Age of talent formation: Neolithic, Quaternary, Pleistocene and Holocene (from 800,000 years ago to the present day)

The ravine-like Venner Bachtal is an example of the interplay between the "inner" and "outer" forces of the earth. Processes within the earth's mantle caused the Rhenish Slate Mountains to rise. The external forces ultimately led to the formation of the Venner Bachtal through weathering and erosion. Around 65 million years ago, the Rhenish Slate Mountains were still a flat undulating hill country that rose only slightly above sea level. At the end of the Early Tertiary period, the mountain block hidden underground gradually rose, while the Lower Rhine Bay collapsed.

Around 500,000 years ago, the upward movement that was already clearly noticeable 800,000 years ago increased considerably. This forced the Rhine and its tributaries to cut deep into the mountains. The gradient from the main terrace plain to the Rhine valley or Marienforst valley increased rapidly. The main terrace was now drained by side valleys that became progressively longer.

The deep erosion is favored by the predominantly loose rocks and the weathered Devonian sandstones and mudstones. This is a prerequisite for the formation of a notch valley. The people of the Middle Ages took advantage of this special natural feature to build a refuge castle on the opposite side of the valley.

Soil type: Loess loam

Soil type: Parabrown earth

Age of soil formation: Neolithic, Quaternary, Holocene (approx. 11,700 years ago to the present day)

In the Venner Bachtal valley, a change in land use can be observed near the forest boundary. The forestry areas of the Kottenforst are followed by agricultural grassland down the valley. These different types of use are partly due to the changed geological situation. The subsoil consists of heavily weathered sandstones and mudstones from the Devonian period, which were covered by a loess layer over 200 centimetres thick during the last glacial period.

The sands and gravels of the main terrace had already been largely removed, so that the loess lies directly on the heavily weathered slate rocks. The development of the soils from the loess in the post-glacial period, the Holocene, took place differently in the valley than on the flat plateau of the Kottenforst. This is due in particular to the steep slope. The rapid runoff of precipitation and seepage water and the slight, yet constant erosion and rearrangement of the upper soil horizon prevented the development of a pseudogley (waterlogged soil). The parabrown soils of the Marienforster Valley are characterized by high performance.

Rocks: Soil from clayey rocks of the Devonian period

Age of spring formation: Quaternary, Holocene (post-glacial period) (since approx. 11,700 years ago)

The stream crossing the Venner Kirchweg has its source in a large spring swamp on the northern slope of the Venner Bachtal valley. The special geological situation on the edge of the Kottenforst plateau means that groundwater seeps out over a large area here. The clayey soils in the Kottenforst make it considerably more difficult for rainwater to seep away. Nevertheless, it penetrates up to 10 centimetres per day through the approximately 200 centimetre thick top layer of Kottenforst loam. After 20 days at the earliest, the seepage water reaches the loose rock of the main terrace. Here they move rapidly downwards until they reach the groundwater. The porous unconsolidated rock of the main terrace is an important aquifer. It is sealed downwards by a groundwater dam, a clay layer.

In our region, this bottom layer consists of either Tertiary-age clays or Devonian-age clayey rocks. As the bed layer has a relief, the groundwater collects at the deepest points and flows in shallow channels towards the edge of the Kottenforst plateau. Where the groundwater zone is cut by a valley slope, the groundwater flow emerges as a layer source.

Building material: Basalt (columnar basalt)

Age of rock formation: Neolithic, Tertiary, Oligocene / Miocene (solidified approx. 25 - 22 million years ago)

The enclosure of the former Marienforst monastery is predominantly made of fragmented basalt columns. Due to its great strength and weather resistance, volcanic rock has been used in many different ways over the past centuries as bank reinforcement and road paving, for fortifications and church construction. Among the earth's volcanic rocks, basalts are the most common. The name "basalt" possibly goes back to the Ethiopian "bsalt", which means "cooked". Basaltic rocks have a lack of or insignificant quartz content and a black to dark grey color in common. They have a very fine-grained structure of microscopically small crystals. Well-formed intergranular crystals are often recognizable in this groundmass.

The columnar secretion of basalt has nothing to do with the crystallization of the rock. During the cooling of basaltic rock melts, pentagonal or hexagonal columns are formed perpendicular to the cooling surface, which are bounded by shrinkage fissures (the volume of the rock decreases during cooling).

