Paleolivelli tardoglaciali e olocenici del Lago di Garda

After the withdrawal of the Pleistocene glaciers, a palaeo Lake Garda established at elevations higher than those reached at present. The large volume of water released by melting of glaciers in retreat, the damming action exerted by moraines laid in the last phase of maximum glacial expansion, and lake threshold only partially eroded by F. Mincio, led to the formation of a palaeo Lake Garda standing over 30 m above present lake level (about 65 m a.s.l.). Along the coast of Benàco are widely preserved sedimentological and geomorphological evidence, referring to Late Glacial and Holocene relict shorelines. Many evidences are observed along the rocky cliffs (to vast stretches still active), other data come from artificial excavations and, finally, relevant information comes from archaeological excavations conducted in perilacustrine settlements. The main tributaries have built large deltas, particularly evident along the western shore. In the lower lake, however, have developed numerous small deltas, which originate from the dismantling of the moraines laid in the last glacial maximum. The gradual lowering of the level of the lake, mainly due to deepening of the lake threshold by the F. Mincio, has led the terracing of the many deltas, with consequent formation of telescopic deltas. The most widespread evidences of relict lake levels are included in a coastal belt that rises no more than 15 m above the present lake. Most of the rocky shores show large stretches of relict cliffs retaining traces of ancient lake levels, well represented by relict wave-cut notches, suspended abrasion platform (rocky lacustrine terraces), palaeo beaches, and calcareous rims. Prominent abrasion platforms, both active and relict, border the main islands and peninsulas. They are common landscape features in the Manerba area, the most spectacular one being located at about 3.5 m above the present lake level. Remnants of beaches above present lake level, made up of rounded and embricated pebbles are preserved at several locations on lacustrine terraces and relict abrasion platforms; gravel patches are locally cemented on rocky cliffs, too. Horizontally arranged concretion levels overhanging the lake, known as calcareous rims, develop in places on vertical cliffs (from 1 to 5.7 m above the present lake level). Made up by algal stromatoliths and other organisms comprising fresh-water gastropods, (Bithynia tentaculata and Theodoxus fluviatilis), they supplied ages ranging from 10,070 ±70 to 6140 ±60 14C yrs BP (11,319-11,839 e 6877-7176 cal yrs BP ). Several prehistoric perilacustrine sites show that at least since about 6000 years BP the lake level always remained below the height of 68 m (+3 m above the present lake). Lake Garda underwent level fluctuations from some decimetres to not more than 1 m during the ancient and middle Bronze Age (4100-3500 BP , cal age), as archaeological excavations conducted in pile-dwellings document. Radiocarbon dating and detailed topographic survey made it possible to recognize that the Garda region registered relative uplift and subsidence, occurred in the last 12,000 years or so. The northern portion of the coast of Lombardy, to the N of Salò, for example, would be raised by about 1.1-1.3 m compared with the southern portion of the lake, with an average rising rate of about 10 cm/1000 years.