Ceský ráj lies at the northern edge of the so called Bohemian Cretaceous Basin, which stretches some 300km from Brno in the southeast to Dresden in the northwest.

In the Cretaceous geological period (which lasted from 145 to 66 million years ago) the Bohemian Cretaceous Basin was a low lying area, protected to the north by a mountainous area (later to become the Jizerské and Krkonoše ranges) and to the south and west by a large elevated block of the Bohemian massif.

The Cretaceous period saw a massive increase in global sea levels, mainly due to accelerated widening of the mid ocean ridges, causing vast amounts of hot new rock to be created on the ocean floors, which in turn helped push the sea onto the continental land masses.

Over this period, the global sea level rose by over 200m, which dramatically transformed the landscape. Low lying basins throughout Europe were inundated and the higher land masses became increasingly isolated islands.

What follows is the geological story of how the Český ráj landscape came into being over this extraordinary period in the earth’s history.

100 million years ago…

It was an extremely warm and humid time. Sub tropical conditions existed throughout the region. Dinosaurs roamed the valleys and hills, foraging amongst the palms, small conifers and ferns. However, dramatic change was on the horizon.

Around 100 million years ago, the first waves rolled up the shallow river valleys of the Bohemian basin. The sea came from two directions; from the northwest flowed the cool waters of the ancient Boreal sea, and from the South East came the warm Tethys ocean.

Over the next 20 million years, the basin effectively became a broad and shallow sea channel between two large Central European islands, the Bohemian massif to the south, and the West Sudetic Island (which incorporated the Jizera and Krkonoše hills) to the north.

The consequences of the sea’s arrival were far reaching. Around 95 million years ago, a small herbivorous dinosaur from the Iguanodon family, which probably lived on one of the rapidly diminishing islands in the Bohemian basin, somehow lost its footing and floated off into shallow tropical sea that now filled the basin.

In 2003, a Czech paleontologist hunting for fossils in a quarry near to Kutna Hora found the Iguanodon’s left femur, buried in some of the basin’s very first sandstone deposits. The bone showed clear signs of being gnawed by the primeval sharks that now occupied the newly created tropical sea.

Around 70km to the north of the Iguanodon’s last resting place, the swirling sands were also starting to build the first layers that would become the famous sandstone rocks of the Český ráj.

94 million years ago…

The rising global sea level in the Cretaceous period also coincided the first movements of several plates in the earth’s crust, in particular the northwards movement of the Africa plate, which would ultimately lead to the formation of the Alps.

The stresses caused by these initial plate movements were felt many thousands of kilometers away, and probably started the process which led to a gradual uplift of the West Sudetic island and a sinking of the northern part of the Bohemian basin. These movements were triggered along a major fault, the Lusatian fault, which ran from the northwest (close to where Dresden is today) to the southeast along the edge of the West Sudetic island.

In the tropical conditions of the mid Cretaceous, raging rivers eroded the uplifted slopes of the Western Sudetic Island, taking with them a mix of sand, rocks and mud and depositing these sediments in the shallow sea now covering virtually the entire Bohemian basin.

The basin gradually filled up with these deposits, but at the same time their huge weight increased the basin’s subsidence, and the reduced weight of rock on the island contributed to its continual uplift.

Around 94 million years ago, the first layer that was deposited in the Český ráj region was made of course-grained sand, created in a very shallow near shore environment. As the basin subsided and the sea level rose, the shoreline receded, and the layers of deposited sand became finer and muddier.

Over the next seven million years, a huge volume of rock was eroded from the West Sudetic island, washed down its flanks by fast torrents and deposited into the sea. There it was spread out on the sea bed in layers of sand, silt and mud of varying thickness, by the strong currents, waves and tidal forces. Incredibly, the sedimentary layers built over this period in the Český ráj region were more than 600 metres deep.

87 million years ago…

Today, in the Český ráj, the majority of those layers are still buried below ground level, and the lower levels are only visible in a few places, particularly where the river Jizera and its tributaries have cut their way through the sedimentary rocks.

