The Sundgau, which is the part of Alsace to the south of Cernay, is divided by the Rhone-Rhine canal into two regions, the physical aspects, geological structure, and tactical value of which are essentially different. The country to the south of the canal, known as the Alsatian Jura, is thickly studded with rounded mammelons, like a nest of giant molehills, intersected by a series of irrigation canals, some of which are two or three yards wide and useful as lines of defence. The country, as a rule, is thinly populated, there are few isolated houses, and the villages are some distance apart. It is watered by three rivers, the Thalbach, the Ill, which flows northward from the Swiss frontier past Altkirch, Mulhouse, Colmar, and Strassburg to the Rhine, and the Largue. On the right bank of the Ill there is a light railway, constructed shortly before the war, running from Ferette to Altkirch, and on the left bank of the Largue an ordinary-gauge line, running from Porrentruy, just across the Swiss frontier, to Dannemarie. There would be a formidable risk of a German flanking movement by this approach on the fort of Lomont, to the south of Belfort, if it were not for the careful watch kept by the Swiss army on their frontier. The general character of the country is suitable for guerilla warfare, but not for operations on an extended scale. It has two main defensive positions against a French attack based on Belfort along the line Petit-Croix, Dannemarie, Altkirch, at Altkirch itself, and at Britzy-Berg. The first of these consists of a series of heights on the south of the spur of the Schweighof (Hill 381), and on the north of a ridge running in the direction of Heidwiller and the junction of the Ill and the Largue. The value of this position is especially great on the south-west side where it commands the important point at which the lines of communication converging on Altkirch meet and the defile in which lie the railway, the river Ill, and the main roads from Mulhouse and Basle. The Britzy-Berg position, three or four miles further north, near Illfurth, commands the whole of the surrounding country to a considerable distance nearly as far as Mulhouse, and also sweeps with its fire all the roads that meet at Altkirch. Both these positions had been strongly fortified by the Germans.