Basalts from blankets and flows solidify predominantly in hexagonal columns. Pentagonal columns occur predominantly in source crests and veins. Above-ground lava outflows can thus be distinguished from subvolcanic (underground solidified magmas) basalts. Due to their columnar formation, such basalt deposits are ideal for further processing into uniform building blocks. In the past, there were numerous basalt quarries in the Bonn area, including in the Bad Godesberg city forest near Schweinheim ("Im Hohn").

Rock: silty, sandy valley deposit of the Godesberg stream

Age of the deposits: Holocene (post-glacial period to the present day) Formation of the Mühlbach: around 1600 AD.

The formation of a meandering watercourse can be observed in exemplary fashion in the Mühlbach, which accompanies the Marienforster Promenade. The stream was artificially created to supply a rapeseed oil mill, later known as the Brungsmühle, which was already mentioned in the early 17th century. For almost 400 years, the Mühlbach has developed a form typical of natural watercourses. The winding course of watercourses and the shape of the embankments are the result of the interaction of frictional forces on the riverbed, centrifugal force, counterforce (centripetal force), coriolis force and flow velocity, in addition to the geological conditions (hard and soft rocks). The cross current running at an angle to the current thread leads to erosion on the steep slopes. On the other hand, material carried by the river is deposited on the flat sliding slopes. Loops form, the meanders (after a river in Asia Minor). The curvature eventually increases to such an extent that only a narrow meander neck remains, which can be severed during floods. The severed bed remains as a gradually silted up oxbow (for example the Gumme). The water flows considerably faster in the breakthrough and has a greater erosive effect. Any undesirable changes to the course of the stream can be counteracted by reinforcing the impact slopes, for example with basalt columns or site-appropriate riparian woodland.

Rock: Conglomerate of quartz pebbles with chalcedony (Vallendar gravel)

Age of deposition: Neolithic, Early Tertiary (formed approx. 40 - 30 million years ago)

The boulders along the path are some of the oldest evidence of the Tertiary period in the Bad Godesberg area. They are river deposits from the Old Tertiary period, the Vallendar gravels (after the town of Vallendar near Koblenz). At the beginning of the Tertiary period, our homeland was a lowland plain. In an almost tropical climate, the sandstones and clay shales from the Devonian period were exposed to deep chemical weathering. Massive clay deposits were formed. Only the hard quartz rocks resisted decomposition.

During a slight uplift in the course of the Early Tertiary period, the weathering crust was locally removed. The quartz rocks were crushed in the resulting river courses and deposited there as sand, gravel and blocks of Vallendar gravel when the water flow was reduced. Due to the massive occurrence of quartz in the Vallendar gravels and the deposits in the Trier region, it is possible that a large proportion of the quartz originated from the Vosges or even from the French Massif Central. The cementation of the gravels into a conglomerate took place through the later overlaying of volcanic ash (trachyte tuff). During weathering, silicic acid was released, which seeped downwards with seepage water and separated there as silica gel. There it solidified under water loss to form freshwater quartzite, with the quartz type chalcedony.

Mineral and medicinal water spring of the sodium-hydrogen carbonate-chloride acidulous type

The formation of the Marienforst valley can be traced back to a geological fault zone in the slate mountain range. It offers mineralized groundwater an upward path from great depths. Here, one is extracted from a depth of 60 meters in the Draitsch well.

The water from the Draitsch spring, which is located on the site of the bottling plant, contains a lot of carbon dioxide gas (CO2). The CO2 gas originates from activity dating back to volcanic activity in the Tertiary and Quaternary periods. At great depths, the molten mantle and cooling magma chambers release carbon dioxide and other volatile substances in particular. The gases penetrate through fault joints in the earth's crust at high pressure into the groundwater area, where the carbon dioxide is chemically dissolved to form carbonic acid. Some of the gas physically remains in the mineral water as "free carbonic acid" (gas bubbles!). It causes the water to rise further to the earth's surface (gas lift). The carbonated water dissolves minerals, especially metal and semi-metal salts, from the rocks through which it flows. The ingredients of the mineral water thus reflect the geological structure of the subsurface.

