Geology of the Middle Saale Valley
The geology of the Middle Saale Valley is determined by sedimentary rocks of the Germanic Triassic . These rocks and the resulting Quaternary sediments and soils of the Middle Saale Valley near Jena and the surrounding plateaus and mountains in the Saale-Holzland district were decisive for the development of the culture and economy of this region.
Early educations
In the late Carboniferous over 300 million years ago when, in a coastal plain that stretched from what is now Belgium on the northern Rhineland, Lower Saxony and Brandenburg to Silesia (Poland) extensive coal spread forests, were mighty in the central Germany Earth's interior mountain-building forces at work. As a result of a continental collision, the earth's crust was laid and raised in large lines of folds running from southwest to northeast (Variscan Mountains). At the same time as the uplift, the erosion began on the surface , which continued in the subsequent period of the Rotliegend , with the erosion debris accumulating in larger depressions within the mountains and in the foothills. At the end of the Rotliegend period, the Variscan Mountains were largely leveled. Subsequently, as a result of subsidence of the earth's crust, the Zechstein Sea penetrated from the north into central Germany (Zechstein transgression ). Therein were limestone , marl and Eindampfungsgesteine as gypsum and rock salt deposited. Calcareous algae and bog animals , which found favorable living conditions on submarine thresholds, built reefs there . While several such fossil bog animal reefs come to light in the Orlatal , in 1906 such a reef was drilled at a depth of 600 m during an exploratory drilling for rock salt near Porstendorf. On the basis of the plank wall near Saalfeld, in which the folded dark slate of the Devonian and Lower Carboniferous are overlaid by light, horizontally lying limestone layers of the Zechstein , the geological processes of the late Paleozoic can be understood.
Towards the end of the Zechstein period, the sea withdrew ( regression ) and the region became mainland again. The red sandstone that is subsequently deposited includes the oldest layers of rock that come to light near Jena. Together with the overlying rock units Muschelkalk and Keuper , the red sandstone forms the more than 200 million year old Germanic Triassic . These rocks build up large parts of the southern half of Germany, including the Thuringian region between Werra and Weißer Elster as well as between Harz and Thuringian Forest (see Thuringian basin with edge plates ).
Red sandstone
The lower red sandstone , about 300 m thick, forms a wide strip that stretches from Saalfeld to Gera and was only found at Jena at a depth of about 200 m below the Saale floodplain. The middle red sandstone (about 200 m thick) is even within the city of Jena. It is a fine to coarse-grained, mostly red-brown and gray sandstone with very few fossils. In its lowest part it contains a lot of kaolin . These layers of kaolin were the reason for the establishment of the porcelain factory in Kahla . Above that lie the Gervillei layers, named after a small clam that occurs in several banks within these layers. The Rothenstein layers above, exposed on the Trumpeter Rock near Rothenstein, are lighter, loose sandstones with thin layers of red-brown shale. The strata carry almost no fossils .
Loose, light sands were quarried earlier in poor clay locations and used as house sand . Above these Rothenstein strata lies the roughly 50 m thick, thick banked, mostly dark red-brown sandstone. In many quarries, these coarse-grained sandstones were previously broken as building blocks. The foundation walls between Kahla and Stadtroda are made of sandstone (regionally called building sandstone). Within Jena, he stands in line at the rock cellar of the former brewery. Vertical rock walls often show a strange network weathering. In the upper part, U-shaped residential buildings of worms, which at that time populated the bottom of water pools , are common ( Diplocraterion luniforme ).
The so-called chirotheria sandstone, named after the trace fossil chirotherium , is part of the Solling sequence, which forms the uppermost part of the middle red sandstone, but is considered by some authors to be part of the upper red sandstone due to its already relatively strongly marine influenced character . These are the footprints and tracks of reptiles, from which the first dinosaurs and pterosaurs and the ancestors of today's crocodiles emerged in the further course of the Triassic. The traces as casts ("reliefs") on the underside of sandstone layers were found on the Talstein, near Waldeck and between Großbockedra and Schiebelau. The approximately 20 m thick, gray to white, carbonate-rich sandstone strikes within the urban area on Burgweg, in front of the devil's holes and at the rock cellar. It also forms the underground of the market and the town church. In many cases, layers come in red meat Cornelian and pseudomorphs after rock salt before. On the layer surfaces there are corrugated ripples and the filling of dry cracks ("net strips"). Brown to black spotted and streaky sandstones (regionally called tiger sandstone) also appear in the sequence of layers. Solid, spherical structures rich in iron and manganese often give the sandstone a crumble cake-like surface when weathered (on Burgau's Schlossberg). From other places in Thuringia, fossilized horsetails , bear moss plants , ferns and conifers have become known. The almost pure white sandstones between Jenertal and the Teufelslöchern were quarried as parlor sand in the deep niches in the rock face.
