more sinuous course. Within the basin, the major peaks are Presanella () and Adamello (). Morphologically, the Garda hydrographic basin can be divided into four areas: the plain of about , the lake surface of about , the western portion of about 500 km² and the eastern one of approximately . To the south of Lake Garda, between Brescia, Mantua and Verona, a large morainic amphitheatre develops: a succession of hilly circles with small flat areas interposed, in some cases marshy, originated thanks to the transport and storage action of the great Garda glacier. These morainic deposits were formed during the glaciations Günz, Mindel, Riss and Würm: very limited morainic deposits are attributed to the two oldest, Günz and Mindel, while the outer morainic circles are attributed to the Riss glaciation and the inner ones to the Würm glaciation. The morphology of the hills is gentle and with delicate lines; from the highest points, it is possible to perceive the relationships that link the hills to the mountains as well as the circular amphitheatre shape of the hills, which seem to embrace the southern part of the lake. The main tributary of Lake Garda is the Sarca River, others include the Ponale River (fed by Lago di Ledro), the Varone/Magnone River (via the Cascate del Varone) and various streams from both mountainsides, while the only outlet is the Mincio River (, at Peschiera). The subdivision is created by the presence of a fault submerged between Sirmione and Punta San Vigilio which is almost a natural barrier that hampers the homogenization between the water of the two zones. If the water level of the Adige river is excessive, water is diverted to the lake through the Mori-Torbole tunnel. Geology Lake Garda area is considered one of the most seismic in Italy. The oldest known earthquake that struck the Garda region seems to have occurred in 243 (or perhaps 245): it was so disastrous that the city of Benaco, located where Toscolano Maderno stands today, suddenly disappeared. The disappearance of the town is probably due to the cracking and landslide of the mountain above Toscolano, which caused the flooding of a small lake enclosed by the mountains, whose waters submerged the populous town. Other earthquakes, more or less intense, followed one another periodically: in 793, when according to the chroniclers it caused great calamities, especially in the Baldo area; in 1457, during