Where is the site located?
GPS: 49°52’4.858″N, 15°56’49.658″E
The area lies in the eastern part of the Iron Mountains National Geopark, where the curved margin of the East Bohemian Table in the northeastern environs of Vrbatův Kostelec passes to the Iron Mountains in the southwest. The site lies at about the midpoint of the trail along the Žejbro Stream, which begins at the foot of Kostelecká hůra Hill and ends at Podskála. The picturesque ambient countryside provides an insight into different intervals of geological history at several sites.
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. The basement is formed by the Železné hory plutonic complex, which is overlain by Upper Cretaceous sediments of the margin of the Bohemian Cretaceous Basin in the northeast. The text below discusses Lower Paleozoic magmatic rocks and younger sedimentary rocks left behind by Mesozoic sea, and subsequent processes affecting the present look of the landscape.
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 – Laurasia 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. This mountain range became 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.
– 100–95 million years
In the course of the Mesozoic, the supercontinent of Pangea successively split into separate tectonic 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, controlled by the greenhouse effect which induced a progressive rise of global sea level. Flooding occurred over a large portion of dry land, first by freshwaters of rivers and lakes. Later on, a large part of the Bohemian Massif became a part of a shallow sea passage. This site was located in a shallow near-shore zone, locally dominated by sand, densely colonized by representatives of bivalves, gastropods, crustaceans and fish.
What does the site display today?
Southern to southeastern vicinity of Vrbatův Kostelec is composed of magmatic rocks of the Železné hory plutonic complex, represented here by reddish-brown fine-grained granodiorite (a rock related to granite) of the Nasavrky pluton. The principal light minerals of this rock include quartz and feldspar, dark minerals are represented by flakes of dark mica – biotite. In its top portions, this rock was subjected to a special type of weathering, called lateritization. This type of weathering is typical for warm and dry climates, indicating desert environment. Rocks of the Nasavrky pluton were exposed to lateritization in the Late Paleozoic and in the earliest Mesozoic. Granites were rapidly disintegrated and their original minerals were changed in their chemical composition: most notably, feldspars were changed into kaolinite (a clay mineral), and the wearthering horizon was enriched in aluminium compounds which give the rock a typical reddish colour. Dykes of gabbro, emplaced (intruded) in the host granodiorite, are exposed within a complex of magmatic rocks near the Žejbro Stream in the northern vicinity of Vrbatův Kostelec. They can be identified on the basis of their dark greenish-grey colour. Gabbro is fine-grained and differs from granodiorite in its mineral composition: it consists mostly of minerals hornblende, actinolite and feldspars. Farther to the north to northeast, these basement rocks are overlain by Upper Cretaceous sediments of the Bohemian Cretaceous Basin. The stratal succession starts with sand-dominated sediments of the Korycany Member, which is Cenomanian in age (the first age of Upper Cretaceous). They comprise light yellow to rusty quartzose sandstone with clay to kaolinite cement, often containing mineral limonite. They were derived from sands deposited in shallow near-shore zone of the sea. They occasionally enclose remains of fossil shells and contain frequent interbeds of claystone and carbonaceous claystone with organic matter and coal. They have been preserved in a narrow strip lining the whole southern and western slopes of Kostelecká hůra Hill. Younger sediments pertain to the Bílá hora Formation, comprising fine-sandy siltstone (opoka) of Lower Turonian age. At the site of Podskála, an interval with this rock forms a long, steep series of outcrops. The cliff on the right-hand stoss bank of the Žejbro Stream is about 300 m long and over 40 m tall. The present position of Upper Cretaceous sediments results from tectonic forces and erosional processes, which were in effect during further geological history in the Tertiary and Quaternary. The original horizontally lying beds of loose sediments became consolidated, bent and fractured into separate blocks. They were shifted against one another: some of the blocks were uplifted, other blocks subsided. The stream of Žejbro made use of the progressively weakened zone in the rocks, incised its channel and created an erosional valley. Upper Cretaceous sediments constitute an environment favourable for the creation and accumulation of groundwater. A two-aquifer hydraulic system can be recognized here. The first aquifer is represented by Cenomanian sand-dominated sediments, which hold water in pores among the individual grains. Moreover, these sediments are tectonically deformed at specific places in the Žejbro Stream valley, where surface water seeps under the ground along joints. The second aquifer developed in joints of the younger Lower Turonian opokas (spiculitic marlstones) where water is retained within the joint system. The recharge area is located between the villages of Vrbatův Kostelec and Skála. Groundwater contained in the opokas is drained through strata-bound springs at the site of Podskála.
What was affected by man?
An absolute majority of geological phenomena around Vrbatův Kostelec can be studied on natural outcrops, formed without any intervention of man. The main job in the discussed area has been done by tectonic forces, erosive energy of water, and weathering. Small-scale excavations like shallow test pits or trenches at a few sites were sufficient to expose the rocks. Simple water sources, such as spring reservoirs or wells, were constructed to retain groundwater. Some of the factors adversely affecting interesting geological phenomena, such as the overgrowth of geosites by vegetation, can be still controlled by man. What lies beyond human possibilities, is to control perpetual weathering and erosion, which primarily inflict scaling of the outcrops of opoka (spiculitic marlstone).
What was discovered?
The fine geological phenomena in this area were noted by scientists in the early 20th century already. The book Příroda Chrudimska a Nasavrcka. Díl I. (Nature in the Chrudim and Nasavrky areas. Part I), published in Chrudim in 1906, reported the occurrences of local magmatic rocks in the chapter Doba žulová (Granite times), mentioned the frequent red coloration of sediments around Kostelecká hůra Hill, and described the finds of fossil shell accumulations in boulders of Cenomanian sandstone in the Žejbro Stream (the numerous oyster shells were called “holubinky”). The book also makes a note on the common complete drying of the stream at this place in summer months. A major geological survey of the Nasavrky pluton of the 1950s reported the most extensive lateritic weathering of granodiorite from the slopes of Kostelecká hůra Hill. It resulted in the formation of a layer of reddish-brown laterite max. 5 m in thickness. Attention was also given to gabbroic rocks in an erosional valley north of Vrbatův Kostelec: spectacular orbicular gabbro was discovered here (a rock in which layers of minerals are arranged into spherical structures). A sandstone cliff representing a fossil sandy beach of ancient sea was documented in immediate proximity of the road from Vrbatův Kostelec to Skála. Photographic documentation from the 1950s even covers a place displaying petrified ripple-marks developed in the sand of the Late Cretaceous sea. In 1970s, finds of boulders of Cenomanian sandstone were confirmed in the Podskála area; they contain abundant accumulations of fossil shells mostly belonging to gastropods and bivalves, and locally even plant remains.
“The Iron Mountains – a geologically significant region” project of 2014
Two information panels were manufactured within the project of “The Iron Mountains – a geologically significant region”. They were installed next to a sandstone outcrop at the northern limit of Vrabtův Kostelec, in immediate proximity of the road from Vrbatův Kostelec to Skála (GPS: 49°51’38.189″N, 15°56’17.620″E). The outcrop was stripped of vegetation and native arborescent plants, and it was cleaned. The site of Podskála is located some 1000 m north to northeast of the panels.