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The river Berounka valley with limestone rock formations and hills in the background.

Lookout onto the Berounka valley with Devonian limestone formation.

Geology of Bohemian Karst

My last post was about the geology of caves in Bohemian Karst, but what was here before? Way back in prehistory, in the geological period called Ordovician (490-435 MA), change was afoot on the ancient continent of Gondwana. This was long before Bohemian Karst: the whole area was still under the sea. Remnants of sandstone and slate sediment tell us that sand and clay were flushed into the sea to form the bed of what would later become Bohemian Karst. And this geology took place far away from the equator, somewhere in the southern hemisphere in the cold climate.

Tropical north

The tectonic plate that was carrying the beginnings of Bohemian Karst started moving northwards to the tropical climate at a speed of a few centimeters a year. After a few million years it hit a granite continent in the north. And the ancestor of today’s Europe was born.

But back to Bohemian Karst… the era which followed Ordovician is called Silurian (435-419 MA). Cracks in the bottom of the sea basin gave way to further volcanic activities. Some of the originally submerged volcanoes were emerging above sea level. Where sunlight could reach the submerged volcano, life could thrive. And the calcareous shells of those prehistoric sea organisms led to the creation of the first layers of limestone.

When two eras collide

From the end of the Silurian era and the beginning of the Devonian era (419 – 360 MA), we know that the limestone layers alternated with layers of slate sediment a few decimeters thick. Geologists come from all over the world to see the stratotype where Silurian meets Devonian. This division is clearly visible at Klonk u Suchomast in Bohemian Karst. Due to the caulk effect of slate layers, water cannot easily penetrate, and therefore the karst phenomena do not fully occur in those layers. Most of the limestone that would make the Karst what it is today would not form until the late Devonian era.

Sea shells come out on top

Bohemian Karst was in a tropical climate by now, which meant a surge of tropical sea fauna. Immense layers of limestone originated from the shells of dead sea creatures. Torrents carried the shells to new areas where they formed the layers of sediment on which the majority of known caves later formed.

The rock formation where the layers of limestone are changing with the layers of slate

Budňanská skála rock formation in Karlštejn – you can see how the Variscan orogeny forced the limestone into corrugated layers. Pic taken by Miaow Miaow.

Then was the Variscan orogeny: a collision of tectonic plates that changed the face of the future Bohemian Karst. During an orogeny event, sediment usually sinks deep down into the depths of the Earth, where it is transformed under high pressures and temperatures into different types of rocks. However this was not the case in Bohemian Karst. The sediment must have been squeezed and pushed up instead, and the layers corrugated into the shapes we see today. One example of this geology is the Budňanská skála rock formation by the river Berounka in Karlštejn.

And as the limestone sediment corrugated, the vertical cracks allowed water to seep in, which was so important for later cave development.

Come and see all this geology for yourself on a hike to Karlštejn in Bohemian Karst!

Source: K. Žák, M. Majer, V. Cílek: Český kras,  Klíč k české krajině, Academia 2014