Rock: Alkali basalt

Age of rock formation: Neolithic, Tertiary, Oligocene / Miocene (approx. 25 million years ago)

The Godesberg is a "ruin" of a volcano whose hard gangue rock, a fine-grained alkali basalt with a flowing structure, was modeled out of the soft surrounding rock by weathering. It is the solidification product of a low-silica magma that came close to the earth's surface in the late Tertiary period in the course of the Siebengebirge volcanism around 25 million years ago.

The magma, which was more than 1100º C hot, penetrated a thick blanket of previously extracted (trachyte) tuffs and expanded there in a more or less club-shaped manner. An underground volcano, a subvolcano, was formed. However, it is uncertain whether the molten rock was transported to the earth's surface as lava and built up a volcano above ground.

The rock contains clinopyroxene, olivine, plagioclase, ore grains and, in places, vitrified minerals. Olivine grains and clinopyroxene and quartz grains dissolved from the melt occur as inclusions. The "Godesberg volcano" has become unrecognizable due to erosion and an exact reconstruction of its former shape is not possible. There used to be small quarries on the eastern slope of the Godesberg, where most of the building material for the Godesburg was probably extracted.

Age of formation of the Rhine: Mesozoic, Tertiary, Middle Miocene (approx. 14 - 11 million years ago)

The formation of the Rhine valley is partly due to a deep fracture zone in the earth's crust. Its formation is closely linked to the Tertiary-period rise of the Rhenish Slate Mountains and the collapse of the Lower Rhine Bight. The causes of the movements of the earth's crust are sought among other things in movements of the earth's mantle. Tensile stresses formed in the overlying earth's crust, causing parts of the earth's crust to collapse. An accompanying phenomenon was lively volcanism, the center of activity of which was the Siebengebirge.

An impressive example of the movements in the earth's crust is the subsidence field of the Lower Rhine Bight, which cuts wedge-shaped into the Slate Mountains near Bonn and can be traced through faults to the trench-like collapse of the Neuwied Basin. From there, there is probably a further continuation of the graben system to the south with a connection to the Upper Rhine Graben. In the northern slate mountains, the Rhine has used the marked fault zone since the middle Miocene period, the birth of the primeval Rhine, and drained the Upper Rhine area
across the Rhenish Slate Mountains to the North Sea.

Age of formation of the valley slope: Neolithic, Quaternary

Main formation phase: Pleistocene (Ice Age) (500,000 - 300,000 years before today)

From the site, the path leads down to the middle terrace of the Rhine (Promenadenweg). The steep valley slope to today's Rhine valley was shaped by the Ice Age Rhine. Around 800,000 years ago, the Rhenish Slate Mountains were strongly uplifted. During the subsequent warm periods, the wide gravel plain formed during the cold period was deeply incised by the Rhine. A new and much narrower valley was formed.

It was not only the loose rocks of the cold period riverbed that were eroded away. The river also dug into the deeper, Tertiary-age clays and sands and the Devonian-age sandstones and clay shales of the Rhenish Slate Mountains. The erosion processes were particularly intense at river bends, where the stream collided with the valley slope. Here, the slope was undermined and split by lateral erosion. The rocks of the Rhenish Slate Mountains emerged on these slopes. Today, Devonian sandstones and clay slates can be traced along the Kottenforst plateau as far as the Venusberg. Where the slope inclination is not very pronounced, a thin blanket of glacial flowing earth was preserved over the Devonian rocks, as here at the site.

Erosion residue of the middle terrace

Age of the middle terrace deposits: Quaternary, Pleistocene, Saale Ice Age (approx. 300,000 - 120,000 years ago)

The promenade path between Aennchenstraße and Pionierweg is laid out on an erosion remnant of the middle terrace. It appears on both sides of the Rhine and nestles here as a narrow ledge on the steep slope to the Kottenforst plateau. The middle terrace is made up of 20 m thick, gravelly sands. At Poppelsdorf and Duisdorf, the middle terrace widens in a north-westerly direction to form a kilometer-wide plain. The middle terrace towers only a few meters above the Gumme (old branch of the Rhine) and the lower terrace to the east.

