INHERITED LANDFORMS AND GLACIAL IMPACT OF DIFFERENT PALAEOSURFACES IN SOUTHWEST SWEDEN Geografiska Annaler · 83 A (2001) · 1–2 67 INHERITED LANDFORMS AND GLACIAL IMPACT OF DIFFERENT PALAEOSURFACES IN SOUTHWEST SWEDEN BY MAGNUS JOHANSSON 1 , MATS OLVMO 2 AND KARNA LIDMAR-BERGSTRÖM 3 1 Department of Earth Science and Geography, Karlstad University, Karlstad, Sweden 2 Department of Physical Geography, Göteborg University, Göteborg, Sweden 3 Department of Physical Geography, Stockholm University, Stockholm, Sweden Johansson, M., Olvmo, M. and Lidmar-Bergström, K., 2001: In- herited landforms and glacial impact of different palaeosurfaces in southwest Sweden. Geogr. Ann., 83 A (1–2): 67–89. ABSTRACT. Landforms are used as analytical tools to separate inherited features from the glacial impact on Precambrian base- ment rocks in southwest Sweden. The study covers three different palaeosurfaces, the sub-Cambrian peneplain (relative relief (r.r.) 0–20 m) with the character of a pediplain, an uplifted and dissect- ed part of the sub-Cambrian peneplain (r.r. 5–40 m) and an etch- surface (r.r. 20–135 m), presumably sub-Mesozoic. The surfaces were recently re-exposed, probably due to a Neogene upheaval with some pre-glacial reshaping. Strong structural control and no alignment with glacial erosional directions other than those coin- ciding with structures, are arguments for etch processes as a most important agent for relief differentiation. This is strengthened by the occurrence of saprolite residues and etchforms in protected positions. The glacial reshaping of the sub-Cambrian flat bedrock surfac- es is negligible. The glacial impact becomes more evident in the uplifted and dissected parts of the peneplain and within the hilly sub-Mesozoic surface. The higher the initial relief the more effect of glacial erosion on individual hills, both on the abrading side, with formation of roches moutonnées, and on the plucking side. Detailed etchforms are preserved in protected positions in spite of erosion by a clearly wet-based ice. The magnitude of the Pleis- tocene glacial erosion is considerably less than the amplitude of the palaeorelief in the entire area. Landscapes of areal glacial scouring have been described as comprising irregular depressions with intervening bosses scraped by ice and labelled ‘knock and lochan’ topography, but we sug- gest that an etched bedrock surface is a prerequisite for this type of landscape to develop. Key words: landforms, glacial erosion, palaeosurfaces, etching. Introduction In southwest Sweden the Precambrian basement is exposed over wide areas, probably due to wet- based conditions during the withdrawal of the Late Weichselian ice sheet (Rudberg 1970a). In the coastal zone glacial forms in the bedrock, such as roches moutonnées, p-forms, crescentic gouges, crescentic fractures and striae, are ubiquitous (Ljungner 1930; Johnsson 1956). From a glacio- geomorphic viewpoint, this area can be considered as a glacial landscape of areal scouring (Sugden and John 1976; Menzies 1997). In recent years the importance of a long-term perspective on land- forms in formerly glaciated terrain has been stressed. Hence it is argued that the bedrock relief in south Sweden is the result of tectonic and denu- dational events during distinct periods since the Late Proterozoic, rather than a record of glacial events during the Pleistocene (Mattsson 1962; Lid- mar-Bergström 1982, 1995, 1996, 1997; Lind- ström 1988; Olvmo et al. 1999). The relationship between relief in the Precam- brian basement of south Sweden and its cover rocks of different ages has been analysed in several pa- pers by Lidmar-Bergström (1982, 1988, 1995, 1996). Exhumed sub-Cambrian and sub-Mesozoic topography makes up large parts of the relief due to Tertiary uplift events and subsequent exhumation. The hilly topography in the Precambrian basement along the northern part of the west coast (Bohus County) is interpreted as an exhumed sub-Mesozo- ic etchsurface because the basement surface disap- pears below Mesozoic sedimentary rocks offshore, kaolinitic saprolite remnants occur onshore (Lid- mar-Bergström et al. 1999), and it resembles etched relief further south with remnants of Meso- zoic cover rocks. The hilly relief is replaced east- wards by the exhumed, extremely flat sub-Cambri- an peneplain, which in its western part is uplifted and slightly dissected along bedrock joints. The study has been performed along an E–W transect crossing the three types of topography: the well preserved parts of the sub-Cambrian peneplain, the uplifted and dissected sub-Cambrian peneplain and the sub-Mesozoic hilly relief (Fig. 1). This study aims at evaluating the qualitative and