The geological development of the Tharandter Wald reflects the entire geological history of Saxony. Classic stratification complete with basement, transition rocks and overlying rocks can be studied in an exemplary and condensed fashion.
The basement is composed of the Ore Mountain Gneisses of Precambrian Times and the Nossen-Wilsdruffer Slate Mountains. During the time of the New Red Conglomerate (Permian) in the region around the river Elbe there were shifts while at the same time extensions occurred. Thus, the Döhlener Becken (region around Freital) came into existence with its rubble from variscian mountains.
The eye-catching, almost fully circular shape of the Tharandter Wald owes its form to a magmatic eruption during the Carboniferous which was followed by a collapse of the caldera. Extrusive igneous rocks formed (rhyolites).
After a break of 220 million years geological history had another peculiarity in store which—thanks to a favorable erosion in the Tharandter Wald area -- was preserved. On the rhyolite slab residues of cretaceous stones have been preserved. They are part of sedimentation in the Saxon Chalk Basin. Before the Ocean washed over the sides of the Erzgebirge about 90 million years ago, a river coming from Bohemia carried the water towards what is today the Elbe valley.
Large movements of crust during the Tertiary resulted in intensive volcanic activity. The basalt formations in Ascherhübel, Landberg and Buchhübel in the Tharandter Wald originate from these events.
Geoparks are home to interesting geological sites worth of protection, so called "geotopes". They offer exciting insights into the geological development of the respective landscape and therefore deserve to be protected.
To preserve and protect these outstanding sites and to provide information on them is one of the main tasks of a geopark.
Precambrian
About 570 million years ago during the Precambrian, the oldest time period of the history of the earth, the area of the GEOPARK was still in the Southern hemisphere of the Earth. Smaller land masses formed a volcanic arc in front of the Northern rim of the ancient continent Godwana.
The weathered stones of the volcanic arc had been mainly transported by rivers into the Oceans where they remained as sands and clays. Due to the surcharge of following sediments they solidified into hard stones, graywacks as they are called.
Later the graywacks turned into paragneiss during the orogeny of variscian mountains (Devonian to Carboniferous) under high pressure and temperatures. The prefix “para-” reflects that the parent rock material was sedimentary.
Paragneiss can be seen at the Burgfelsen in Tharandt on the GEOPARK area. The rocks here are called “eye gneiss” as well, due to their particular structure.
Cambrian
During Cambrian the area of the GEOPARK was still situated on an active continental rim close to the South Pole. Magma seeped into the sediments that had accumulated there due to the melting of the continental crust. The magma solidified as granodiorite in large, dome-shaped structures such as the Freiberg Dome. In the same move as the graywacks the granodiorites later turned into gneiss. To illustrate their magmatic origin they are being called “orthogneisses”.
A noteworthy rock sample of orthogneiss is the “Dorotheenfelsen” (Dorothy’s Rock) at the Klingenberg dam. Oral history has it that the beautiful miller’s daughter met with her lover there. At the spot where the mill once was, there is the dam, now, but the rock next to the hiking trail can be explored even today.
Ordovician and Silurian
During the period from Ordovician to the Silurian the GEOPARK was situated at a passive continental rim at the shore of the Rheische Ocean. The graywacks and the igneous rocks that had intruded into them now served as the basis for further sediments, chiefly sand and mud. In the areas where the crust expanded there was strong under water volcanism.
By way of metamorphosis (change) later phyllites, sericites, siliceous and alum shale, which can be seen at the upper Triebisch River Valley, developed from the shallow sea sediments.
Devonian
During Devonian the current area of the GEOPARK drifted slowly towards the North. Clays, lime stone and siliceous stones formed the sediments on the continental slope there.
About 400 million years ago volcanos erupted on the ocean floor. From below and from the side molten magma from the fractures intruded into the sea floor. From the solidifying lava diabase stones formed. The railway cut near Mohorn/ Herzogswalde is a geological outcrop that strikingly illustrates the effusion of volcanic stones in the sediments of the ocean floor. The two different types of rock can be discerned even today at the outcrop.
