site 40

Where is the site located?

GPS: 49°47’37.06″ N, 15°34’53.81″ E

The area lies in the northeastern part of the Hornosázavská Hilly Land, near the border with the Iron Mountains which form a forested line in the northwest. This site is located in largely agriculturally used land. Its origin is connected with a specific basement rock, which was transported from great depths of the Earth, and with the activity of quarrymen.

What is the geological position of the site?

The site is located in the Bohemian Massif, at the boundary between two major regional units – the Moldanubicum forming the basement in the southwest, and the Kutná Hora Crystalline Complex in the northeast. The contact between the units is a tectonic one, mostly hidden beneath younger, Upper Cretaceous sediments of the Long Furrow, deposited along the Doubrava River. The basement of the two units mostly consists of metamorphic rocks here.

What happened at this site in the past?

– 550 million years

The end of the Proterozoic and the beginning of the Paleozoic is marked by a rearrangement of lithospheric plates, resulting in the Cadomian Orogeny. The original rocks, produced by marine sedimentation and submarine lava effusions, were subjected to metamorphism by high pressures and temperatures. A deep-reaching fracture originated in the Earth crust, which affected geology of the region for a long time. Fragments of magnesium-rich ultrabasic rocks were lifted up from the depths of the Earth mantle during the orogenic processes.

What does the site display today?

The Borek area is formed by metamorphic rocks of the Moldanubicum, which result from regional metamorphism. Grey to greyish-brown paragneisses contain minerals like biotite and sillimanite, locally also muscovite and garnet. The gneisses host an oval body of serpentinite and serpentinized peridotite – magnesium-rich abyssal magmatic rock subjected to a specific type of alteration (serpentinization). This rock is dark grey to black in colour, and displays a typical net texture. The southwestern margin of the serpentinite body is formed by a tabular body of metamorphic rock eclogite. This is a distinctly banded rock with alternating rusty garnet-rich layers and layers rich in dark pyroxene (a mineral of the silicate group). This rock is highly durable.

What was affected by man?

The Borek Cliff – a natural outcrop of serpentinite – has been protected since 1956. Rare plant species restricted to serpentinite occurrences were protected as early as in 1927. At the beginning of World War II, a pit quarry was opened in the immediate proximity of the cliff. It was active until the 1990s. Fortunately, the quarry respected the nature in its close surroundings and did not inflict any serious injury to the natural serpentinite biotope. Quarried stone was crushed to aggregate in the quarry itself and used mainly for road construction. Water accumulated in the deepest part of the quarry had to be perpetually pumped; the inflow of water was strong for the whole lifetime of the quarry, and caused severe problems in quarrying. After the quarry had been abandoned in the 1990s, powerful pumps stood still and the quarry started to get flooded. The dusty pit of the quarry was turned into a lake of uniquely clear water, over 30 m deep. The lake is still getting bigger, flooding the adjacent areas.

What was discovered?

The opportunity to find good samples of local rocks and minerals is now reduced due to the advance of the lake. The geologists and mineralogists therefore passed this locality on to divers. Garnet, hydromagnesite, fibrous chrysotile and other specimens are housed in mineral collections now but the sites of their origin disappeared under water level. Today, the Borek Quarry is featured on Facebook where the fans of diving share the photos of their catch: karp, pike, jellyfish and other forms of aquatic life. They also document their sporting achievements in the discovery of sunken items, such as an articulated bus and others.

“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 is located in the western vicinity of the Borek village.

Serpentinite (Daniel Smutek)