WHAT KIND OF ROCKS CAN BE FOUND IN THE RALSKO NATIONAL GEOPARK?
The Ralsko Geopark is dominated by Mesozoic sedimentary rocks. In addition to the most common quartz sandstone, we can also find ferruginous sandstone (containing ferruginous cement) and calcareous sandstone (with carbonate cement). In many places, these soft sediments are penetrated by igneous rocks (basalt and polzenite). These are significantly harder and determine the creation of raised landforms (peaks or ridges). In the Ralsko Geopark’s territory, rocks of Proterozoic and Paleozoic age can only be found deep beneath the surface (about 200 metres beneath the surface and deeper).
OUR STORY BEGINS ON THE SEABED
90 million years ago, in the Mesozoic period called the Cretaceous, the Geopark area was flooded with a shallow sea. Its bottom was gradually covered with sediments of quartz sand and gravel which over time, as a result of the weight of the overburden, turned into a solid rock called sandstone, with a thickness of up to several hundred metres. After the retreat of the sea (about 65 million years ago), the sediments remained here as part of the Czech Cretaceous Plain.
At the end of the Mesozoic and in the Tertiary, tectonic pressure led to the fracturing of sandstone plate in many places. The resulting cracks were filled with hot magma from deep within the Earth, hardening in the form of rocks (polzenite, basalt), which are much harder than sandstone and thus more resistant to erosion. For this reason, they form the basis of most local steep hills.
EXPOSED TO THE ELEMENTS
In the Quaternary, today’s soil cover was formed. The sandstone plain was exposed to external forces (water, temperature changes, frost, activity of organisms) that literally modelled it into an intricate mosaic of shapes. This is where you can now see the “sculptures” of nature – rock towers, rock gates, rock clocks and honeycombs.