Closely entwined with submarine volcanism are the lime stones occurring at the Nossen- Wilsdruffer Schiefergebirge. Along the volcanic slopes calcareous algae, sponges, corals and shells settled whose skeletons consist of calcite. Solidification of the residues of these organisms led to the emergence of lime stone which was an important regional resource for centuries be it as mortar, fertilizer, or lime plaster – which, incidentally, acts as a disinfectant.
Carboniferous
During Carboniferous the ancient continents Laurussia and Gondwana collided and thus triggered the variscian orogeny. Pressure and temperatures rose extremely so that rocks in the ground core melted. Strong movement in the earth’s crust led to fractures in the rising mountains via which molten lava could rise up. This is where the volcanic complex around the Tharandter Wald originates from.
When the pyroclastic flow and clouds cooled off rhyolites formed. An impressive geotope on this topic is the porphyry fan at Mohorn-Grund. Its fan-like shape derives from the fact that it cooled off very slowly and shrank in volume.
Another famous site at the GEOPARK is the pitchstone of Spechtshausen, a protected geotope. These rocks, however, must have cooled off extremely fast since the ground mass is glassy. The contained balls are fragments from adjoining rocks that have been rounded off by way of the slow flow of lava.
Permian
To the North East you will find the Döhlener Becken which originates from a trench like depression of the area about 300 million years ago.
After the depression heavy sedimentary layers accumulated in the basin for 15 million years. Worthy of mention is especially the Döhlen Formation with its seven hard coal beds.
Under the then tropical conditions the hard coal derived from dead plant matter. It functioned as energy source and basis for the local steel industry from 1542 to 1967. The partially existent elements of uranium ore in the hard coal beds were extracted by the SDAG Wismut from 1947 to 1989.
The geotope “Backofenfelsen“ in Freital-Hainsberg displays a 50 meters high rock and therefore is considered the largest above-ground geological outcrop dating from the Permian in the Döhlener Becken.
Cretaceous
The variscian Mountains and also the Tharandt volcanic complex were flattened by millions of years of abrasion and weathering. Only the basic outlines of the former volcano remain today.
During the Cretaceous, approximately 100 million years ago the area of the GEOPARK was a vast river delta. The river carried gravel and sand which were deposited in and around the Tharandter Wald.
Over time the region lowered and was flooded permanently by the ocean. Rivers from the nearby continent brought abrasion rubble into the area. In the resulting sand stone formations one can detect fossils and petrifications of various marine creatures to this day.
An important sand stone quarry is the geotope at the Flügel Jägerhorn near Grillenburg. It is well documented that the rocks served as cutting stone for the “Golden Gate” at the dome of Naumburg and its donors’ figurines of Ekkehard II and his wife Uta.
At specific spots in the GEOPARK there are still residues of the sand stone that once covered the whole area. You can also find them at the geotopes “Einsiedlerstein“ and “Götzenbüschchen“.
Tertiary
During Tertiary the region rose again as a consequence of the collision of the African Plate with the South rim of Europe. The north of the Ore Mountain Tectonic Splinter was only lifted slightly while at the southern end fault-throws of up to 1000 m were reached. The rise of the Erzgebirge again led to deep fractures in the crust of the earth because of which hot basaltic magma surfaced and formed the basalts (olivine nephelites) at the Ascherhübel, Landberg and Buchhübel. The column-like structure at the Acherhübel is due to cooling off procedures just like at the porphyry fan.
Quatenary
The Quaternary is divided into the Pleistocene (Ice Age) and the Holocene, which we live in today. Only the ice shelf of the Elsterien Period reached the territory of the GEOPARK. The most important sediment dating from the ice age in the GEOPARK area is bluff, a fine dust, which sank to the ground through fall winds during the ice age. Later it was decalcified and today mostly prevails as bluff clay. Areas with bluff are considered especially fertile.