Il Quaternario del Trentino sud-occidentale. I - Carta geomorfologica e dei depositi quaternari tra la Val Gardena e Cima Tosa Castel del Camosci (Gruppo di Brenta)

Abstract

This cartographic survey was performed on a 1:25,000 scale over an area of some 100 sq.km; it covers part of the territory extending north of Lake Garda, between the Rendena and Adige valleys. The study of the entire territory is about to be completed (Fig. 1). During this study the directions of glacial flow towards the Benaco area have been identified (Fig. 2). The mapped quaternary deposits are not very thick and outcrop discontinuously with continental fades varying from glacial to postglacial. Morainic deposits or strips of relic or skeletal morainic material attributable to the last glacial peak are present in Rendena Valley between 700 and 1,600 m a.s.l. In the Algone Valley, further to the east, similar deposits outcrop between 950 and 1.750 m a.s.l. In these valleys only morainic deposits are present and, although they are lined up at the above elevations, they do not seem to represent real morainic ridges. The antiquity of these deposits is proved by the lithological fragments evidencing a glacial provenance from the entire uphill area. Retreat late glacial phases in the Rendena Valley are present as a continuous alignment of morainic deposits made up of local materials, as the glacial input from uphill was gradually decreasing. All this is cut by the affluent streams of the left bank. The front of this glacial pulse could have reached Pelugo, but it was probably eroded and covered by the alluvial fans of the wide valley bottom. The remnants of such deposits deeply eroded are discontinuously present on the right bank. More recent stadial circuses are present in two of the affluent valleys of the Rendena Valley slightly on the south of Pinzolo: i.e. in the Farine Valley ending at Giustino; and the other ending uphill of Vadaione. The Manez Valley, further to the east, does not contain stadial morainic deposits, whereas on the ramparts of Cima Tosa there are recent and contemporaneous circuses. At the retreat of the glaciers, a strong erosive activity was triggered all over the area. Erosive processes are particularly active on the eastern bank of the Nambi Valley near Cima Tosa, from which radiate the last semiextinct vedrettes. Significant, albeit not as strong, is the unbalance in the Rendena Valley. Secondary valleys open on active conoids supplying enormous amounts of material to the bed of the stream Sarca. This stream, despite its strong erosive power, is incapable of removing such large amounts. The apparent balance of the river Sarca is controlled through a constant programme of maintenance. 'In the paper isostasy problems are also discussed. This phenomenon is at times overlooked during the study of vertical movements, although the alpine region was subjected to repeated glaciations and deglaciations including the present one. Also recent tectonic phenomena have been carefully considered; in this area, however, only clues of questionable validity have been found, no firm evidence has been identified yet.