The entire middle red sandstone results in a loose soil, which is mostly covered with coniferous forests and is well suited for agriculture where there are clayey intermediate layers. The more solid parts form rocky steep slopes in places, z. B. Raven bowl and Felsenberg near Maua and Trumpeter Rock near Rothenstein.
The middle red sandstone is considered a continental deposit. The weathering debris came from the south and south-east. The forces of wind and water spread the rubble in the large German red sandstone basin. In inland lakes and wide-span ponds with varying, little or no water cover, both wind-cut pebbles and conglomerates were deposited. Fine stratification and cross stratification as well as vortex formations are common. The desert-like character of the sedimentation areas is attested by the multiple appearing red colors.
Red weathering blankets are only found in tropical and subtropical areas today. On the whole, the conditions in the Red Sandstone Age were similar to those in steppe and semi-desert areas today.
After the chirotheria sandstone had been deposited, a shallow, partially dry sea approached from the northeast. The layers of the Upper Buntsandstein (red: about 100 m) are introduced near the base through a 10–20 m thick gypsum store . The gypsum was apparently deposited in smaller, almost completely constricted, shallow bays, in which the evaporating water was only more or less episodically supplemented by influx from the sea. The gypsum appears whitish to gray and reddish, coarsely crystalline as gypsum spar and fiber gypsum (devil's holes) and dense with numerous dark individual crystals in it as so-called Jena alabaster (Katzenwinkel near Kunitz). The latter was occasionally used as decorative stone (stair cladding and wall covering in the university building). For example, plaster of paris used to be broken at the painting mill. There he was baked into plaster of paris. The fine, grated plaster of paris served as sand to erase the ink. The strong folding of the plaster layers is noticeable at the Teufelsloch and at the Erlkönig. The conversion of anhydrite free of water of crystallization to gypsum containing water of crystallization causes an increase in volume. The plaster layers may have creased as a result. As a result of leaching by the water, cavities and sinkholes form in some places.
Red
The clayey, often gypsum-containing layers of the Upper Buntsandstein are called red . The fossil-free gypsum stones of the Lower Röt lead over to greenish-gray and blackish-gray marls and shales , which are often still criss-crossed with plaster of paris tape. There are still several decimeter-thick banks of dolomite sandstone and heavily layered sandstones. The dolomites lead to a large number of fossils of mussels, snails, ammonites and bone fragments, which are not to be listed in detail. However, the so-called dinosaur sandstone with Nothosaurus bones is worth mentioning . Further up, a dolomite bench was named after the “ Rhizocorallium bench” on the underside of the crab housing . These are sloping, openly intersecting beads with a net-like surface. The slate and marl used to be quarried in the Wogau brickworks. In the landscape, the Lower Röt is characterized by steeper valley slopes. The salt springs of Bad Kösen have their origin in the salt deposits of the Lower Röt.
The 50 m thick Mittlere Röt above it consists of a series of red marls, shale clays, a few layers of gypsum, and quartzite and dolomite banks. Gray marl reappears only in the upper part. In the terrain, that is, on the middle slopes of the Saale valley, it forms gentle slopes and provides heavy soil. The springs emerging in the lower and middle red contain a lot of gypsum, so that they can hardly be considered as drinking water.
The upper end is formed by the approximately 15 m thick Obere Röt, formerly known as myophoria or celestial layers. The sequence of layers consists mainly of light-colored marls with embedded limestone plates and layers of celestine . At the base are two benches with glauconite parts . Glauconite is a greenish iron silicate that is formed when marine sediments form. The limestone plates carry a lot of mussels and snails, the back of the shell with the curved side upwards, which is considered the most stable position of the mussel shells when they are deposited. On the south side of Jenzig there used to be tunnels in which celestine, a sky-blue, fibrous, heavy mineral, was once mined and used in sugar production. A celestine deposit at the foot of the Dornburg castle rock was already known to Johann Wolfgang von Goethe (letter to von Leonhardt, 1816). Two bright, flat limestone form the end of the upper red.
