The geological map sheet Sakskøbing on a scale of 1:50 000 comprises the topographical map sheet 1411 I and the southern part of the topographical map sheet 1412 II in southern Denmark. The map covers the central part of northern Lolland and the archipelago of Smålandsfarvandet to its north. The geological map sheet documents the distribution of Quaternary deposits in a region dominated by glacial dynamics related to the latest, Weichselian glaciation of southern Denmark. The map sheet area consists of lowland with elevations mainly between 0-10 m above sea level (a.s.l.) and shallow marine areas with elevations between 0-7 m below sea level (b.s.l.). The area of Birket and Ravnsby Bakker constitutes a marked exception to the flat landscape and includes the two highest points of Lolland, Bavnehøj and Ravnsby Bakker around 30 m a.s.l. Along the trend formed by Sakskøbing Fjord, the seabed of Smålandsfarvandet is incised by the up to 16 m deep trench of Lindholm Dyb - Ståldyb. Two shallower, parallel trenches are formed by Femø Sund and the extension of Guldborg Sund farther to the north-east. Although the topography of the map area is relatively flat, the depth to the top of the pre-Quaternary bedrock is quite variable. The thickness of the Quaternary deposits is greatest to the west, where it exceeds 50 m in the area around Lindholm Dyb between Fejø and Skifterne (Figs 14, 16), and decrease to only 5 m between Sakskøbing and Havløkke in the centre of the map area. The bedrock predominantly consists of Maastrichtian chalk, which is cut by several SE-NW-striking faults along elongate horst and half-graben structures (see inset figure Depth to top of chalk on the map sheet). Bryozoan limestone of Danian age with thicknesses of up to 20 m is preserved in the Nøbbet, Kragenæs and Femø Sund grabens. The most pronounced horst is the Merretskov Horst, where Cretaceous chalk occurs 0 m a.s.l. (Figs 14, 16). During the Mesozoic and Cenozoic the map area was under influence by salt tectonics as well as fault movements along the southern margin of the Ringkøbing- Fyn High (Fig. 3). The predominant SE-NW direction of the Mesozoic-Cenozoic faults re-occurs in the Quaternary geomorphology, suggesting Quaternary rejuvenation of the fault system. The Quaternary deposits that overlie Maastrichtian chalk and Danian limestone vary in thickness between 5 and 75 m. These deposits are differentiated into four litostratigraphic formations, which were deposited during glaciogeological dynamic events at the glacial maximum during the last part of the Weichselian ice age about 25 000-17 000 years ago. Two of these formations, namely the Tebbestrup and Midtdanske Till Formations, have previously been established and are recognised regionally in the central part of southern Denmark (Houmark-Nielsen 1987, 1999, 2003; Houmark-Nielsen & Kjær 2003). The two other formations, the Birket and Lolland Till Formations, are new (Fig. 6). The Birket Formation is interpreted as comprising recessive meltwater deposits overlying the Midtdanske Till Formation, and is mainly found in tunnel valleys formed during the retreat of the Main Weichselian Advance (Nordøstisen). The Lolland Till Formation comprises the predominant till that was deposited during the advance of the Ungbaltiske Is. Scattered occurrences of extramarginal, glaciolacustrine deposits overlying the Lolland Till Formation terminate the glacial deposition. These are informally named the Grænge lagene (beds), after bogs located along the Sakskøbing Fjord Lineament south-east of Sakskøbing. Here, remnants of Bos taurus ursus (Andersen & Møller 1946) have been found in peat beds covering these deposits. Holocene terrestrial deposits are thin and comprise undifferentiated freshwater deposits, mainly peat, located in depressions and drainage systems. Holocene marine deposits comprise beach sand and gravel in beach ridges that build out marine forlands by accretion around points and necks. A good example is the angular spit at the southern end of Femø. Thin layers of marine gyttjas have mainly been accumulated in the deepest parts of Smålandsfarvandet. Marine deposits also occur in areas reclaimed from the sea behind artificial dams, e.g. in the southern part of Askø. The late Holocene evolution of the Smålandsfarvandet archipelago was characterised by the Atlantic transgression around 8000 years ago. The drowning of the landscape is documented by artifacts and other traces of Stone Age settlements of the Ertebølle culture, which have been found on the sea bed about 5 m b.s.l.
