Předboř near Maleč


site 36

Where is the site located?

49°46’6.556″N, 15°42’9.561″E

The area lies in the southern extension of the Iron Mountains National Geopark, on southwestern slopes of the Iron Mountains which progressively descend to the Doubrava Basin with the winding Doubrava River. The close vicinity of the villages of Maleč, Jeníkovec, Předboř and Lhůta provides important sources of groundwater, which were installed by our ancestors in perfect quality and which serve their purpose until these days. This site is located near the Ostrovec groundwater source, within sight of the Blatnice well.

What is the geological position of the site?

The site is located in the Bohemian Massif, in the southeastern part of the Bohemian Cretaceous Basin. The basin fill has been preserved along the whole southwestern margin of the Iron Mountains in a structure called “the Long Furrow Cretaceous”. Upper Cretaceous sediments were modelled by subsequent geological processes, creating a structure very favourable for the accumulation and utilization of groundwater.

What happened at this site in the past?

– 95 million years

In the course of the Mesozoic, the supercontinent of Pangea was falling 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, controlled by the greenhouse effect which induced a rise of global sea level. Flooding occurred over large portions of dry land.

In the Late Cretaceous (a part of the last period of the Mesozoic), this site was a shallow sea passage, progressively flooded by warm sea waters. It was located in a shallow near-shore zone, locally dominated by sand, abundantly colonized by bivalves, gastropods, crustaceans and fish. With prolonged flooding, progressive deepening of the depositional area occurred.

What does the site display today?

This site is built by sediments of Upper Cretaceous age, which constitute the extension of the Bohemian Cretaceous Basin elongated along the southwestern slope of the Iron Mountains as the Cretaceous of the “Long Furrow”. They comprise formations of largely marine sediments, originally unconsolidated, which were subjected to diagenetic changes (hardening) through geological time.

The deepest sediments are fine- to medium-grained sandstones and claystones of marine origin, around 10 m thick, pertaining to the Korycany Member. This unit is of Cenomanian age (the first age of Upper Cretaceous). These are overlain by ca. 20 m thick succession of marlstone and sandy marlstone of the Bílá hora Formation, deposited in the Early Turonian (the oldest part of the second age of Upper Cretaceous).

The original almost horizontally lying formations left behind by Late Cretaceous sea were later affected by tectonic forces, responsible for their present position. The pile of sediments was divided into separate blocks, and mutual movements of these blocks occurred along faults. The largest displacement occurred at the Železné hory Fault, along which the block of the Iron Mountains was uplifted by several hundred metres.

Most of the sediments have been removed by erosion. The remaining strata are somewhat bent, forming an asymmetrical concave-upward structure limited by the Železné hory Fault against the Iron Mountains.

Upper Cretaceous sediments in this area display favourable characteristics for the accumulation and transport of groundwater. The individual formations contain rocks with aquifer properties, more or less naturally separated from each other by rocks with sealing properties, almost impermeable for groundwater.

As such, they constitute a two-aquifer hydraulic system. The lowermost aquifer A is bound to sandstones of Cenomanian age, followed by aquifer B formed in sandy siltstones of Lower Turonian age. Groundwater flow is controlled by faults to a high degree: each block of Upper Cretaceous sediments has an independent regime of groundwater circulation. The Maleč Block has been defined in the Maleč area. A certain proportion of groundwater in this block comes from the adjacent crystalline rocks, being guided by faults. Drainage of groundwater from both aquifers in this area was mediated primarily by strata-bound springs.

What was affected by man?

The history of groundwater utilization on this side of the Iron Mountains reaches back to the first half of the 20th century. Construction of the Blatnice well was completed at around 1935 by the respectful Chmelík Company. As suggested by archival data, inflow of groundwater was so strong that it could not be eliminated by two steam pumps. The well, 2 m in width, was lined by bricks. The technical device above the well has retained the character of a small, ornamental house until these days. It reminds us of the times that even such small buildings were constructed by skilled workers in consideration of aesthetical aspects. Construction of the Ostrovec withdrawal centre was also completed by the Chmelík Company. It is an object of a different technical type, called withdrawal incisions. The first incision dates to 1935. As it failed to draw all water, an additional incision ca. 45 m long was realized in 1965. Water from the incisions is drained to, and accumulated in, a two-chamber spring reservoir.

What was discovered?

Over years, these objects and their whole vicinity were subjected to several campaigns of hydrogeological survey designed to answer the questions on the origin and flow of groundwater, its protection and quality. The surveys also suggested, where possible, the potential of the area for additional yield of groundwater to be withdrawn.

The Blatnice well mostly withdraws groundwater of the first aquifer – aquifer A, while spring incisions at Ostrovec withdraw groundwater from the higher-positioned aquifer B.

Both water-supply objects enter a wider system supplying water for the Chotěboř area and a part of the Havlíčkův Brod area.

“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 by the road between Předboř and Jeníkovice, in the proximity of the closed area of the Ostrovec spring. The Blatnice well is located some 400 m south of the panels. Access to the withdrawal areas is permitted only when accompanied by the operating company staff.