The northern part of the lake is narrower, surrounded by mountains, the major of which belong to the Gruppo del Baldo. The shape is that typical of a moraine valley, probably having been formed under the action of a Paleolithic glacier. Although traces of the glacier action are evident today, in more recent years it has been hypothesized that the glacier occupied an already existing depression, created by stream erosion from 5 to 6 million years ago.