The water seeping through the shell limestone accumulates on the impermeable, clayey marl layers of the Upper Röt. Therefore, smaller or larger springs emerge at this point in many places (Ammerbach, Lutherkanzel, Fürstenbrunnen ). Since these marls weather more easily than the shell limestone above, a cove is often created at the Röt-shell limestone boundary, for example below the castle rocks in Dornburg . In connection with the inclination of the Triassic strata to the northwest, which can be observed near Jena, the water stagnation is the cause of many landslides in Jena's area, for example on the Dohlenstein near Kahla, on the west side of the Johannisberg , on the Gleißberg and on the Kernberg . The shell limestone parts slide easily on the soggy myophoric layers. Such landslides can therefore mainly be observed on western and chalk slopes. The layers of the upper red are mostly overrun by limestone rubble.
Due to the larger proportion of limestone layers in the Upper Röt, it becomes clear that stronger marine influences occurred, even if clay marl layers with dry crack network structures and lumpy gypsum indicate multiple drainage.
Shell limestone
With the beginning of the shell limestone , which adjoins the Röt, the sea deepened and had the character of a shallow sea. There are no indications that sea bays are drying out. The gray shell limestone near Jena, about 170 m thick, is divided into a lower, a middle and an upper section.
The lower Muschelkalk or wave limestone begins with a 0.5–1 m thick bank of yellow to brown, foamy, oolithic or conglomerate limestone, the so-called yellow border bank . About 40 m of a fine-fibred to even-surfaced wave limestone adjoins this, which contains a particularly large number of snake-like bulges of worm-like structures ( Rhizocorallium commune). In the upper part follows a zone of fine conglomerate and solid limestone. It forms the lower of the three constant rock belts that emerge in relief on all the bare limestone slopes of the central Saale valley. The rock belts on the steep slopes of the Kernberge and Jenzig are clearly visible, in the lower part above the denser vegetation zone. Separated by slightly weathered limestone, the other two banks of the three rock belts are made of oolithic limestone. When the oolithic spheres weather out of the calcareous binder, a pore-rich foam lime is created . Actually, the oolithic formation of these layers only begins northwards (Dornburg, Freyburg, Bad Kösen). In Jena, the layers known as oolite banks consist primarily of firmer, chunky limestone. The benches are very rich in fossils. There are stalks of a small sea lily , shells of small snails, mussels and arm pods. In places, solid, very fossil-rich plates can be found ( Jenzig , Kernberge ).
Above that, in turn, wave lime follows. In the upper third of the slopes of the Jenzig and the Kernberge you can see the next rock belt from afar, which includes the bright Terebratula banks. They consist of firmer, crystalline limestone, which contains masses of the shells of a pod called Coenothyris vulgaris . In places they are even rock-forming. Coenothyris lived firmly by attaching herself to the ground with a muscle stalk that was stuck out of a small hole in the back shell. In addition, many species of mussels and snails lived at this time. In the terebratula banks, larger brown plaster teeth of the approximately seal-sized dinosaur Placodus gigas and smaller shark teeth can also be found.
The terebratula benches have been a valued building block for centuries. Well-known quarries are located on the north-eastern slope of the forest and northwest of Zwätze. The building material for the castles in Jena's surroundings is largely Terebratula limestone ( Kunitzburg , Fuchsturm , Lobdeburg ). From newer buildings z. B. the university and the public bath built from it.
After a few more meters of corrugated limestone, the Jena mountain slope plateau begins. There the upper rock belt, the limestone banks, is clearly visible (core mountains). There are several foamy, formerly oolithic, rust-stained limestone banks of varying strength, which in turn are very rich in fossils. The foam lime was previously broken in many places and used partly for lime burning, partly as stone . In addition to terebratula lime, the Fuchsturm also consists of foam lime. Almost all of the castles and palaces in the middle Saale valley are made up of firmer banks of the wave limestone.
The rocks of the lower shell limestone form steep mountain slopes, which differ very markedly from the flatter slopes of the lower lying clay rocks of the Röt. The rocks of the 30-40 m thick Middle Muschelkalks above it consist of lime, dolomite and marl, which are predominantly less stable and more easily weathered. On the level above the Saale valley, which is cut deeper, this series of rocks mainly forms gently sloping slopes. At its base lie dolomitic plate limestone, which was quarried as stone on the Jägerberg and was mainly used as slabs. The plate limestone contains remains of the marine reptile Nothosaurus mirabilis . In the Middle Muschelkalk the sea became much shallower. Gypsum layers and salt deposits (Bad Sulza) even indicate complete evaporation phenomena.
