Geology Trail
The underground layers of rock are most often invisible to us, but they impact our daily lives in manifold ways. Our soils’ fertility largely depends on the mineral elements present in the geological substrata. For our water supply and for all of our building endeavours, too, geological factors play a vital part. For this reason, the Open Air Museum illustrates fundamental contexts of geological conditions with the Geology Trail.
At the outset of the trail, there are display boards on topics such as The Structure of the Earth and The Cycle of Stones. In the second part, visitors have the opportunity to take a walk through the various geological periods, inasmuch as they are represented in Hesse. The granites of the western Odenwald recall the first origin of earth and the solidification of the flowing mass of magma below the earth’s surface.
The Palaeozoic (500 to 230 million years ago) is represented by the chronological sequence of Silurian sericite gneiss, found in the southern Taunus, and Devonian slates, micaceous sandstones, quartzite and limestones from the Taunus proper. Carbon slates from the Lahn river region, sandstones from eastern Hesse and conglomerates from the Hesse-Thuringia border region, which come from the Permian, follow.
The only representative of the Mesozoic (230 to 65 million years ago) in Hesse is the Triassic. It contains red sandstones from the eastern Odenwald and the Reinhardswald, shell-bearing limestones from the Diemel region and the Eastern Rhön mountains and, finally, Keuper sandstones from the Netra valley. The Jurassic and Cretaceous periods cannot be shown as what is Hesse today was dry land at the time, so no sediments formed.
The Cainozoic (65 million years ago to today) is represented by stones from the Tertiary. These are coralline limestones, but also the softer sandstones of the Wetterau region and, in particular, basalts. The latter are of importance not only in the Westerwald, along the northern edge of the Odenwald and in the Knüll mountains, they also form the central Hesse Vogelsberg massif. This is the largest contiguous basalt mass in Europe. The end of the trail is marked by a sandstone column from Rockenberg (Wetterau) which consists of very young, not fully solidified fine sand sediment.
Formation:
Basalt is a hard, dark, often black rock with a uniform appearance. It is the most common volcanic rock and is formed when lava cools rapidly on the earth’s surface or in oceans on the seabed. The main constituents are silicates that are rich in iron and magnesium, but also feldspar and olivine.
Occurrence:
Thanks to its geological formation, the Vogelsberg has the largest basalt deposit in Hesse as it was covered over a large area with mighty lava blankets. However, smaller basalt quarries can be found in many regions, such as the Westerwald, the Rhön, and the Knüll.
Use:
As basalt is a very abundant rock it is mainly used as crushed material for the construction of roads and concrete, and is also used as railway ballast. Near basalt deposits, houses have also been built from basaltic rock, giving these villages a dark appearance. Basalt is also often the source rock for copperores, iron ores and sapphires. Basaltic lava provides a lot of plant nutrients and beneficial clay minerals from the depths to the earth’s surface, which is why basaltic soils are very fertile.
Basalt was also used for the renovation of Cologne Cathedral.
Formation:
Clay slate can be categorised as both a sedimentary and metamorphic rock. It is formed by the initial transformation and consolidation of mudstone. Fossils are often found in clay shale, which provides information about life at the time the sediment was deposited.
Occurrence:
In Hesse, the clay shale of the Ems stage (Lower Devonian) is mainly found in the Hintertaunus region. Outside Hesse, however, it is found in many places throughout the Rhenish Massif, for example in the Hunsrück.
Usage:
In the Hunsrück, clay slate has been mined, processed and exported for centuries because of its regional fineness and evenness. For this reason it is a visual distinguishing feature of the cultural landscape, as the roofs and sometimes also the facades of houses of entire settlements are panelled with this stone. Slate facades were also common in Hesse, especially on the side of houses most subject to weathering.
Until the introduction of large-scale industrial paper production, its use as slate was of great importance.
The word “slate” only describes the visual appearance of many different rocks. What they have in common are the parallel, flat slabs that can be split.
