Dripstones in the Sontheim Cave
Actually, stalactite growth is only the reversal process of cave formation by lime dissolution, i.e. the precipitation of CaCO3 dissolved in drip water on ceilings, walls or floors of the cave room. The cause of precipitation is the release of CO2 from the drip water into the cave air. The CO2 pressure in the dripping water is much higher than that in the cave air. As soon as a droplet comes into contact with the cave air, e.g. at the cave ceiling, both pressure systems strive to equalise, i.e. CO2 escapes from the drip water into the cave air. As a result, the amount of lime dissolved in the drip water can no longer be kept in solution and precipitates; dripstone growth begins.
The most well-known and classic forms of dripstone are stalactites (growing from top to bottom) and stalagmites (from bottom to top). As a mnemonic, you can remember: „T“ hangs and „M“ stands. Stalagmites in Germany grow up to 4-5 m tall. There are, for example, massive, conical stalagmites or slender and cylindrically round forms, so-called candle stalagmites. When stalactites unite with underlying stalagmites, columns are formed.
CaCO3-laden water threads running diagonally down rock surfaces build up layer upon layer of thin plumes that can throw folds like curtains and are usually so thin that they allow light to pass through.
The colours of the different stalactite forms are determined by admixtures of e.g. iron, copper, manganese or humic acid dissolved in the seepage water. Iron and humic acid colour the whitish-clear sinter red-brown, manganese grey-black and copper blue-green. The black-grey colouring of dripstones in Sontheim Cave may also have been caused by soot, because after all, visitors to the oldest show cave in the Swabian Alb were guided by torches for a long time. Since the installation of electric light in 1957, however, white stalactite layers have been growing again in various places.
Fig. 29: Larger stalagmite with clearly visible new white growth layers
The fact that there are fewer stalactites to be seen in Sontheim Cave is also related to its long show cave history. Unfortunately, many stalactites were cut off in historical times and taken away as souvenirs.
Worth mentioning are, for example, a group with larger, massive stalagmites directly after the weather boundary, the stalactite curtain called „Organ“ to the left of the guide path. In the same hall, but to the right of the guide path, is the „Schäferwand“ (shepherd’s wall), a large-scale wall sintering with numerous stalactite formations. Probably the most beautiful large-scale wall sintering is on the left cave wall towards the end of the final hall. Some visitors might be reminded of a petrified waterfall or viscously flowing cake dough.
The most striking stalactite of Sontheim Cave is also located in the final hall, the so-called bell (Fig. 34). It is located on the right side at the end of the guide path. It is a stalagmite that had grown on a former sediment filling. Since the base is fused to the cave wall at the back, the stalagmite remained in its original position even after the clay deposits under its base were cleared out, so that today it resembles a free-hanging bell, which led to its name.
Fig. 34: The bell
Several dripstones have also been dated in Sontheim Cave using different radiometric methods, the first time as early as the 1970s. In summary, it can be said that in Sontheim Cave at least the last four great warm periods, i.e. the period for the last 400,000 years, are attested by dripstone generations. With an age of about 300,000 years, for example, a stalagmite of the already mentioned group from the first hall directly after the Bat Gate dates to the third last warm period. Of course, various dating samples were also taken from the bell. The results indicate an origin in the last warm period about 126000 years ago. The fact that several climatic phases can also be contained within a single stalagmite is shown by a 35 cm high stalagmite, 20 cm thick at the base, from a side shaft of the entrance hall (Fig. 33).
Fig. 33: Section through a stalagmite
Finally, the Hintere Kohlhaldehöhle should be briefly mentioned. With its wealth of stalactites, consisting mainly of sinter columns and candle stalagmites (Figs. 30 and 35), this is certainly one of the most beautiful sintered cave rooms in the Swabian Alb. The beginning of growth of the stalactites there could be dated to 46,000 years before today, the end of growth to 2,800 years before today. A more precise analysis of the growth structure shows that the dripstones grew over a period of 28 000 years.
Fig. 30: Candle stalagmites and sinter columns in the Hintere Kohlhalde cave.
According to the unfavourable climatic conditions for stalactite growth at that time, the growth rate was very slow or averaged about 4 mm per 1 000 years. It was not until the climate improved after the peak of the last ice age about 18,000 years ago that the growth rate increased again significantly. Between 18 000 years ago and the end of growth, the stalactite grew 114 cm, which corresponds to an average growth rate of about 75 mm per 1 000 years.
Fig. 32: Sinter plumes and tubes in the Hintere Kohlhalde Cave
Dr. Wilfried Rosendahl (abridged)
Further information in the brochure on Sontheim Cave, available at the cave ticket office.