site 20

Where is the site located?

GPS: 49° 49′ 28.47″ N, 15° 49′ 51.68″ E

The site lies in the centre of the Iron Mountains National Geopark, in the granite core of the Iron Mountains. The area of the Ctětín Quarry is used not only for the production of stone but also reveals interesting geological phenomena.

What is the geological position of the site?

The site is located in the Bohemian Massif, in the marginal area of the Central Bohemian Region which is represented by the Železné hory plutonic complex at this site. This text is therefore focused on rocks which constitute a complex magmatic body solidified at great depths. By the effect of geological processes and geological time, this body got to the proximity of the earth surface.

What happened at this site in the past?

– 350–300 million years

Carboniferous and earliest Permian times (the fifth and sixth periods of the Paleozoic era) were marked by unrest due to major orogenic processes (Variscan Orogeny). Progressive collision of two continents – Laurussia in the north and Gondwana in the south (this area lies at its margin) – created the supercontinent of Pangea. This area became a part of a large mountain range of the Variscides (Hercynides), which extended from todayʼs Spain across southern England, France and Germany, as far as to central Europe, becoming a solid basement of the Bohemian Massif. The orogenic processes resulted in bending, fracturing and shifting of the existing rock masses, inducing the rise of huge volumes of hot magma. Some magma batches ascended along deep-reaching fractures and reached the earth surface. Other portions of rising magma remained under the surface and constituted large, loaf-shaped bodies.

What does the site display today?

Rocks around Ctětín belong to the Železné hory plutonic complex – an approximately triangular body lying among the towns of Seč in the east, Skuteč in the north and Trhová Kamenice in the south. This site is located in its northern part, pertaining to the younger Nasavrky pluton. The rock exposed here is granodiorite, a rock related to granite, called the Skuteč granite by the stonemasons. In geological terms, the rock can be described as light grey fine- to medium-grained amphibole-biotite granodiorite. Its principal mineral components include light grains of quartz and feldspar (a silicate mineral), black grains of amphibole (a silicate mineral) and dark lustrous flakes of biotite (a silicate mineral). The magmatic body was constituted in several stages and is accompanied by dykes. One of the dyke rocks is pegmatite – a rock similar to granite but of coarser grain size, containing phenocrysts (larger grains or crystals) of quartz, light feldspar and biotite. The age of this rock was determined at approximately 360 million years.

What was affected by man?

This is an area with a long tradition of granite extraction and subsequent stonemason production. A number of bigger or smaller quarries were left behind by our ancestors here. At this site, a pit quarry ca. 300 × 200 m in size finally originated, supplemented with a stone processing unit. The quarry was repeatedly abandoned in the past, and the extraction was last re-started in 2007. The extracted material is cut into paving stones of various sizes at this very place. Larger compact blocks are dimensioned for the production of large stonemasonry segments to be used, for example, in garden architecture.

What was discovered?

The large pit quarry is a scar in the landscape. Nevertheless, it provides the geologists with an opportunity to study mutual relationships of various kind. The exposed quarry face displays a system of joints characteristic for granites, sometimes referred to as granite-tectonics. It consists of mutually perpendicular joint sets, which render easier disintegration and subsequent processing of stone.

The rock is, of course, not evenly jointed at all places. Zones of strong deformation can be observed, where the weakened rock mass was subject to stronger pressures, contrasting with areas of rather compact rock. The joint system in rocks is measured by geological compass and the results are then used for the study of geometries and histories of rock bodies.
The top of the quarry face provides an instructive example of the effects of erosion on a granite body. The origin of eluvium can be observed. Eluvium is the weathering product of the rock resting at the place of its origin and gradually passing into solid, unweathered substrate below.

At its top, eluvium is represented by completely disintegrated granodiorite, having more or less the appearance of clay (feldspar grains easily weathered to clay minerals) with quartz grains (quartz is resistant to weathering). Somewhat deeper, loaf-shaped granite boulders appear in the weathered material. They tend to leave the place of their origin and start their way down the slope. Hard substrate is then formed by blocks of orthogonally jointed granite, showing a better roundness of the apices at edge intersections in top portions.

Some of the boulders display onion-like peeling of individual layers – a feature called exfoliation. Such spherical disintegration is particularly common in homogeneous fine-grained granites. This feature is attributed to elevated overburden pressures exerted on the original rock in the subsurface. As the rock appears on the surface and gets unloaded, microfractures become rapidly dilated and weathered. In this process, arrangement of the individual mineral particles plays a role. Another factor is the contrasting response of various mineral constituents to temperature changes, especially to insolation (some minerals expand to a higher degree than others if heated).

“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 at the entrance to the mining area from the road between Ctětínek and Nasavrky. The quarry is an active one, accessible only after a prior consent by the company operating the quarry.

A granodiorite block cleaved by wedges, 1955 (Jindřich Vodička)