In the Closewitz – Cospeda – Rödigen – Isserstedt area one encounters the rocks of the Upper Muschelkalk. The 3–6 m thick, firmer trochitic limestone forms a steeper terrain there. It indicates that the sea was deepening again. The trochitic limestone got its name from the stalk members (trochites) of the sea lily Encrinus ( liliiformis ), popularly also called Bonifacius pfennige. In addition, limestone shells ( Plagiostoma striata ) and remains of dinosaurs were found in it, and at Isserstedt even the remains of a flying fish. In addition, there are also multiple corrugations. Trochitic limestone can occasionally be found as a building block in the villages in the area mentioned.
In the upper part, the approximately 35 m thick layers of ceratite form shallower slopes and plains because they consist mainly of softer slate with only occasional limestone banks. A well-known example of this is the plateau around the Windknollen northwest of Jena. Rainwater can often accumulate on the arable soils of this area, so that these areas have to be well drained. The key fossil Ceratites, the namesake of the strata, is one of the extinct ammonites with a rolled-up chamber housing. The size of the different ceratites increases in this sequence of layers up to a diameter of about 30 cm.
Keuper
Over the shell of the rocks are Keupers developed that form in the direction Apolda surfaces with lower inclinations. Initially only the marls, Latvians , sandstones and ocher limes of the Lower Keuper appear here. At this time, Apolda even formed an impure coal, from which the name Kohlenkeuper comes . The sandstones and Latvians in particular have remnants of ferns , horsetail and conifers. The Muschelkalkmeer withdrew and continental influences increasingly prevailed.
Like the Middle and Upper Keuper in Central Thuringia, younger Jura and Chalk rocks were probably also common in Thuringia. The bowl-shaped Thuringian Basin gradually formed through subsequent uplifts and large-scale erosion, together with more pronounced fragmentation of the area by earth-internal, tectonic forces in the context of younger, so-called Saxonic mountain movements . The stratification of the Triassic in the Jena area today falls generally flat to the northwest into the interior of the Thuringian Basin. Fold and fault lines on which layer complexes have shifted can also be observed in the Jena area. Faults occur at the railway cuts in the Mühltal. At the Devil's Holes, both red plaster and middle red sandstone are exposed at the same height. The local mountain with the fox tower is on a saddle.
Regarding the north-westerly inclination of the strata: With an average inclination angle of 2 °, the shell limestone strata are about 70–80 m deeper west of the Saale than east of it. It is interesting that this also influenced the formation of the side valleys of the Saale. While the valleys east of the Saale have cut into the softer clay stones of the Röt and are sunk like troughs, the resistant shell limestone on the other side has mainly allowed ravine-shaped valleys.
Tertiary
A long mainland time with large erosion processes is only known from the Tertiary , about 60 million years ago, but with only small remnants of deposits. While in the northeast, huge forests eventually led to the formation of lignite, on the heights on both sides of the Saale, gravel, sand, clays and quartzite were only deposited in places. The quartzite from the Camburg area made an excellent paving stone in Saalstrasse, Jenergasse, Frauengasse and other places in Jena. The term "potato plaster" was common.
The tertiary deposits were deposited at a time when the present-day valleys had not yet been cut. The Saale-Ilm and Saale-Elster plates were still connected. Only at the end of the lignite age did the Saale valley deepen in a certain rhythm. It continued into the Ice Age. The phases of deepening are reflected in the remains of gravel terraces on the slopes of the Saale . The terraces are on top of each other. Numerous different finds of plant and animal remains as well as the different types of rocks that were found in the gravel terraces on both sides of the Saale valley prove a multiple climate change. It is known that during the Ice Age, the inland ice from Northern Europe advanced in several advances southwards into central Germany. During the cold period, when little rainfall can be expected, the gravel is graveled by the river, mainly from material from its catchment area in the upper reaches. In the warmer interglacial periods, both meltwater and more precipitation occur. This is the time when a river can cut its way deeper.
quaternary
Pleistocene (ice ages)
The first ice advances have not yet reached the Middle Saale area. During this time, only rocks from the upper reaches of the Saale were deposited. Several pre-glacial terraces were created around 600,000 years ago, e.g. B. on the Plattenberg near Porstendorf. When the ice finally penetrated into the Middle Saale Valley during the glacial period , it brought Scandinavian, red granite , gneiss and flint with it, which it deposited in its moraines . The latter can be found as unlayered till with blocks up to three quarters of a meter in diameter.
In front of the ice, the damming of the water flowing from south to north resulted in reservoirs in which finely layered, often very rich band clay was deposited. In some places, interim, gravel-filled channels interrupted the banded clay paragraph. In the 1970s, the band tones on Closewitzer Strasse in the northern district of Jena were open several times. During the development of the northern quarter, several such glacial gravel terraces were encountered , staggered in height . They are made up of sandy, alternately silted fine to coarse gravel and contain both Nordic debris and indigenous rocks.
