Sudetic Tors Formed under Periglacial Conditions

Abstract

The flat-topped low ridges (ca 640 m above sea level) of the SW part of the Kaczawa Mts. in the vicinity of Bolków (West Sudetes), built of Cambrian and Sylurian greenstones, were nunataks during the Pleistocene glaciation. They were, therefore, subject - until the Holocene - to an almost uninterrupted influence of the periglacial climate. But the most visible traces have been left here by the periglacial conditions of the last glaciation.

This is confirmed by the specific land-forms and covers. There are frost-riven cliffs, tor-like forms, rubble fields, small altiplanation steps and terraces, and characteristic interrelated block-, loam-debris and loam-silty covers. Such facts prove the intensity of frost weathering of the greenschists as well as a vivid congelifluxion and downwash. The strongly schistose greenstones constitute a material susceptible to exogenic processes.

The distribution and the type of land-forms and covers of this area are in close relation with the geologic structure and the initial morphologic situation.

On the basis of numerous references and of the common occurrence of those forms, the periglacial genesis of tor-like forms of metamorphic rocks has been established. Metamorphic rocks have a specific system of fissures particularly favourable to frost weathering; the weathered rock-material easily underwent processes of slope transportation. In less cracked and thus more resistant places the frost cliffs, parallely backwearing, formed series of isolated tors, later modelled by processes of nivation and subaerial weathering.

At present, those relict forms are masked by Holocene accumulation and by low- and high-growing vegetation. The final morphogenic stamp on this area has been impressed by human activity: fields and pastures have contributed to the fixation or the obliteration of some Pleistocene land-forms.