The Black Forest lies on the eastern side of the Rhine Valley, facing the Vosges, some nineteen miles away. At the beginning of the quaternary epoch, the two ranges formed a single chain of mountains with peaks over 3000m high. Then there was a collapse, similar to that for a rift valley, leaving two fairly symmetrical ranges divided by a truly vast river valley. On the eastern side, the Schwarzwald adjoins the Swabian Alps. Principal cities of the area are Mannheim, Heidelberg, Baden-Baden, Karlsruhe, Ulm and Freiburg.