During the last ice age, the Netherlands was not covered by inland ice. However, it is so cold that hardly any plants grow on the permanently frozen ground (permafrost). This makes it easy for the wind to blow a lot of sand across the barren landscape. At the end of the Weichselian glacial period in particular, a hilly, metre-thick layer of covering sand is deposited on the boulder clay from the Saale glacial period. These high piles of sand are known as cover sand ridges.
The cover sand ridges are archaeological treasure troves, as they often contain the remains of campsites that were temporarily used by hunters and gatherers from the Early and Mesolithic periods. For example, 60,000-year-old hand axes were found near Lonneker!