, creating a mound. This hill will slowly grow larger, because new groundwater is constantly being brought in from below and the ice lens is growing.
With a warmer climate, the permafrost will eventually disappear. The soil will slide off the ice core, and after the ice core melts, a hole will remain in the ground that fills with water. This creates a small lake surrounded by a ring-shaped wall of slipped material. Later, the embankment will usually disappear because the sand washes away or blows away and then only the lake remains. Such a lake is called a pingo ruin.

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