Geomorphological Development of the Area of Bělá nad Radbuzou
Authors: Lochmann Zdeněk 
Published: October 2023

The area under investigation of an extent of about 97 km2 takes up the northwestern part of the Chodské hills in the drainage area of the upper portion of the Radbuza, and their transition to the southern extremity of the Tachov basin (Photo 1). The valley network in the area has been arranged along three structural directions. In the north-north-east to south-south-west direction (the oldest direction) flows the majority of tributaries to the Radbuza, skirting the foot of the amphibolite ridges projecting individually from mica schist gneiss series of the Domažlice Crystallinicum. There are two tectonic directions which have affected the development of the valley network. In the first case it is the faulted zone of the Bohemian quartz range (Quartzphal), and the parallel fault of the Mariánské Spa. Both of them are geomorphologically conspicuous tectonic lines of the first order bordering the Cheb-Domažlice basin. Their direction (north-north-west to south-south-east) influences the course of several brooks (the Bezděkov brook, the middle course of the Slatina, sections of the Radbuza, the Křakov brook). The second tectonic direction (the youngest) runs from west to east. It gave rise to those valley sections of the Radbuza which originated in places of amphibolite ranges (Plate 1). West of the Dehetné (at altitudes of 488-492 m and 495-497 m) and northwest of Borek (at an altitude of 470 m) fragments of Palaeogene peneplain were ascertained which at the time of origin of the asymmetric fault gap of the Tachov basin subsided to its bottom. More extensive remains of the peneplain have been preserved in the neighbourhood of Bor, some 10 km north of the Dehetné. It is situated at an altitude of 470-487 m. Denudation plateaus in this area form either plains on watersheds or in gentle slopes. Terrace plains have been preserved only in fragments (the Radbuza, the Slatiny brook, Slatina). An outstanding feature of the valley network are abrupt structural slopes typical of the valley asymmetry. Their inclination makes more than 15°, where as opposite slopes usually makes 5°, maximum 10°. Abrupt denudation slopes (over 10°) form the transition from monadnocks to gentle denudation slopes (up to 10°) which prevail. Out - standing structural phenomena are amphibolite ranges projecting individually from a series of softer gneiss and mica schists, and quartz monadnocks which have survived as fragments of the filling of the tectonic line of the Bohemian quartz range (between Újezd Sv. Kříže and Pavlíkov). In the depression between Slatina and Štítary at the foot of quartz monadnocks remains of fossil mantle rocks were ascertained. Granites, mica-schists and mica-schist-gneiss were capped with white kaolin waste, whereas on amphibolites red-brown waste similar to laterite horizons has been preserved. Judging from physico-chemical analyses and the morphological position of the mantle rocks, we have come to the conclusion that in both cases it is the question of kaolin waste developed at the same time. Their different colour and different proportion of basic chemical components are due to the heterogeneity of parent rocks. According to the results of palynologic analyses of the surrounding Tertiary sediments - under which the fossil mantle rocks crop out - the upper line of weathering may be placed to the Latest Miocene. From the time of the extraglacial Pleistocene climate frost scarps and small stone fields have been preserved on top of the monadnocks. Survey of the geomorphological development: The development of the relief of the north-west part of the Chodské hills was influenced by the fault tectonics and by changes in the resistance of rocks building the Domažlice Crystallinicum. From the series of epizonal and mesozonal metamorphous mica schists and mica schist gneiss, amphibolite material was removed by selection. At the present they form structural ridges running from NNE to SSW. They also enabled the origin of a network of subsequent tributaries to the Radbuza. In the Saxonian reactivation of Varisian (as well as older) faults took place. It gave rise to a valley predisposition of some brooks (Bezděkov brook, Křakov brook) or sections of their course. Their direction coincides with the Bohemian quartz range. At the same time a new west-east direction comes into being, interrupting in places the old tectonic lines. This direction also displays the tendency to valley formation. Abrupt structural slopes are gradually subject to denudation. Along the fault of the Mariánské Spa a subsidence took place in the Saxonian of the Palaeogene peneplain, remains of which have been preserved north and north-west of Borek and west of Dehetné (southern margin of the Tachov basin). Through the subsidence of these peneplain the lower erosion base of the brooks was lowered down; consequently, their erosion activity against the relatively higher relief of the Chodské hills was revived. Fossil weathering - which affected the whole area - gave rise to a thick eluvial mantle which was then gradually denuded. Its remnants have been preserved in several places (Slatina, Štítary, at the foot of monadnocks). The periglacial climate demonstrated itself in the destruction of rock outcrops and in the formation of small stone fields. The modelling of the valley network was finished in the Pleistocene. From the geomorphological analysis of the surface phenomena it becomes evident that the relief in the area of Bělá nad Radbuzou is polygenetic since a whole series of endogenous and exogenous processes took part in its modelling. We can therefore consider it a tectonically affected denudation relief.