Heidelberger Felsenmeer

The Heidelberger Felsenmeer is an approximately 5 ha large area at the northern slope of mount Königstuhl above the village of Schlierbach. It accommodates many rocks of variegated sandstone, called Bunter sandstone, which have assembled to a so-called stone run. The area is an habitat for rare plants and animals and is thus designated a nature reserve since 1956.

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Rock at Mount Königstuhl

At mount Königstuhl several hundred metre thick layers of Bunter sandstone reach the Earth's surface. Bunter sandstone can be found across a wide area in Central Europe: from France and Luxembourg in the west to Poland and Belarus in the east as well as Switzerland in the south to Scandinavia in the north. In the area of the Heidelberger Felsenmeer there is middle Bunter sandstone. It can reach a thickness of up to 300 metres and formed approximately 249 to 245 million years ago. The ocean, which was present at that time in Central Europe withdrew due to movements of the tectonic plate. Consequently, large rivers alternated with desert-like dry periods. Different types of sediments formed, mainly sandstone and conglomerates, i.e. rock consisting of single grains bound together by finer sediments. During the dry periods, cracks formed in a web-like fashion. They can still be found at present date. There are almost no fossils found in Bunter sandstone, which leads us to believe that is was a harsh time period for life.

1) Question:
Up to how many years passed during the period of formation of the middle Bunter sandstone, until one millimetre of the stone we see today was formed?

Felsenmeer/Stone Run

Due to the above-mentioned web-like cracks, water can easily enter the rock. During long periods of cold weather, the water freezes and fractures the rock. Larger rocks can be fractured into smaller and smaller pieces by repetitive freezing.
When temperature started to rise towards the end of the last ice age, there has been several melting phases. During such phases, the water in the upper soil layer melted first. It could not drain through the still frozen lower soil layers, rendering the surface wet and slippery. As consequence, rocks started to slide downhill and often assembled in so-called stone runs, which are also called Felsenmeer in German. This process is known as solifluction and it takes place even at very flat slopes. Due to solifluction, rocks can slide between 5 and 10 cm per year. The speed differs depending on the size of the rocks, often leading to a sorting of rocks by size.

2) Question:
Due to solifluction some rocks slid all the way down into the Neckar river. At the first stage of the cache, (S1) you find a small green sign showing the altitude of the Felsenmeer. The Neckar river has an altitude of 114 metres above sea level. Which altitude do rocks have to traverse from the Felsenmeer down to the Neckar?
Per metre of altitude, rocks have to slide 4.4 metres across the slope. Which time do the rocks need to slide down the slope all the way into the river?
3) Question:
If you walk down the path, you reach the Felsenmeer (stage S2) on the left hand side. How does the size of the rocks change along the path? What is the reason for the change?
4) Question:
Walk the path further down until you reach a junction. A couple of metres downhill (stage S3) you find an extraordinarily large rock next to the path. You can easily recognise the above-mentioned cracks in it. Describe the cracks. How many are there, how far are they apart and how long are they? Can you spot them all over the rock? Are they connected to each other or separated?

Plant Life

As it is usual for stone runs, the fraction of pebbles and sand is rather small in the Heidelberger Felsenmeer. Thus, it can easily drain, such that drifted soil is washed away by rain. In this rare and harsh habitat only a few specialists can survive. Among them are luminous moss, green shield moss, fir club-moss, and bristly club-moss, as well as common polypody and bilberry bushes. There are also a few trees surviving, such as Celtic maple, Spanish chestnut, durmast oak, and Scotch pine.