However, what is visible in the landscape today is geologically something rather extraordinary. The (up to 140 metre) layer of hard quartz sandstone that created the famous Ceský ráj rock formations is unusually ‘clean’ and homogeneous. The difference between this visible layer and those underneath (which are less homogeneous, much more layered and generally made of softer rock) is probably due to a change in how and what sediments were reaching that area of the Bohemian basin.

The change in the Český ráj deposition history occurred around four million years after the seas first flooded the basin (so, approximately 90 million years ago). This coincides with a remarkable global event, the reaching of the so called Cretaceous Thermal Maximum. This marks the point when the earth’s climate reached a ‘super greenhouse’ stage, where the amount of carbon dioxide in the atmosphere was nearly three times the levels of today. Global temperatures soared, and the global sea levels also reached their maximum levels around this time.

The dramatic climatic conditions probably accelerated the erosion of the island, possibly accessing deeper and harder rock types. Torrents poured increasing amounts of sediment into the basin, forming massive river deltas at the seashore. Thick ribbons and fingers of rocky sediment were pushed by the river deltas deep into the sea, to be caressed and woven by the strong currents into layers of high quality quartz sand on the sea bed. It took not much more than a million years for the huge layer of delta created sand that became the ‘rock cities’ of the Český ráj to be laid down.

The huge weight of the new sand layers caused accelerated deepening of the basin, once again creating deeper offshore conditions. Over the next million years or so, a significant muddy layer of offshore deposits was laid on top of the delta created quartz Český ráj sand. This ‘final’ muddy layer is still visible as the top most section of rock in the southern part of Český ráj and is dated to around 87 million years ago.

66 million years ago…

What happened next in the Český ráj geological history remains a bit of a mystery, simply because there is nothing left to see from this period. It has all been stripped away by millions of years of erosive forces.

What is evident is that there was a gradual shallowing of the sea globally towards the end of the Cretaceous period. At the same time, the process of general uplift and deformation of the Bohemian basin as a whole started, due to increasing pressure of the northerly moving African plate. Both factors probably led to an acceleration in the retreat of the sea from the basin.

It is reasonably clear that before the sea finally left there were significant further deposits of sand, mud and silt, probably in similar cycles to those experienced when the sea first invaded the basin. As much as several hundred metres of layers, that have now disappeared, may have been laid down over the next several million years.

What is certain, however, is that by the time of the major extinction event that marked the end of the Cretaceous, some 66 million years ago (which killed off the dinosaurs and up to 75% of animal and plant life species at the time) the Bohemian basin was no longer a tropical sea.

After its retreat, the sea left behind a massive plateau of mud and sand up to 1km thick across the Bohemian basin. This was compressed over several million years by its own enormous weight into layers of hardened sandstone, siltstone and mudstone. With the huge solid mass ready to be sculpted by the forces of nature, the next phase of the evolution of the Český ráj geology was about to begin…

Seismic ripples…

At the very end of the Cretaceous period, and throughout the next geological period, there was increasing seismic activity in the region, particularly along the Lusatian fault. This caused deformation and warping of the underlying sandstone layers, including a sizeable uplift at the edge of the basin of the lowest layer of course-grained sandstone. Extensive hardening of this layer occurred due to crushing forces that were exerted during the uplift, and we can see the effect as this hardened layer is revealed today as an impressive serrated rocky ridge in the Suché skály at the northern area of the Český ráj.

The ripple effect of this seismic activity also affected the broader Český ráj region, tilting the whole mass southwards and causing large individual blocks of sandstone to subside and some to uplift. Vertical cracks started to emerge in individual blocks. These cracks and segmented blocks would ultimately lead to the creation of the individual ‘rock cities’.

Streams running off the elevated area to the north (the former West Sudetic island) now had a gently sloping and undulated terrain to work with. Slowly, over millions of years, shallow valleys were carved out, and the soft and exposed top layers of Cretaceous sediment were stripped away, carried off in the rivers towards the German border and finally the North sea. The first landscaping of the Český ráj had begun.

