Descriptive text to the Geological map of Denmark, 1:50 000, Mon 1511 I, 1511 IV and 1512 II

The geological map sheet Mon covers the island of Mon, the smaller adjacent islands Lango, Lind -holm and Nyord as well as adjacent parts of Sj ae lland and Lolland. It comprises the geodetic map sheets 1511 I and 1511 II and areas on bordering sheets. Mon is surrounded by the Baltic Sea with the bay of Hjelm Bugt to the south, the straits of Gronsund and Ulvsund to the west, and the bays of Stege Bugt and Fakse Bugt to the north.Mon is divided into three glaciomorphological areas, namely a high, hilly landscape of Hoje Mon to the east, a hummocky to parallel ridge landscape to the west and areas of marine deposits around Nyord and Ulvshale. The composite ridge landscape of Hoje Mon constitutes a glaciotec- tonic complex comprising four individual glaciodynamic sequences, with the hill Aborrebjerg as the highest point (143 m a.s.l.). The parallel ridge hills consist of thrust-fault-displaced chalk sheets with superimposed glacial deposits. The thrust sheets are up to 80 m thick, of which 60 m constitute Maastrichtian chalk. The vertical displacement of the thrust sheets is about 150 m measured from the primary, undeformed pre-Quaternary surface located 25-30 m below sea level. The pre-Qua- ternary surface consists of chalk of Late Maastrichtian age, which forms a carbonate platform in the subsurface of Mon about 27 m below sea level. Chalk displaced by glacial tectonics is not restricted to Hoje Mon but also appears in smaller thrust sheets and rafts in the small-ridged landscape around Stege Nor. In the chalk sheets along Mons Klint, most of the Late Maastrichtian succession is exposed. Cliff sections with chalk are also exposed at Hvideklint along the south coast of the island. However, here the glaciotectonic shear deformation has commonly altered the lithology into a chalk glacitectonite.The oldest Quaternary units deposited on the pre-Quaternary unconformity are Saalian till as well as sand and clay from the Eemian Interglacial. These units are overlain by Early Weichselian sand. The next Quaternary succession, the Ristinge Klint Till Formation, was deposited during the Ristinge ice advance in the early Middle Weichselian about 55 000-50 000 years ago. Then followed the Kraneled Formation (new formation) consisting of fluvial and lacustrine deposits. The follow- ing Klintholm Till Formation (adjusted formation) was deposited during the Klintholm ice advance 35 000-32 000 years ago. The Klintholm Till Formation is overlain by a more than 10 m thick unit of greyish glaciolacustrine clay with dropstones. Glaciofluvial sand with thin-layered intercalations of laminated mud and diamictites of the Kobbelgard Formation (new formation) are related to this unit and interpreted as deposited in a huge, partly ice dammed lake covering a large part of the present Baltic Sea and the southern part of Kattegat 32 000 to 28 000 years ago. The Kobbelgard Formation is overlain by sand and gravel of the Stubberup Have Formation (new formation) and tills of the Mid Danish Till Formation deposited by the NE Ice Advance from central Sweden about 23 000-20 000 years ago.Relatively shortly after the NE ice had melted away, the Young Baltic Ice advanced from the east-ern part of the Baltic area. North-directed compressive deformation during this advance created the glaciotectonic complex of Mons Klint including the new unit Mons Klint Glaciodynamic Sequence. In the southern part of the complex, a steeply inclined imbricated fan was formed; towards the fore- land to the north, the thrust faults became gently dipping and the tip-zone of thrusting is located under the landslides at Liselund. The composite ridges form a characteristic hilly landscape with elongate crests trending E-W.The curved coastline along Hjelm Bugt was formed by a glacial lobe, north of which a push moraine was built up. A number of spillways striking radially northward from the lobe were formed by meltwater discharged from its glacier ports, including the Borre, Maglemose and Roddinge depressions. Deposition of sand and gravel of the Ny Borre Formation (new formation) took place at this time. During the advance of the Young Baltic Ice over southern Denmark to the Eastern Jutland stationary line, a relatively thin lodgement till of the Lolland Till Formation was deposited, which is rich in chalk due to its truncation of the upthrusted chalk sheets.Towards the end of the Weichselian glaciation c. 17 000 years ago, the Young Baltic Ice melted back, leaving a residual ice cap in Skane from where a recessive ice advance towards south-west reached Mons Klint, resulting in superimposed glaciotectonic deformation. During the Late Weichselian, freshwater lakes in the Hjelm, Tovelde and Hoje Mon areas were filled by clay and gyttja, with deposition that continued into the Holocene.During the Holocene, the former spillways were turned into fjords during the Atlantic transgression. Marine deposits mirroring the Littorina Sea are thus found in Maglemose and Borre Somose. After the Atlantic transgression had established a sea level more or less corresponding to that of today, accretion of marine forelands and formation of a spit system started. In particular, this is the case for the areas of Ulvshale and Nyord. At the same time, vegetation migrated out into the numerous fjords, and peat began to accumulate. The last phase of sedimentation is confined to the formation of beach ridges in the coastal areas, typically covered by aeolian dunes, as can be seen on the coast at Klintholm Havn and Rabylille as well as along the north-eastern coast of Ulvshale.