During the cold times, the names of which are not listed here, the course of the hall changed several times. After the pre-glacial Saale had flowed around the Heiligen-Berg to the west, it cut into today's valley. To the west of this mountain are old gravel pits. The wind blew the finest particles out of the moraines that were released when the ice retreated and deposited them in places other than yellow-brown loess . Loess loam areas are widespread on the western slopes of the Saale valley. When dry, it is almost dusty and has a loose structure. When soaked, it swells up in a certain way and works almost like soft soap.
The gravel of the lower terrace group was opened up while excavating the construction pits for the thermal power station north of Winzerla-Burgau. They are about 2–5 m above the level of today's Saale-Aue. The people of the Stone Age must have seen the sedimentation of these Saale gravel more than 20,000 years ago. The floodplain of the Saale and the side valleys form the youngest, sandy gravel, which in historical times was finally covered by a brownish alluvial clay 1–2.5 m thick.
Holocene
In the geological present, the Holocene or Alluvian, of course, all geological processes continue, starting with flood deposits , debris formations at the mouths of the side valleys in the Saale valley, moors, tufa formations, also sinkholes and landslides . The course of the climate and changes in the vegetation can best be followed by analyzing the pollen grains of the plants in the moors. In this way, the connection to the Ice Ages can also be established.
The fact that the climate in the Holocene has not been uniform up to now can also be seen from the young limestone tufa deposits that were formed by streams in the side valleys and which in some cases still continue today. Pennickenbach, Leutrabach near Maua, Ammerbach, the Jenaer Leutra from the Paraschkenmühle to the university clinic and the Wiesenbach between Laasan and Kunitz are to be mentioned.
In the lime-rich spring waters, the lime content is dissolved as hydrogen carbonate . Through the assimilation of the plants and through the air, which absorbs carbon dioxide, poorly soluble calcium carbonate remains and is precipitated on the aquatic plants (formation of travertine / tufa). As a result, plants are encrusted with lime and after the organic substance has died, the often well-preserved impressions of the plants remain. Limescale is deposited in the air even on dead objects. In the Pennickental, cascading steps of the limestone tuff have formed over a length of 1500 m and a height of 5–15 m. Tools and shards from the Younger Bronze Age were found in the upper layers. During this time, the lime separation was mostly over. Today these processes are only going on very slowly, as the water supply has decreased significantly since then.
The flora and fauna at that time were very extensive. a. Leaves of willow, alder, poplar, beech, oak, hazel, birch, linden, needles and cones of pine, fruits, grasses, pond horsetail, as well as the remains of snails, wild boar, deer and beaver. In the Pennickental, tufa was still mined by the Otto Schott glassworks . In the past, the loose material was also used to make Chlorodont and Blendax toothpastes.
The tufa was also used on various occasions for the production of so-called air bricks (in the vernacular "Dreckbachsteine"). Clay and other material were added to the loose limestone tuff in certain proportions, both were soaked together and mixed well to form bricks, which were dried in the open air under a protective roof. After several months of dryness, they could be used. Because of their low strength, they were mainly used for interior walls. However, numerous houses in Wenigenjena were destroyed during a flood around the turn of the century (1900) because air bricks were used in the construction of the houses.
The landslides in the area of the shell limestone have already been mentioned. The north-westerly inclination of the strata and stratified water stagnation over red clay stones are the cause of numerous older fragments of limestone clods in the vicinity of Jena, which mainly occur on slopes sloping northwest to west. In addition to the clearly visible example on the Dohlenstein near Kahla, demolitions on the northern side of the Jenzig, on the western side of the Kernberge and below the Kunitzburg are particularly noteworthy.
Attractions
In the area of the local mountain there is the first geological educational trail of Jena , at the core mountains and in the Pennickental the second Jena educational trail . Some of the geological features described here can be studied along these two educational trails.
literature
- Rudolf Daber (Ed.): Geology experienced and explored . Leipzig, Jena, Berlin (Urania) 1965
- Werner Hiller: Lecture manuscript . Jena 1997
- Lothar Lepper, Wolfgang Heinrich (ed.): Jena - landscape, nature, history . Jena (Ahorn-Verlag) 1999, ISBN 3-934146-01-5
- Gerd Seidel (Hrsg.): Geology of Thuringia . Stuttgart (E. Schweizerbart) 1995, ISBN 978-3-510-65205-1