Das Problem der Lößbildung und die Lößmollusken
Abstract. At present, the view is generally adopted that loess does not originate only by the accumulation of wind-borne dust, but also by a particular soil-forming process (loessification), that impresses to dust-accumulations typical features of loess, especially the structure, calcareous-ness and colour. Loessification effects the entire area of suitable regions, but requires special environmental conditions, which are not available in present-day Europe. This hypothesis is fully supported by the loess molluscan fauna showing a peculiar composition and including several species and races confined to loess deposits. Therefore, we are justified in speaking about loess assemblages in the ecologic sense and about loess environment, distinguished by specific climatic, sedimentation and soil conditions, as well as by a characteristic fauna and flora. The period of loessification, which could be called loess phase, differs sharply from other sections of the Quaternary climatic cycle. Eolian deposition and loessification are almost contemporaneous. The wind redeposits materials that are already partly loessified and undergo further loessification during and after their accumulation. This also holds for fine-grained deposits of non-eolian origin, particularly for slope and proluvial deposits; these assume a number of features consistent with those of loess, so that they can be termed loess-like (= loessoid) sediments. The malacofaunal contents of loesses and loess-like deposits are the same. It should be emphasized that during the loess phase sedimentation of eolian dust fully prevails, even in places where during other phases quite different processes, e. g. deposition of coarse screes, exist. This phenomenon along with a typical fauna evidences the particular natural conditions of the loess phase. In the higher altitudes and more humid areas loess is substituted by non-calcareous loams belonging to a different soil zone of the loess phase. In considering the sediments of the loess phases, distinction should be made between true eolian loesses and loess-like deposits originated either by loessification of fine-grained non-eolian deposits or by redeposition of loesses. The correlation of malacologic and sedimentologic results witnesses to the Tightness of V. A. Obrucev's hypothesis on the loess origin.