The beech forests of the Kellerwald National Park surround the Edersee near Kassel on the eastern side of the Rheinisches Schiefergebirge mountains. The summit regions rise to 675 m (2.215 feet). The Kellerwald encloses one of the largest remaining beech forests in Central Europe. They originated as extensive primeval forests about 12.000 years ago after the end of the last ice age and the associated northward retreat of the glacial ice cap.
The mist surrounding the trees, as well as the abundance of autumn colored foliage on the forest floor, conceal an even more fascinating geological history of this region. It is only revealed when the earth provides a window into its past by taking a close look at the exposed rocks.
These rocks on which this beech forest grows were formed more than 380 million years ago, during the Devonian period, at the bottom of a long vanished ocean teeming with prehistoric armored fish, coelacanths and early sharks. As if this were not astonishing enough, Germany, and with it the Kellerwald, was located far south of the equator at that time and formed a tropical ocean with the well-sounding name Rheia.
The Rheia Ocean stretched between the former major continents of Laurussia to the north and Gondwana to the south. Laurussia comprised the union of North America with Baltica and Siberia, while Gondwana enclosed today’s South America, Africa, Antarctica, Australia and China. In the Devonian, the region of the Kellerwald and thus also the entire north of Germany belonged to the southern margin of Laurussia and bordered on the Rheia Ocean. However, this was not always like this. Even earlier, in the Silurian age 440 million years ago, northern Germany was a small continent of its own in the vast Iapetus Ocean and was located almost at the South Pole. This small continental mass was named Avalonia. In the Silurian, Avalonia drifted steadily northward, crossed the equator, and eventually collided and merged with Laurussia. In the Devonian, the southern beaches of former Avalonia bordered the Rheia Ocean.
Rheia included all oceanic ecosystems from the deep sea to the shallow seas of the continental shelf, where coral reefs and stromatopore reefs grew. The latter reef-builders belonged to the sponges. The habitats of this ocean provided ideal conditions for an abundance of fish species. Thus, a prolific and novel fish fauna evolved in the Devonian Sea, conquering not only saltwater but also brackish and freshwater habitats. Among them were the impressively looking armored fishes, both jawless and jawed. Their rugged outer armor of scales and bone plates gave them a bizarre appearance and protected them against predators. Simultaneously with them, streamlined neck-joint fish species up to eight meters in length evolved, as well as the cartilaginous fishes and thus the first sharks. Furthermore, the bony fishes evolved, giving rise to 99% of the fish species living today. They include the ray-finned, lung-finned and coelacanths. The latter were pioneers among fishes as they ventured onto land, laying the foundation for all land animals, including ourselves. This variety of fish fed on bivalves, snails, and brachiopods. Brachiopods look like shells at first glance, but they belong to the arthropods. Furthermore, the fish hunted stretched and curled ammonites, the bottom-dwelling trilobites, as well as sea urchins, starfish, crabs, horseshoe crabs, sea cucumbers, jellyfish, and sponges. It is amazing how many of these species are still familiar to us today. They are all living fossils and belong to the most successful species ever.
At the same time, the conquest of the continents by plants and animals took place. While lycopods and ferns conquered the world, scorpions, spiders, dragonflies and millipedes followed them. In the shallow waters near the shores of the Rheia Ocean, coelacanths made their first appearance on dry land. The barren land turned green for the first time.
Driven by the heat in the interior of the planet, plate tectonics relentlessly shifts all continents across the surface of the planet. The continents on which we live are ancient and count after billions of years. The reason for this is that they are preserved during the tectonic movements and are merely rearranged. Exactly the opposite happens with the ocean floors.
They dive back into the earth’s interior at the astonishingly young age of at most 190 million years, and are continuously recycled there. That is why entire oceans basins opened up and closed repeatedly throughout the history of Earth. The different behavior of continents and ocean crust is explained by the fact that they consist of completely different rocks material. Continental granite is lighter than the volcanic basalt of the ocean floors. Therefore, the lighter continents have greater buoyancy compared to the heavier ocean floors. Continents behave in the earth’s crust much like ice cubes in water.
In this way, we understand why the Rheia Ocean slowly disappeared in the interior of the earth during the approach of Gondwana and Laurussia. In the Devonian about 390 million years ago and the following Carboniferous age, these two large continents finally collided with each squeezing various small continental fragments and islands located between them. This gigantic collision at snail’s pace compressed the earth’s crust tremendously and stacked several rock layers on top of each other as a nappe structure. What happened here had global extent and initiated the beginning of the union of all continents in the supercontinent Pangaea.
However, not only the continental rocks of Gondwana in the south and Laurussia to the north were included in the resulting Variscan mountain range. All sediments, which had been deposited on the bottom of the Rheia Ocean in the course of millions of years, were scraped off during the descent of the ocean plate into the Earth’s interior and folded into the resulting mountains of the continental collision. Thus, these claystones and sandstones as well as the limestones were folded into the rising mountain range and with them all fossil remains of the fish fauna and coral reefs. Fantastically preserved fossils of the armored fish originate from the Kellerwald. The Variscan mountains rose to a mountain range comparable with the today’s Himalya. Also there, the fossil remains of ammonites are found up to the highest summit regions.
With the end of the tectonic collision, also the Variscan mountain formation ended. As with any high mountain range, the water cycle unfolded its erosive effect. Snow turned into ice and formed glaciers, which gradually carved and eroded the mountains. Since the Upper Carboniferous, about 320 million years ago, the erosion of the mountains began and over time, a rugged mountain range transformed into the hills of the Rheinisches Schiefergebirge and the Harz Mountains. After the end of the last ice age, the dense primeval forests of deciduous mixed forest migrated northward following the retreating ice until it reached the region of the Kellerwald.
It is fascinating to imagine the landscape of the Kellerwald in four dimensions. This journey through time takes us back to the age of the armored fish and we dive between the foggy fall-colored beeches into the tropical Rheia Ocean. One can almost taste the salty warm water and sense the armored fish in the mist between the trees as they chase after their primeval prey.