Pinpointing a Historical Dew Pond in the Kerekbükk Natural Lake Area, Cserhát Mts., N-Hungary by Geodetic Survey and Interpretation of a Medieval Boundary Charter

Abstract

The mesa-like volcanic Kerekbükk hill in the Eastern Cserhát Mountains (N Hungary) hosts numerous small lakes linked to postglacial landslides. Double Lake (Kettős-tó), near the mesa summit, occurs in a geomorphologically atypical position and has therefore been debated as potentially artificial, with uncertain hydrological behavior and function. This study aimed to answer these questions by integrating geodetic and geological field observations with archival historical sources. A detailed geodetic survey was used to build a high-resolution terrain model and reconstruct basin morphology to distinguish natural landforms from anthropogenic modifications. In parallel, we examined 13^th–14^th-century boundary charters and historical maps (18^th century to present) to assess landscape continuity and the persistence of local reference points. Double Lake comprises two basins within a shallow west–east trending glen: the western basin is a natural depression, whereas the eastern basin shows clear human modification, including artificial deepening and a small embankment dam. Resistant andesite bedrock enhances hydrological isolation and restricts incision, allowing water retention despite a minimal catchment. Cartographic representations vary through time, consistent with seasonal water availability and changing survey precision. Field reambulation indicates that a Medieval charter likely documents Double Lake as a stable boundary point since the early 14^th century. Overall, Double Lake is a hybrid feature produced by erosion and weathering of the lava cap, and human interaction for local water management, likely for the construction of a dew pond.