How old is loam?

Answer

The loam we know in our regions is a residual product of the ice ages. Glaciers descending south from the Scandinavian Ice Sheet scraped the rocks and, as they melted, left these products in the Northern European Plain. The large blocks were left behind as erratic boulders, the sand was blown away by high-pressure winds from this barren area over the ground as far as there were no obstacles and thus remained behind in the low plain, to which the north of Belgium belongs, but could not cross the relief threshold that corresponds with the border between Low and Central Belgium. The finer fraction of the crushed glacial debris, the size of dust or flour (less than 0.065 millimeters) can be lifted high into the air by the wind and carried over greater distances and over relief obstacles (testified by the dust storms in the Sahara occasionally depositing a layer of dust). Close to the source, the northern European plain, a meter-thick package of dust-like sediment could be deposited by the wind. This is known as loess, with a composition similar to that of the Scandinavian rocks and also containing a lot of lime. Due to soil processes and infiltrating rainwater, the loess leaches somewhat, especially decalcifying, thus losing some volume and silting up, with a slight increase in the clay content. The result of this weathering process is the loam which may comprise either the superficial layer or the entire loess package.

In addition, loess or loam layers can become washed away during wet periods or undergo soil formation during warmer periods (especially interglacial periods) and become permeated with plant roots, between cold dry periods with new loess deposits. In most places in the loam region, not much of this long history has been preserved, so mainly loess/loam from the last ice age has been preserved. This brings us to old age. The coldest phase of the last ice age has been dated to 22,000 years ago. That is therefore also the average age of the loess/loam layer that covers the landscape everywhere in the loam region (purists can say that the weathering from loess to loam is a continuous process and that this layer is therefore constantly changing, but the deposition itself is thus old). In some places a thick loam layer occurs (up to more than 20 meters where this is an average of 3 metres). There, the youngest deposit also contains deposits from older ice ages, separated by interglacial weathering phases. These can be several hundreds of thousands of years old, depending on the local preservation potential, but these older loess/loam layers are therefore not visible on the surface.

However, a good part of the loam has been washed off the loam plateaus since its deposition, especially in periods of population expansion and deforestation such as the Gallo-Roman period and the Middle Ages, a process that now seems to be repeated in the era of great earthmoving with tractors and bulldozers. The result of this erosion is that the loam layer, especially on slopes, has become very thin or has completely disappeared and that the loam has raised the alluvial plains, again on average by 3 metres. Loam in the valleys is therefore located on a secondary location, was deposited there during floods and these layers are therefore several centuries up to a maximum of 2000 years old.