Geografie 1962, 67, 99-112
On the Geomorphology of the Northern Part of the Tachov Basin and the Bohemian Forest
In 1960 the author carried out a quarternary and engineering-geological investigation of the area that is represented on sheets M-33-73-B-b, d and M-33-74-A-a, b, c, d of the new state map of Czechoslovakia compiled in scale of 1 : 25.000 The present paper treats of the geomorphological results of this investigation carried out in the northern part of the Tachov Basin and the Bohemian Forest. The maped area measures 330 km2. The field work was started with the investigation of the peneplain of the Bohemian Forest which lies at an approximate altitude of 650-750 m. Individually preserved dominants, composed of quartzitic migmatitic gneiss protrude above the peneplain surface (Tišina, Ve skalkách, Štokovský vrch etc.). The basic structural lines of the peneplain body of the Bohemian Forest were formed as early as the Varisian Orogene. After the Varisian faults, movements were resumed at the time of the saxonian tectonic disturbance which made the whole mountain range assume its present character. Towards the end of the Varisian Orogene, old fault lines were filled with hydrothermal quartz or blocked with clastic material. The quartz filling of the faults has the character of morphologically prominant veins or grabens, stretching from the northern environs of Tachov over Tři Sekery (Three Axes) towards Dolní Žandov and over Hazlov to Saxony. Selective stream erosion affected before all healed-up fault lines of the Upper-Rhine (longitudinal) direction as places rendering the least resistance. Valleys of the consequent streams heading for the subsided Tachov Basin are younger. Along the young Marienbad fault line valleys were founded tectonically (sections of the Hamerský and Kosový Brooks). The author consideres the sedimentation depression of the Tachov Basin - filled in its northern part with Tertiary sediments (gravel, sand, clay) - to belong to the accumulation phenomena. Pleistocene terraces have been ascertained along the Hamerský and Kosový Brooks. Two levels could be distinguished (12-18 m and 7-12 m), the thickness of the terrace accumulations reachins 2-4 m. Chronologically they are most probably older than Würm. On their surface they are covered with loeses. The unique occurrence of volcanic pyroclastic rocks of Železná hůrka in the vicinity of Boden is also considered an accumulation phenomenon. The next chapter treats of wheathering processes. Pleistocene weathering asserted itself most strongly in the summit areas of the Bohemian Forest where it formed extensive stone seas. In the Borský Mass it deeply affected the surface granite. Besides the Pleistocene weathering also Tertiary weathering (Chotěnov, Chodová Planá, Hamrníky) took active part especially in chemical decomposition of the alumosilicates in the rocks, Decomposed rocks occur in the form of whitish clayey eluvium along the whole course of the Marienbad fault. They also occur in the vicinity of mineral water springs with exhalations of CO2. The last chapter is reserved for periglacial phenomena. It describes solifluction and cryoturbation which deforms the upper layers of the weathered mantle. Periglacial phenomena are rather scarce in the area under investigation.