Formation and composition:
Conglomerates are coarse-grained sedimentary rocks comprised of boulder material (>2mm) in a fine-grained rock mass. They are round in shape because of the attrition of the boulders as they are transported in rivers and seas, as well as from the motions of the waves and surf after deposition. The coarse-grained components and the fine-grained groundmass are often cemented together with the binder calcite to form a solid, non-uniform sedimentary rock. The gravels can originate from many different rock types or just one. Conglomerates can vary in terms of consolidation. Unconsolidated conglomerates are often geologically young formations, while strongly consolidated conglomerates are usually considerably older.
Occurrence:
Conglomerates mostly occur where sandstone is also found, however, they are much rarer. Conglomerates of volcanic origin are few and far between, and are often found in former shorelines and vast river systems.
Use:
Thanks to their varied patterns, conglomerates are used as decorative natural stones for wall cladding, as building stones, flooring, and work tops. However, only strongly consolidated conglomerates can be worked as the rocks break apart easily with softer binding agents.
Conglomerates used to be called “sausage stones” because they resemble blood sausage.
Formation and composition:
Diorite belongs to the category of intrusive igneous rocks (plutonites). It is closely related to granite, but has a different composition. The main minerals are plagioclase, green-black hornblende and biotite, with quartz and potash feldspar are usually involved as secondary minerals. Diorite is formed by very slow cooling and often occurs at the edge of large granitic rock bodies.
Occurrence:
The southwestern Odenwald largely consists of different diorites. The deposits in the “Felsenmeer” near Lauterbach are particularly impressive.
Use:
Diorites are mostly used as building materials. In the Hessian Odenwald, they were mined in larger quarries for road construction as gravel or paving stone or as concrete aggregate. Dark diorite is also used as decorative stone.
Diorite is also called black granite, although it is not actually real granite.
Formation:
Gneiss is a metamorphic rock that forms at high temperatures and under high pressure from sedimentary or igneous rocks.
Like granite, gneiss consists mainly of feldspar, quartz and mica. However, the minerals are arranged in patterns. The quartz often makes it coarse-grained and does not produce smooth surfaces. Gneiss is usually grey, but can also be pinkish, brownish or greenish in colour.
Gneiss forms the core of mountain ranges and can be found in some of the oldest rock formations on earth. As intrusive rocks, they only come to the surface either when the rock above them is eroded or they are pushed to the surface by movement of the Earth’s plates.
Occurrence:
In Hesse, gneiss occurs mainly in the crystalline Odenwald and the crystalline Spessart. The largest occurrence is found in the Böllstein Odenwald.
Use:
Gneiss has been a sought-after natural stone for centuries and was used for building paths and houses because of its hardness and weather resistance. Recently, it has also been used as a decorative stone for kitchen worktops or bathroom panelling.
The name “gneiss” is an old miner’s term and describes the sparkle and shimmer of the rock.
The name granite is derived from the Latin word “granum” for grain. Granites are coarsely crystalline, bulky rocks and are white, light grey, pinkish, yellowish or greenish in colour. The main components of granite are feldspar, quartz and mica.
Granite is formed by the very slow solidification of magma within the earth’s crust, usually at a depth of more than 2 km. They therefore belong to the group of intrusive rocks (plutonites). The longer the hardening and crystallisation process takes, the larger the visible mineral grains in the rock.
Almost all granite in Germany was formed in the Carboniferous Period about 360-300 million years ago.
Occurrence:
In Hesse, granites occur mainly in the anterior, crystalline Odenwald.
Use:
Due to its hardness and durability, granite has been quarried since Roman times (for example, Felsenmeer in the Odenwald). In addition to its decorative patterns, its grindability and polishability make it a classic building stone. Even today, modern building facades are often clad with granite. It is also used for wall cladding or kitchen worktops, and in sculpture, granite is a popular raw material.