Volcanic intrusions…

While the erosive forces of rivers and streams were stripping the top layers of sedimentary rock, there was equally significant events happening far below ground. Much of the tertiary geological period (66 to 2 million years ago) was characterised by significant volcanic activity in the region. As the forces that eventually created the Alps and Carpathian mountains started to intensify, old faults in Bohemian massif were reactivated. Magma erupted through channels and vents across the region, bursting through layers of sedimentary rock. Mini Stromboli-like volcanos were created in numerous places across the Bohemian basin.

In the Český ráj, with some of thickest layers of sediment anywhere in the basin, these magma intrusions probably did not reach surface level at that time, apart from at Kozákov, at the northern most tip of Český ráj. The Kozákov volcano has benefited from its proximity to the Lusatian fault for several periods of volcanic activity, up until around 4 million years ago, and its lava strewn slopes still bear testimony to its violent past.

However, the magma intrusions have become visible today in several places in the Český ráj, as the top layers of sedimentary rock that covered the intrusions have been eroded away, leaving the hard basalt dykes exposed. The characteristic twisted shapes of the cooled magma are visible at Mužský, Sokol, and Vyskeř, but most famously in the twin pillars of 30 million year old basalt that created the iconic towers at Trosky.

The impact of these volcanic intrusions on the landscape is not just seen in the dark twisted forms of the basalt outcrops, but in the more subtle strengthening and hardening the heat and infused minerals caused within layers of the surrounding sandstone. This contributed to an environment where sandstone blocks were eroded in a myriad of different ways, creating the mosaic of forms we see today.

2 million years ago…

At the same time as our earliest ancestors were evolving in East Africa, the final stage of evolution of the Český ráj landscape was beginning.

Around 2 million years ago, most of the region would have looked like a relatively solid and continuous sandstone plateau, penetrated by the volcanic intrusions of the previous geological period. By this time almost all of the softer sedimentary layers deposited on top of the hardest (90 million year old) quartz sandstone had been eroded away. Now was the time that the first ‘rock cities’ started to form.

As the streams and rivers which flowed down from the Jizerksé hills to north met the porous quartz sandstone, they started to carve out deep gullies and canyons in the plateau. Large blocks were separated from each other, as the Jizera river and its tributaries including the Klenice, Libuňka, and Žehrovka streams, cut their paths through the sandstone.

The seismic activity of the past 70 million years had left the sandstone blocks titled to the south, with massive vertical and perpendicular cracks. The blocks separated by the Jizera and its streams were then selectively eroded by the action of wind, rain and, most catastrophically, by ice. Most of the damage done to the individual sandstone blocks has been caused by the effect of water seeping into individual cracks, gullies and notches and then freezing. This ice-induced erosion was probably most intensive during the periodic ice ages of the past 2 million years.

Each individual sandstone block has been eroded in subtly different ways, impacted or protected by a large variety of factors including its proximity to the Jizera and its tributaries, the different titling forces of the Lusatian fault, the hardening affect of the basalt intrusions, and the variable formation of protective rock crusts on the sandstone surfaces. Some of the blocks are still relatively solid, like the Hrubá Skála plateau, others are more mature and have already been broken into a maze of individual pillars and towers, such as at the Prachovský Skály. Several blocks are already reaching the end of their lives as distinct sandstone rock formations. Gravity has toppled over individual pillars, filling the canyons with massive rocks and boulders, which have slowly eroded away into pools of powdery white quartz sand.

10,000 years ago…

When humans first settled in the Český ráj region, perhaps some 10,000 years ago, the landscape broadly looked how it does today today, at least on the higher plateaus and within the rock city areas. As our hunter-gatherer ancestors sheltered in the rock caves along the banks of the river Jizera, they must have looked up at the towering pillars of sandstone with the same sense of wonder as we experience now.

It took 100 million years to create the unique sandstone landscape of Český ráj, and it is likely that in a few hundred thousand years most of today’s rock cities, pillars and arches will just be unrecognizable fragmented boulders strewn around a sandy plain. In these final days of Český raj’s geological story, we are fortunate to be able to enjoy the sandstone landscape mosaic at its most beguiling and picturesque.