It is so hard to imagine that our country was buried in the distant past by miles of pack ice and advancing glaciers. The fact that the Netherlands was once a polar desert is proven by a vertical sand wall in Drenthe, recently christened the Steilrand Donderen. The sand wall reads like a book about the Ice Ages.
Sequence of three ice ages
It is only 10,000 years ago that the last ice age (glacial), the Weichselian, came to an end. In the Round Thunder the cover sand profile from this cold period is perfectly visible. But not only the last ice age comes to the surface here. The glacials before it, the Saalien and the Elsterien, are also less than three meters below the Drenthe farmlands. Due to the geological importance of this 'sand pit' for the Northern Netherlands, the province of Drenthe decided that the steep edge should become a Geographical Monument.
Swallows
“The fact that this steep edge is still here is thanks to a colony of sand martins that nested here about twenty years ago,” says Gerrie Koopman, soil scientist and closely involved in this geological monument. The swallows have left, but the steep edge has remained. In fact, the vertical sand wall is a remnant of a sand quarry from the last century (1962-1990).
The cover sand heads in the landscape, located here on the eastern flank of the Rolderrug (named after the nearby town of Rolde), had to be leveled to make the land suitable for agriculture. The sand was used in road and housing construction and as grit sand against slipperiness.
Wavy blanket
The Hondsrug and Rolderrug (both located in a NW-SE direction) were formed during the second-to-last ice age (the Saalian), when a large meltwater lake emptied within a short time, causing part of the land ice to move in a southeasterly direction. The sand ridges on the Drents Plateau are part of the Hondsrug system. Cover sand was deposited by the wind during the last ice age (Weichselien) in a tundra landscape and as an undulating blanket over older deposits.
Recognize cover sand
Cover sand can be recognized by its reddish-brown colour, but also by its stratification, in which loamy layers alternate with less loamy and sandy layers. Koopman:“If clear phenomena are also observable, such as boulder clay as a base, more loamy at the bottom (old cover sand) than at the top (young cover sand), old soils and podzolization in the topsoil, then you know for sure that the about cover sand.”
Superstorms and pillars of salt
There are also shifting sand complexes on the Drents Plateau, but these were only created in the Middle Ages as a result of human intervention. Drift sand is mainly blown-up cover sand, which can also be seen in the Steilrand Donderen. The formation of drift sands is also thought to be due to natural causes, such as super storms around the year 1000 or the presence of salt pillars in the subsoil.
The salt pillars in the deep subsurface pushed the soil upwards, which led to desiccation and increased the chance of dispersion. Koopman:“For the people in the Middle Ages, those dust storms were a terrifying natural phenomenon!”
“Every time you discover new things”
“Every layer in the steep edge is intriguing,” says Koopman enthusiastically. "And every time you discover new things! We even find the meteorite dust iridium in some layers, which may indicate a meteorite impact. In a layer of sediment from an above-average warm period in the Weichselien, the Allerød layer, we find small pieces of charcoal There must have been major forest fires then, between 11,000 and 12,000 years ago.
The steep edge consists of snow-white sand to the bottom of the sand pit, which experts call 'pussy sand' because it feels so soft. This Peelo sand, originating from the Peelo Formation, continues a few meters below the visible soil profile. Underneath lie old river deposits.
Luminescence
The profile of the Steilrand Donderen has already been mapped in detail by soil scientists and geologists. However, not everything is known yet. For example, it is not certain whether the white Peelo sand (the bottom layer of the escarpment) dates from the first phase of the Saalian, between 370,000 and 130,000 years ago, or from the pre-glacial period, the Elsterian, between 475,000 and 410,000 years ago. .
“Perhaps this could be investigated with luminescence dating, a technique in which the current radiation of an object, such as a grain of sand, can say something about its age,” Koopman suggests. What is certain is that the snow-white Peelo sand is a 'melt water deposit'. Koopman:"We know this almost certainly because of the criss-cross layering, which is also beautifully visible in the Steilrand Donderen".
Sickle dunes wiped off the map
“Although a lot has already disappeared in the Netherlands, and perhaps because of that, we are now more economical with the landscapes that we still have,” says Koopman. "The Nederlandse Aardolie Maatschappij (NAM) has even rerouted a gas pipeline to save the Steilrand Donderen.
Nevertheless, in the past century we have already lost so many 'fossil' landscape forms in our country, such as several crescent dunes in the Hunzedal. These have been completely wiped off the map." Koopman already has new ideas for geological monuments. Such as the Havelterberg, a moraine from the Saalian that also contains dolmens.