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Karst formation - where rocks and water disappear

When it rains on the Swabian Alb, the rainwater dissolves the limestones. Cracks in the mountains are then widened to become passages and shafts. Finally, these processes led to the development of large cave systems within the rock. The water does not stay in the bedrock for long but quickly drains away via aquifers or even underground rivers. Since the end of the Tertiary, a barren karst landscape evolved − the Swabian Alb.

A fabulous underworld

As a result of the dissolution of limestone inside the Swabian Alb, extensive and impressive cave systems were formed over millions of years. These are often adorned with rich stalactite and stalagmite formations. Because when the water containing calcium carbonate drips down from the cave ceiling, the calcium carbonate is precipitated again. Thin stalactites gradually grow down from the cave ceilings. When the water drop lands on the ground, stalagmites start to grow upwards over thousands of years until they both meet in the middle and form a column.

Rocks formed from water
The opposite of what happens in cave formation is also possible and rock can be formed from water. Dripstone forms in the caves and tufa forms on the earth‘s surface. The calcium carbonate in karst springs, which mostly rise in the deeply carved valleys at the edge of the Swabian Alb, is mainly precipitated on mosses and algae. Although the plants become increasingly „petrified“ from below, they can continue growing upwards so that several metres of thick tufa deposit can develop. This highly porous limestone is quite striking on a number of older buildings. It can easily be cut with a wet circular saw and has a strong insulating effect − it is therefore no surprise that it used to be a popular building stone.

Water plunges down - and re-emerges
The extensive underground cavities of the Alb are famous for the large number of show caves and notorious for the dolines, fairly small sink holes which in extreme cases may suddenly cave in when a tractor passes over them. And finally there is another phenomenon to mention concerning the „hollowed out“ Alb − despite a fairly high level of precipitation, the Swabian Alb is the most arid landscape in Germany. Before the Alb water supply system was put in place, many inhabitants of the Alb were dependant on rainwater collected in tanks and ponds, and during a summer drought, water sometimes had to be transported in barrels from far away over the plateau for months. At the same time, the Alb has the most productive springs in Germany, with the Aach spring and the Blautopf. During flooding, the Aachtopf fills up at a rate of up to 24,800 litres per second and the Blautopf at up to 32,000 litres per second. This can be explained by the water draining away into the hollowed-out underground. What is particularly impressive is the sudden disappearance of the entire water of the Danube near Immendingen, the so-called Danube seepage. The water of the Danube resurfaces in the Aachtopf spring about twelve kilometres away.

River valleys without rivers
There are impressive river valleys all over the Alb plateau but the rivers you expect to see are nowhere to be found. Where are the rivers buried by these valleys? These valleys were formed during the Tertiary and the Ice Age. At that time, large rivers surged over the Alb until they disappeared into the extensive systems of caves and crevices that developed as a result of the karstification. Water flows along mysterious channels for kilometres in the „subterranean streams“. It was only when experiments were carried out with coloured water and diving expeditions were made through caves that some of these channels could be detected and retraced. The water normally accumulates in two karst levels. The water in the lower level flows towards the River Neckar while the water from the higher level mainly ends up in the Danube. Both karst water reservoirs are extremely important for the supply of drinking water in the Swabian Alb which is intrinsically a very arid area due to karst formation.