Die Entstehung der dänischen und norddeutschen Rinnentäler (Tunneltäler) -Glaziologische Gesichtspunkte

Abstract. The original tunnel valley theory was put forward by Ussing 1907. Its validity has, however, been doubted by most German geologists since then, especially because many so-called tunnel valleys are surprisingly wide (1 km or more). It was claimed that so wide tunnels or cavities under the ice were impossible. Since 1950, progress in glaciology (especially Nye 1952 and Shreve 1972) has caused a revival of the tunnel valley concept. Long and narrow ice tongues which have been postulated for North Germany by some researchers, according to Nye are only possible in mountainous terrain. On the other hand, Shreve's picture of the subgla-cial stream pattern matches precisely the known pattern of eskers and valley incisions in Denmark, Holstein, and Mecklenburg. Küster & Meyer (1979) and Ehlers & Linke (1989) interpret the buried valley incisions in northern Germany as eroded by subglacial streams. Until now, however, their large width has not been sufficiently explained. A group of Danish scientists including Ole Humlum has discussed this problem. A successive origin of the valleys appears to be most probable: during the winter with its reduced meltwater flow, the tunnel is narrowed by ice pressure. Therefore, it cannot carry the vastly increased amount of water during the spring, and a new tunnel has to be eroded, presumably partly into the substratum. Annual repetition of this process causes wide valley bottoms, only small parts of which were ever water-filled at any given time. It is shown that the morphology of the Danish tunnel valleys can be explained in every detail by such a sequence of events, and that no alternative theory can offer an equally satisfactory explanation.