Assessing the genesis of periglacial ramparted depressions through a macroscopic and microscopic analysis of their internal structures

Perennial frost mounds developed across northern Europe following retreat of the late Quaternary ice sheets (c. 23–19). Their relict forms comprise depressions with surrounding ramparts (periglacial ramparted depressions - PRDs). Although PRD surface geometry is well-documented, their origin is less well-understood. There is little agreement on: i) definitive identification of PRDs, ii) PRD formation processes, and iii) the relationship between different frost-mound types (i.e. pingo, palsa and lithalsa). For the first time, this research characterises the internal structure of a relict lithalsa in the Ardennes (Belgium-German border), at macro- (e.g. coring, logging) and micro-scales (thin sections) and contextualises this with observations on the hydrological, lithological and topographic setting. Micromorphology enables the study of sedimentary environments and processes of formation. This investigation identifies diagnostic suites of microstructures indicative of frost action, landform development and environmental setting. The results are then applied to suspected PRDs in Norfolk (Walton Common) and Wales (the Cledlyn Valley), for which a likely frost-mound origin is confirmed. This approach: i) identifies the internal structure of PRDs, ii) considers the potential for change in deformation with depth and lateral extent within the rampart, and iii) considers the differences and similarities in micro-textures and structures in a variety of grain sizes across the sites where PRDs occur. Key microstructures identified, indicative of cryogenic origins, include: i) a vertical to subvertical microfabric (e.g. frost-jacked grains), ii) platy-prismatic, sub-angular aggregates, iii) planar deformation (e.g. fragmented domains, frost-cracked grains), and iv) evidence of pore-water movement on thawing of ice and associated grain translocation (e.g. silt and clay cappings). Microstructures attributed to PRD development include: i) a sub-vertical microfabric of similarly inclined elongate grains, associated with tilted strata, ii) microstructures linked to mass-wasting during frost-mound growth or rampart formation (e.g. grain concentrations, grain coatings of silt and clay, curvilinear grain arrangements, skelsepic plasmic fabric), iii) planar structures (e.g. grain lineations, linear concentrations of grains and fragmented domains and fractured grains, that may reflect shear strain during rampart-formation processes), and vi) multiple domains, interpreted as re-homogenisation of sediment caused by frost-mound heave, and subsequent rampart-formation processes. Consequently, this research identifies and characterises PRDs, which: i. provides a better understanding of the genesis of PRDs, for the classification of different types of ice-cored hills, ii. informs palaeoenvironmental reconstruction, since ice-cored hills are diagnostic of former permafrost (frozen ground conditions), iii. informs civil engineering projects where sediments are disturbed by PRD development (e.g. heave and subsidence)