ABSTRACT
After the discovery of erratic blocks and moraines far away from to–days Alpine glaciers by Swiss pioneers, Morlot and Heer (1858) could demonstrate the presence of gravels and lignites between moraine layers. This has lead to a first subdivision of the Ice age. Further refinements of this subdivision were made at the end of the last century. The Higher and Lower Cover Gravels were attributed to the Günz and Mindel periods, and the fourfold system of Penck & Brückner (1901) was documented also in Switzerland. This system was not enlarged by the recognition of still older gravel sheets and boulders in the Basle area: the Sundgau gravels (oldest Quaternary to Pliocene), the “Wanderblock” Formation (Pliocene) and the Vosges Gravel (Miocene/Pliocene boundary) in the Delemont basin. This proved the existence of pre–Günzian cold climatic periods. The Sundgau gravels are glacio–fluvial deposits with transport–controlled pebbles. The “Wanderblock” Formation is represented by polished erratic boulders and cobbles of the Mottled Sandstone derived from the southern border of the Black Forest massif; they lie in a deeply weathered matrix. The Vosges gravels, cobbles and bebbles from the southern Vosges have been transported by muddy debris flows caused by ruptures of dammed lakes. All these pre–Gunzian sediments are restricted to an area not covered by Rissian ice. Still earlier, analogous sediments seem to occur in the conglomeratic sequence of the Jura–Nagelfluh (middle Miocene), “torrential” deposits derived from the Black Forest and deposited on the Swiss Table Jura. Similarly, several Alpine Nagelfluhs, without the imbricated structures typical of fluvial transportation, can be found in the proximal parts of the Lower and Upper Fresh Water Molasse fans. The finegrained sediments between the conglomerate layers with remains of palms and Lauraceae and the fauna of the Upper Marine Molasse (which separates the two Fresh Water Molasses) prove a warm climate. The associated conglomerates have generally been considered as deposited in a warm phase also, but this is rather doubtful. The reconfirmation of the Milankowitch curve shows that the Earth's orbital elements are considerably involved in the mechanism of alternations of cold and warm periods. This mechanism seems to have been restricted to the Quaternary, but in the Tertiary the amplitudes were smoother and the temperature level was higher.
Southwest of Lake Garda, ice–formed rocks and even gravels and calcareous boulders of pre–Middle Quaternary age occur higher up than the uppermost moraines. This must be due to the en–bloc uplifting on the border of the southern Alps.
At the southern end of Lake Como, huge Bergell crystalline boulders were deposited at the Oligocene/Miocene boundary. They seem to give the pre–Quaternary cold periods a general importance in the Alps. The Oligocene–Miocene glaciers advanced from the high areas of the Bergell massif and the adjacent Pennine and Austroalpine Nappes of the Bernina mountain area (with altitudes of 5000 metres) into the Lake Como basin. They should also have initiated ice flows farther to the northeast, in the Inn valley and to the north, over transfluence passes, into the middle Grisons area. From there, the debris have been transported by meltwaters into the Hohronen–Etzel Molasse fan (uppermost Oligocene and lowermost Miocene), south of the upper Lake Zurich. The high content of crystalline pebbles, especially of the Bernina type, shows that the Bernina group seems to have been the source area. In the Inn valley, on a higher level, the ice first 194reached Zernez and then, after the downcutting of the Clüs, moved further down and was connected at Susch with the Lower Engadine Ice from the Grialetsch mountains. The glacier ended possibly near Landeck, where lateral meltwater flowed through the lowermost Pitz valley into the Upper Inn valley.
In the Iller system, crystalline pebbles of the Silvretta nappe could reach the Middle and Upper Miocene molasse fan of the Adelegg, west of Kempten, and the Quaternary gravel sheets further to the northeast. This was possible only through transfluences over the Flexen pass and higher passes into the source area of the Iller river. The front of the Silvretta nappe lies on the Arlberg pass. As late Wiirmian moraines of the Iller system do not contain crystalline elements, the Miocene to old–Quaternary ice must have reached even larger dimensions.
