Rockfalls at chalk cliffs in northern Europe

The Cretaceous and early Tertiary carbonates are exposed along several steep coastal cliffs in northern Europe, where they repeatedly constitute a serious geohazard risk. Often the cliff collapses take place in the late winter to early spring, when the ground water saturation is highest and the action of freeze and thaw triggers the rockfalls. Two representative examples of carbonate cliff collapses are presented, both located in SE Denmark. At the first one, cliff collapses appear along a 100 m high, nearly vertical cliff, the Møns Klint, where rock avalanches with a frequency of 3–5 years occur. The other example appears along a shallower cliff height (40 m) in eastern Denmark, Stevns Klint, where the famous Cretaceous/Tertiary boundary is exposed in the middle of the cliff. Here the wave erosion undercuts the cliff by removing the soft Maastrichtian chalk and leaving the hard Danian bryozoan limestone as an overhang, which breaks off in a cliff fall.