| Summary: | The article depicts the problem of river system development during the Middle Pleistocene Interglacial in the Bystrzyca River Valley (Sudetic Foreland, south-western Poland). Ten research sites located within the Świdnica Plain are presented, in which the structural, grain size (granulometry), petrographic, quartz grain morphoscopy, and heavy mineral analyses were carried out. The study results show the formation of piedmont fan deposits 2–8 km to the NE of the Sudetic Marginal Fault. The location of the fluvial deposits between the Sanian and Odranian tills indicates that they were deposited during the Holsteinian Interglacial (Krzczonów Formation, Mazovian; see Table 1). According toxthe lithofacies analysis, vast alluvial plains, composed of angular gravel grains in the south and of sands in the north, were deposited in the Sudetic Foreland in the environment of a very dynamic river. They are covered with a discontinuous layer of Odranian till. The petrographic spectrum shows 90–99% of local rocks, namely, Sudetic porphyry, Sowie Mts Gneiss and milky quartz, and 1–10% of Scandinavian rocks. In the proto-Bystrzyca river system, the existence of an oxbow lake in the distal part of the Krzczonów fan has been proved, which was developing at the end of the Holsteinian Interglacial. The continuity of the alluvial deposits is interrupted in the vicinity of Świdnica due to both the tectonic movements and the formation of the narrow tectonic graben of Roztoka–Mokrzeszów.
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