Phenomena Due to Weathering and Erosion of Granite in Northern Part of the Nová Bystřice Hills
Authors: Votýpka Jan 
Published: November 2023

The northern part of the Nová Bystřice Hills - situated in the Javořice system - is composed of two types of granite. The Čímeř type (called "Einsgarn type" by austrian geologist) is two-mica; middle to finely-grained granite. The Landštejn type is two-mica, coarse-grained granite, porpháric in places. In the area under investigation two types of phenomena due to the weathering of granite may be distinguished. Phenomena dating from the warm Tertiary climate display rounded forms, in some inistances also surface sculpturing (rocky dishes). There are loaf-shaped boulders of considerable size (ranging from 2 m to 12,5 m), the size of individual boulders being closely dependent from the jointing of granite (sheet to rectangular jointing). All these boulders are in fact more resistant "cores" of granite which have not been chemically affected and as such have survived in the weathered out mantle. At a later date, the weathered material was removed by denudation and consequently the "cores" got to the surface. Some boulders were torn by mechanical activity of fros in Pleistocene. Phenomena datinf from the Quarternary were sculptured by mechanical weathering in Pleistocene and only slightly altered in Holocene. Phenomena which originated in the Periglacial era of the Pleistocene are represented here by psudocirques and rock currents. This paper describes three pseudocirques. The height of their hinder wall makes 8,5 m to 15 m. The diameter of the dish-shaped depressions ranges from 17 m to 130 m. Two of them are orientated to the North and one to the West. All of them occur at an altitude of 630 m. Rock currents streaming from them reach the length of 120-700 m, their width ranging from 40 to 150 m. More often occur smaller phenomena - the socalled frost scarps and stone fields - which are due to the frost-weathering. Frost scarps occur only at altitudes over 600 m. Their height ranges from 1,80 to 11,5 m. The dip angle of the frost scarps makes 75° to 90°: Sometimes part of the wall hangs over. In the Čímeř type of granite, scarps are higher and more regular than in the Landštejn type. Three types of frost scarps may be distinguished. The first forms a "step", the second a morphologically characteristic wall, and the third type is irregular. The orientation of the scarp depends before all upon the prevailing direction of jointing. Tone fields cover almost all areas situated above 600 m. In the Čímeř granite type boulders are smaller, sharp-edged, mostly covered by a continuous mantle of vegetation. On the other hand, in the Landštejn type, boulders are larger (up to 4,5 m) and partly rounded. Blocks of stone currents and stone fields have been transported by solifluction. In Holocene slight sculpturing of past phenomena took place. A characteristic feature of this area are "rocky dishes" with their outlets. Four stages may be distinguished in their development according to general sculpturing. In the youngest stage a primary depression originated. In the second stage the process of deepening and formation of vertical to overhanging walls took place. The third stage is characterized by the origin of a run-off rill and its gradual deepeing down to the level of the floor of the depression. The later passes into the last stage of extinction since the water cannot stagnate any more. When determining the age of the dishes, we must consider their size (diameter and depth) and the general sculpturing of the very boulder in which they are situated. In the area of question, there are dishes dating from the end of the Tertiary (mostly torn by frost in Pleistocene) of depth of 85 cm, and dishes dating from the olecene and reaching the depth of as much as 40 cm. In the Holocene, slight sculpturing took place characterized by the origin of "rill lapies" and by further widening of all former fissures and cracks.