Formation:
Gypsum is both a naturally occurring mineral and a chemical sedimentary rock. It consists of calcium sulphate and belongs to the category of evaporate rocks (evaporites). These are formed when sea basins slowly dry out because to evaporation and the minerals dissolved in the water crystallise. (precipitation). Depending on the addition of various other minerals, gypsum stone can be grey, white or even red in colour.
Occurrence:
The largest deposits in Germany were formed during the Zechstein in the so-called Zechstein Sea approximately 257 million years ago. Other gypsum deposits originate from the upper Buntsandstein, from the Muschelkalk and from the Keuper. The most significant deposits in Hesse are in the Witzenhausen-Eschwege, Alheim and Sontra area.
Use:
Gypsum is not only used in the construction industry as a building material, but also in the food industry, in medicine, in dentistry, and in the creation of moulds in art.
Alabaster also belongs to the category of gypsum rocks.
L for limestone from the remains of animals and plants
Formation:
Many Limestone are biochemical sedimentary rocks consisting mainly of calcite or other carbonates. The largest limestone deposits on earth were formed at the bottom of oceans. There, the calcareous remains of dead algae, snails, mussels and sea urchins were deposited and transformed under high pressure into rock.
But not all limestone has a biological origin. Some were also formed from minerals that dissolved in water and then solidified once again.
Limestones are less hard in nature and weather quickly. Rain or flowing water lead to soft curves and depressions, which can form entire landscapes and huge, underground caves.
Use:
Limestone can vary in terms of their composition, properties, and the way in which it’s used. It is a natural stone that is easy to process and is an important raw material in industry, for example in production of concrete, cement, or glass. In agriculture, it is also used as fertiliser or to regulate the pH value of the soil.
Refined marble can be created from limestone under high pressure and at high temperatures.
Formation:
Picrite is a volcanic rock that was formed in Hesse, mainly during the Lower Carboniferous Period, and is closely related to basalt. It gets its green colour from large amounts of olivine and augite. It also has a high chlorite content.
Occurrence:
This type of rock occurs in Hesse, mainly in the Dill syncline of the Lahn-Dill area, and is also called ‘Hessian olivine diabase’.
Use:
Picrite is a hard stone that is not easily worked, but thanks to their rarity and beautiful patterns and formations, they are nevertheless often used as decorative or gravestones. Picrite is most often used in sculptures or for foundations and plinths in grandiose buildings in the region. Later, during the course of industrialisation, there were more possibilities for machine processing, and this made it possible to use picrite as facade cladding and for windowsills. Picrite also is very resistant to weathering.
Especially the so-called ‘Hessian New Green’ from Hirzenhain in the Lahn-Dill region is appreciated for its beautiful shine, good polishability and resistance to frost.
Formation:
Quartzite is usually formed by the transformation and strong consolidation of quartz-rich sandstone. It is a very hard, mostly light-coloured, fine- to medium-grained rock that consists of at least 80% quartz. The colour varies from white to grey, though is sometimes also bluish, pinkish or greenish in colour, depending on which other minerals are mixed in. Unlike sandstone, because of the high degree of compaction and new formation of the crystal lattice, there is no longer any recognisable graining. During its formation, the initial pore spaces of the original rock disappear almost completely. For this reason, the rock is extremely resistant to weathering, which is why in natural conditions it erodes much more slowly than other rocks. This is why quartzite often forms massive rock bodies such as cliffs, ridges and hills in the landscape.
Occurrence:
In Hesse, quartzites occur mainly in the Kellerwald and along the main ridge of the Taunus, where they have formed striking rock formations. The Eschbacher Klippen are a particularly impressive quartzite formation that is very close to the Hessenpark.
Use:
Today, quartzite is considered one of the most precious natural stones. Brightly coloured quartzite in particular is popular for flooring and wall or facade cladding. As it is very hard and resistant to environmental influences, quartzite is well suited for interior and exterior areas that are subject to heavy wear. However, high-purity quartzite is also used in the manufacture of special optical glass or as a filler in the production of refractory material for furnace linings. Coarse, base quartzite is also used as chippings and gravel in road construction.
