site 26

Where is the site located?

GPS: 49° 53′ 54.14″ N, 15° 49′ 22.72″ E

The area lies in the central part of the Iron Mountains National Geopark, near its northern border. It also lies near the boundary between the East Bohemian Table – a flat region towards Chrudim – and the Iron Mountains, which constitute the forested slope in the direction to Nasavrky.

What is the geological position of the site?

The site is located in the Bohemian Massif, in the marginal area of the Bohemian Cretaceous Basin. The rocks discussed in the text below witnessed the last sea flooding in the latest Mesozoic.

What happened at this site in the past?

– 95 million years

In the course of the Mesozoic, the supercontinent of Pangea fell apart into separate lithospheric plates, which drifted atop the elastic asthenosphere.
In the latest Mesozoic, this area was lying on the Northern Hemisphere, becoming a part of the Eurasian Plate. The climate was very warm, being referred to as the greenhouse effect. This induced the rise of global sea level and, consequently, flooding of a large portion of dry land.

In the Late Cretaceous (Cretaceous is the last period of the Mesozoic era), this area was a shallow sea passage, which progressively turned into a shallow and warm sea. This site was located on a sandy beach with protruding shoreline cliffs, breaking the waves. The sea was colonized by various bottom-dwelling organisms, such as bivalves (e.g., oysters) or brachiopods, and different kinds of crustaceans. Sharks were among particularly abundant sea inhabitants.

What does the site display today?

The southern vicinity of Škrovád is composed of Upper Cretaceous sediments on the margin of the Bohemian Cretaceous Basin. The basin is filled with sediments deposited by the sea in northern, central and eastern Bohemia, i.e. in the region which subsided along faults of the so-called Elbe Zone.

Sediments in the Škrovád area are Cenomanian in age (Cenomanian was the first age of Upper Cretaceous) and are classified within the Korycany Member. This means that they witnessed the first marine flooding in the Late Cretaceous times: they contain grains of mineral glauconite – a green mineral indicative of marine environment. These sediments, now preserved in a thickness of about 10 m, overlie much older rocks of the so-called Lukavice Series. Towards the south, the latter rocks form the neighbouring body of porphyroids, porphyries and sericitic schists.

Cretaceous rocks at this site are dominated by light yellow to yellowish-grey quartzose sandstone, called Quadersandstein (pillar-jointed sandstone) by its typical orthogonal, thickly bedded disintegration.

The sandstone is coarse- to medium-grained, locally with a prominent gravelly admixture; this evidences deposition at places with stronger sea currents. The originally unconsolidated sand-dominated sediments have been consolidated through geological time – they were subjected to diagenetic processes. Sandstone is composed of quartz grains; its matrix is calcareous, clayey or ferruginous. It is light green to brownish-yellow in colour, rusty brown pigment is due to the presence of iron oxides.

What was affected by man?

Today, the sites of former intensive extraction of sandstones around Škrovád are difficult to distinguish. Former quarries now function as a natural recreation area with weekend-houses. Sandstone cliff faces are used as a training range by rock climbers, who designed climbing paths of various degrees of difficulty on the cliffs. The artificial origin of the faces is indicated, among others, by their almost perfect planar surfaces. Such surfaces result from the most common method of quarrying – cutting by giant frame saws.

The oldest records of sandstone quarrying at Škrovád date to the 16th century. As much as 10 quarries, now abandoned, were operated here between the mid 18th and mid 20th centuries. The quarries were manually operated, with faces mostly 6–8 m tall.

Sandstone was usually used as building stone and material for raw stonemasonry. If homogeneous blocks happened to be isolated from the quarry face, they could be used for challenging works in architecture and sculpture. Sandstone from Škrovád was frequently used in Chrudim: its ashlars can be found at several places in city walls and were largely used for the construction of the archbishop Church of the Assumption of Virgin Mary on the main square in Chrudim.

Sandstone utilization in architecture can be documented by the panels with figural and ornamental elements used for the decoration of the facade of the Mydlář House and by the stone portal of the Khom House on the main square in Chrudim.

The most notable trace of sculptural use of the sandstone is the Column of Transfiguration on the Chrudim main square – a Baroque plague column created by the local stonemason school in 1719–1732.

What was discovered?

In the early 20th century, the Škrovád quarries were still in operation. Nature scientists of that time had the opportunity to have a look into the rock interior – an opportunity we are lacking today. Studies and discoveries of that time were recorded in written reports and photographs, which have been preserved till today. The key work for the documentation of geological setting was the book Chrudimsko a Nasavrcko. Díl I. Obraz přírodní (Chrudim and Nasavrky areas. Part I. Nature) authored by teachers from Chrudim and published in 1906.

As has been recorded in this book, a bed rich in calcareous shells of bivalves and sponges and in shark teeth was lying in the top part of the sandstones. Cavities were locally filled with specific fine material, which was called “mozekˮ (brain) by the stonemasons. In the book, it was reported as presumable remains of sea weed. Today, it is explained as a complex of tunnels produced by marine organisms like crayfish and crabs. The most abundant fossil fauna concentrated to the lowermost bed of the sandstones. It yielded shells of typical Upper Cretaceous bivalves of the genus Inoceramus, teeth of a big shark and an imprint of the base of a shark vertebra.

“The Iron Mountains – a geologically significant region” project of 2014

An information panel was manufactured within the project of “The Iron Mountains – a geologically significant region”. It was erected in the centre of the Škrovád village, near the pub. The most spectacular outcrops lie about 500 m to the south, along the Chrudimka River.

Baroque plague column on the square in Chrudim (Daniel Smutek)