The Tharandt Forest is one of the biggest forests in Saxony located at only 20 km west of Dresden. These two reasons gave us sufficient motivation for a one-day trip during the prolonged Pentecost weekend. Nevertheless, there is at least one more reason for going to Tharandt: the Geological Open-Air Museum, another time journey through the geological Earth’s history. The so-called museum is actually a nature trail with 23 info points in the forest that witness the dramatic modifications that Tharandt suffered over the past 570 millions of years: formation of mountains, volcanic eruptions, river deltas, sea flooding, ice ages etc.

Stones at Tharandter Open-Air Geological Museum

This could be described on short as following (on the left – the approximate geological age in millions of years ago – mya-, on the right – the resulting rock type):

570 mya
400 mya
390 mya
370 mya
300 mya
96 mya
10 mya
– sands and clays start depositing on the sea bottom
– eruption of volcanoes on the sea bottom
– accumulation of calcareous organisms rests
– immense pressure and temperatures from continents
– huge cracks produces by earthquakes, cooling of erupted magma
– again sedimentation of sands in the newly formed river delta
– eruption of basalt-reach magma through the cracks
| sandstone
| diabases
| limestone
| phyllite
| porphyry
| sandstone
| basalt

We walked along the hiking trail and stopped at several information boards. Each one focuses on a certain aspect of the geological transformation and you can observe some real hints just behind the panel.

Board no. 8 – Cretaceous epoch in Saxony

Our first stop was at a former quarry from the Cenoman period (part of the Cretaceous epoch – about 93 mya). The small quarry provided sandstone for the construction works in the surrounding area. Now vegetated, the quarry allowed me to take a small sand sample for the Sand Atlas: the orange colored sand reminded me of the sands from the Elbe Sandstone Mountains, fact confirmed by the information on the board – the sandstone belongs to the same plate as the formations south of Pirna about which I wrote in another post back in 2008.

Board no. 9 – Basalt volcanism in Tertiary epoch

The repetitive earthquakes created long cracks down to the earth mantle. The basaltic lava erupted and reached the surface in form of volcanic outbursts. Sometimes one can find several other minerals (including sandstone fragments) in the basaltic rocks such as pyroxene, olivine or feldspar. The basaltic fracture just behind the board no. 9 is very impressive:

Basaltic fracture in Tharandter Forest Basaltic fracture in Tharandter Forest
Basalt quarry in Tharandt Open-Air Geological Museum (about 10 million years ago)

Board no. 7 – Spherical pitchstones

Like a game of nature, the several pitchstone boulders behind the information board no. 7 are very nice to look at and even more interesting to learn about. Pitchstone is a volcanic rock with dull, glassy aspect and very resistant to erosion. In comparison with obsidian (a volcanic rock with similar characteristics), pitchstone contains about 8% water in its structure.

Pitchstone in Tharandter Forest
Pitchstone in Tharandt Open-Air Geological Museum (about 310 million years ago)

The main color of pitchstone is black and, in case of Tharandt, the stone contains small red patches. Their presence in the main block is not yet clearly explained, some theories suggesting that they are foreign rock particles (most probably hematite – a red iron ore mineral) assimilated by the molten lava on its way to the surface. An important note: the pitchstones are protected by law in Tharandt Forest and cannot be collected. The photos above were taken without any disturbance of the stones’ natural position.

Board no. 2 – Phyllitic shale stones

Phyllite is a foliated metamorphic rock found in the surroundings of stones of the pre-Cambrian epoch. In Tharandt Open-Air Geological Museum we found phyllite behind the board no.2 in Mohorn-Grund (north of Tharandter Forest).

Phyllite in the north of Tharandter Forest
Phyllite in Tharandt Open-Air Geological Museum (about 370 million years ago)

Due to the high content in mica, phyllite has a nice silvery aspect but is very fragile and breaks easily just under the pressure of fingers (sample no. 2301 in Sand Atlas).

Phyllite stone surface
Magnified phyllite stone surface

Board no. 4 – Porphyry fan

Maybe the most spectacular geological formation is represented by the porphyry block at Mohorn-Grund in the northern part of Tharandt Geological Open-Air Museum. The area is just a small part of the volcano whose eruption 300 mya covered the whole Tharandt Forest area. The outburst had the form of glowing clouds involving gasses under enormous pressure.

Porphyry fan at Tharandter Open-Air Geological Museum
Porphyry fan at Tharandt Open-Air Geological Museum (about 295 million years ago)

Porphyry formations at Tharandter Open-Air Geological Museum
Porphyry formations in Tharandt Forest

The pyroclastic material sent into atmosphere by the dramatic explosion returned to the earth surface and melted by taking different shapes. The huge amount of material left behind an empty room inside the volcano body which filled with material from the surface. The rock fracture is impressive and attracts lots of tourists and visitors. Looking at the stone wall I notices an interesting area on the left side and climbed several meters to have a better look. One could see the single blocks of material similar to a basalt formation. From the top, the pluvial waters washed out the material and transformed it into a fine material, right perfect for a sample for the Sand Atlas (sample no. 2302):

2302 DE-SN Porphyry at Mohorn-Grund
2302 DE-SN

Unfortunately, we did not manage to visit the rest of information boards. We promised ourselves to do it next time since they offer even more interesting themes: ore mines (including silver ore), quartz deposits of Tertiary age, sands and gravels from Cretaceous, chalk pits and many more.

Note for the German readers of the Sand Atlas: a detailed geological map of the region and additional information about the geological park can be found here (in German only).