The Eifel and its western continuation into Belgium, the Ardennes, are the remains of a Variscan truncated upland, much of it 400 million years old, that is part of the Rhenish Massif (Rheinisches Schiefergebirge). In the area of the Stavelot-Venn Saddle (Hohes Venn) are the oldest layers of rock, which originate from the Cambrian and are around 550 million years old. The Eifel is one of the few volcanically active areas of Germany, as is evinced by numerous discharges of carbonic acid, for example into the Laacher See. The last eruptions, which gave rise to the most recent maars, occurred about 11,000 years ago.