The mineral quartz is so hard that it can scratch window glass.
Formation:
Rhyolite is one of the more rare volcanic rocks. It is characterised by a high silicon dioxide content and is usually light in colour with red, red-grey or grey-violet tones. Rhyolites are formed from viscous rock melts that rarely flow out at the earth’s surface but rather harden in the volcanic vents. The Hessian rhyolites were formed during the Rotliegend about 270-290 million years ago.
Occurrence:
Rhyolite is almost always formed in connection with continental volcanic activity and crystalline melting. In Hesse, it is found in large numbers in the anterior crystalline Odenwald. Rhyolite is particularly common in neighbouring Rhineland-Palatinate along the Nahe River, where it is protected from extensive erosion in the low-lying Saar-Nahe trough.
Use:
On account of its resistance, rhyolite is mainly used as crushed stone, chippings and paving stone. Despite its similar composition, rhyolite is less hard and resistant than its counterpart granite because it cools more quickly when it is formed and crystallises at a shallower depth.
The name rhyolite is derived from the Greek words for “flow” and “stone.
Formation:
Sandstone is one of the most common sedimentary rocks and is formed by the natural consolidation of loose sand. The time this process takes varies and can take between a few decades and several million years. Sandstone occurs on all continents and is preserved from almost all geological eras.
The most significant areas of formation are mainly shallow seas near the coast. The sand usually originates from the mainland and is transported via rivers to the sea, where it is displaced by ocean currents to its final place of deposition. However, it is also possible for sandstone to form on the mainland.
Sandstone consists of two components: the grains themselves and a binding agent, such as lime, clay or silica in the interstitial spaces. The individual sand grains have a size of 0.063 to 2 mm and consist mainly of quartz, feldspar, or other small rock fragments.
Occurrence:
The largest Hessian formations are in the Spessart and in the Buntsandstein-Odenwald. They also occur frequently in the Taunus and the Reinhardswald.
Use:
Sandstone is a historically widely used building material because it is decorative and easy to work with. It comes in both red and yellow varieties. Many magnificent sculptures and old buildings are made of sandstone, for example Frankfurt’s Paulskirche or Frankfurt Cathedral. Sandstones of lesser grandeur are used as millstones, boundary stones, or in dry stone walls.
A distinguishing feature of sandstone is the grains of sand that can be seen with the naked eye.
Formation:
The source material of this rock is dead wood that did not decompose but rather was covered by sediments such as clay, sand, or volcanic ash, and thus preserved.
The organic substances were gradually replaced by silica dissolved in water. This process is called silicification.
Occurrence:
Fossils such as silicified wood can be found in strata from all geological periods. It is preserved where it has not been eroded away with the surrounding rocks, but rather has been covered by younger sedimentary layers.
Use:
Fossils provide information about creatures that existed in the past and the environmental conditions at that time. By analysing fossilised bones and plants, palaeontology can explore the past and thus reconstruct life in prehistoric times. However, fossils are also popular with jewellery designers, collectors and amateur archaeologists.
Silicon dioxide dissolved in water is also called silicic acid.
Formation:
Trachyte is an alkaline volcanic rock consisting mainly of alkali feldspar. It is formed by the rapid cooling of lava on the earth’s surface and can be yellowish, brownish, or light to medium grey in colour. Gas bubbles are often trapped as the rock hardens faster than the gases can escape.
Occurrence:
As a typical volcanic rock, trachyte is common in many volcanic regions. In Hesse, for example, it can be found around the Vogelsberg, in the Odenwald, in the Westerwald and in the Rhön Mountains.
Use:
As trachyte is a hard rock, it was used by the Romans as paving stones. Their high mechanical strength also made them popular building materials in the Middle Ages, for example, in churches such as Cologne Cathedral. Nowadays, trachyte is hardly ever used as a building stone because of its porosity and susceptibility to weathering.
When trachyte is broken, it is jagged and rough (the Greek ‘trachys’ means “rough”).
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