In the past, paths, roads and railway lines were built on the narrow strip of terraces in the Rhine valley, which is why it is also known as the valley road terrace. The Rhine terraces can be distinguished from each other not only by their different altitudes. They also show clear differences in the way they carry boulders and volcanic heavy minerals. For example, the deposits of the Main Terrace contain 50 to 60 percent quartz boulders, those of the Middle Terrace 30 to 40 percent quartz boulders and those of the Lower Terrace 20 to 35 percent quartz boulders.

Erosion residue of the Middle Terrace and fine sandy valley deposits of the Gumme

Age of the Gumme: Quaternary, Holocene, Preboreal to Boreal (approx. 11,700 - 7,000 years ago)

North of where the Pionierweg joins the Promenadenweg, the middle terrace of the Rhine is cut off by a silted-up channel of the post-glacial Rhine, the Gumme. It can be followed below Godesburg Castle along the steep slope as far as Poppelsdorf. There, the shallow riverbed bends in a wide arc, through Bonn city center to the west towards Bornheim. The "sagging" of the roads leading east from the edge of the mountain near Friesdorf to Friedrich-Ebert-Allee and the "Kumme" road leading north from Hochkreuzallee indicate the course of the former valley profile.

Following the last glacial period, around 11,700 years ago, the Rhine emerged from the narrow valley of the Middle Rhine like an alpine whitewater river. On the glacial valley floor, the later Lower Terrace, it meandered in wide bends into the Lower Rhine Bay. At the edge of the Kottenforst plateau, the river collided with the steep slope near Friesdorf and quickly dug into the soft deposits of the Lower Terrace. Here, bank erosion also led to the erosion of the narrow middle terrace. During the Holocene period, the riverbed shifted to the middle of the valley and gradually took on its present appearance.

Rock: Sandstones and clay shales of the Rhenish Slate Mountains

Age of deposition: Palaeozoic, Lower Devonian, Upper Siegen Stage (deposited approx. 412 - 405 million years ago)

The rocks exposed along the Pionierweg are sedimentary rocks from the Lower Devonian period. During the Carboniferous period (coal age), they were uplifted about 300 million years ago from the floods of the northward receding sea by compression of the earth's crust and included in the formation of the Rhenish Slate Mountains.

In terms of large-scale tectonics, this and the worldwide formation of mountain ranges can be traced back to the collision of the Gondwana and Laurussia continents.

The Rhenish Slate Mountains are part of one of these mountain ranges, which in their entirety are known as the Variscides. Named after the Germanic tribe of the Variscans, these mountains stretched across central and western Europe from eastern Germany via southern Belgium and France to Wales. Streams and rivers transported the erosion debris from the mountains into a moraine depression to the north. Huge coastal swamps with tropical rainforests stretched out there. They formed the basis for the formation of hard coal, for example in the coal basins of the Ruhr area and the Aachen coalfield. By the end of the Palaeozoic, in the Permian period, the mountain range had been eroded, leaving only a flat undulating landscape 225 million years ago.
landscape remained. It was only at the end of the Tertiary period and during the Quaternary period that the rump of the Rhenish Slate Mountains moved upwards again and for around 500,000 years has appeared as today's low mountain range landscape.

Rock: Sandstone and clay shale from the Rhenish Slate Mountains

Age of deposition: Palaeozoic, Lower Devonian, Upper Siegen Stage (deposited approx. 412 - 405 million years ago)

During the Carboniferous period, the seabed was increasingly narrowed from the south. As a result, the stacked sedimentary rocks were folded, fractured and raised to form a mountain range. The resulting rock folds consist of "wave valleys", the geological troughs, and "wave crests", the geological saddles. Folds of all sizes can be found in the Rhenish Slate Mountains. From centimetre-sized special folds to kilometre-wide and tall large folds.

The bending of the layers led to stresses which, when the rock strength was exceeded, led to the fissuring and faulting of the layer packages. The fissuring is clearly visible in the sandstone layers along the Pionierweg. There are fine cracks and fissures that run perpendicular to the layer surface. In the central and southern slate mountains, other surfaces also occur in the fine-grained, clayey deposits. There, narrow surfaces divide the rock into lamellae, the slate surfaces.

The slate surfaces intersect the bedding planes at different angles. In the northernmost part of the Schiefergebirge, the schistosity is hardly pronounced. Rock fissures and slate surfaces provide a suitable point of attack for frost and root penetration, facilitating the development of soils.