Contribution to the Knowledge of the Genesis of Erosional Forms

Abstract

In the present paper the author presents a genetic analysis of a 430 m long "balka" depression developed in loess loams in the northeastern slope of the Kyšice Hill between Kyšice and Ejpovice in the Plzeň area. In consequence of an irregular development the longitudinal profile of the "balka" may be divided into three levels: catchment, erosional and accumulation level. The catchment (upper) level is bowl-shaped and was hollowed out solely in loess loams. Erosional level (middle), in which the "balka" reaches its greatest thickness (4 m), has been excavated partly in loess loams and partly in Tertiary clays which form the substratum of the loess mantle. The erosional level passes smoothly into the accumulation (lower) level characterized by the accumulation of rain wash and spring sediments. In the erosional part erosion cut through the loess mantle denuding the Tertiary substratum built of Oligo-Miocene impermiable clays. In this way also the lower water-level - situated between the Quarternary and Tertiary strata - was exposed, which resulted in the origin of copious springs at the foot of the slope of the "balka". In melting seasons and during heavy bursts of rain springs are continuously shifted backward eating into the slope of the "balka" and producing a wide and flat floor. Consequently the "balka" in question is the result of a strong erosion which in the second phase of its development changed into retrograde erosion exercised by the springs, and not into lateral erosion by a running stream as is usual with erosional phenomena of this kind. The author succeeded in discovering several other "balkas" of a similar origin in the neighbourhood of Tuchořice near Žatec.