Ottensee

Useful Information

Location:	74673 Mulfingen. Follow the road from Mulfingen to Ochsental, from the crossing 500 m before Ochsental its a 1 km walk on a single lane road through fields. (49.354867, 9.818770)
Open:	no restrictions. [2022]
Fee:	free. [2022]
Classification:	Karst Lake Tufa Deposits
Light:	n/a
Dimension:	L=200 m, W=50 m.
Guided tours:	self guided
Photography:	allowed
Accessibility:	yes
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As far as we know this information was accurate when it was published (see years in brackets), but may have changed since then. Please check rates and details directly with the companies in question if you need more recent info.

History

09-NOV-1984	declared a Natural Monument.
2013	explanatory signs added.

Description

The Ottensee is a natural monument, a biotope, and a geotope, which was developed about 10 years ago with trails and explanatory signs. A 200 m long and 50 m wide depression in the fields fills with water after heavy rains, which takes only a few hours. The water does not flow into the lake from the surrounding, it comes out of springs in the ground. After a few days the water is normally gone. Most of the year there is only a shallow depression. Due to the regular flooding the ground is soft, wet and very acidic, so only special specialized plants, mostly plants which grow in wetlands, grow inside the lake. Karst lakes are very rare, in Baden-Württemberg there are only two, the Ottensee and the Eichener See in the southern Black Forest.

The depression is a doline, which is rather common in karst areas. However, in this case the situation is a bit more complicated. The plateau is formed by a thick layer of limestone which belong to the upper Triassic, and as it is a fossil bearing limestone it is locally called Muschelkalk. The upper and lower part of this Middle Triassic is limestone, but the middle layer consists of gypsum with a lot of clay. Gypsum is easily soluble by water and at the location of the doline this gypsum, was dissolved by water which followed to fracture zones. This process is called subrosion. Now only the clay remains, and the layer is only half as thick as before, with the result that the limestone above went down forming the doline at the surface. By this movement cracks opened between the limestone blocks and were filled with clay.

The next point is the water in the doline. The underground layers are not completely horizontal, and the drainage follows the bedding downhill. On the higher side the rainwater of the fields enters the cracks in the limestone below, flowing downhill. Despite draining to one side, there is sometimes too much water and so the level of the groundwater rises. On the uphill side of the doline it flows out of intermittent spring and fills the depression. It is drained through a ponor or swallow hole on the other side, but this takes much longer. So it fills fast and empties slowly.

The karst lake is interesting all over the year. However, if you want to see a lake, you have to visit in early spring when snow is melting or after heavy rains. Most time of the year there is only a bowl shaped meadow.