Liege is the Hohes Venn, a long table-land topped with lakes and marshes. The Hohes Venn is larger than the Schnee Eifel. Its northeastsouthwest course is defined by a line through Malmedy and Monschau on the German face and by a line through Eupen and Spa on the Belgian. In the northeast the Hohes Venn is prolonged by the Hurtgen Forest. As the Schnee Eifel forms a barrier covering St. Vith on the east, so the Hohes Venn is a large outer bastion for Liege. Although the Hohes Venn is geologically a part of the Eifel it is somewhat removed from the other parts of the complex and represents the gradual and sometimes indefinable blending of the Eifel and Ardennes.
The Eifel is thickly covered with forests and provides good cover from air observation even in the fall and winter. The area has no large towns but rather is marked by numerous small villages, requiring extensive dispersion for any large forces billeted here. The road net is adequate for a large military concentration. The rail net is extensive, having been engineered and expanded before 1914 for the quick deployment of troops west of the Rhine. The railroads feed into the Eifel from Koblenz, Cologne, and the lesser Rhine bridgeheads between these two. The main rail line, however, does not cross the Eifel but follows the Moselle valley on the southern fringe of the Eifel. Rail and road systems throughout the Eifel are marked by meandering courses and numerous bridge crossings thrown over rivers and deep ravines.
The Ardennes, like the Eifel, is not a single and well-defined bloc. The general area may be defined as a wedge with the point between Aachen and Duren. The northern edge is a diagonal: Aachen, Liege, Maubeuge, Landrecis. The southern edge (much debated by geologists) is a more pronounced diagonal running from Aachen southwest to Arlon. The base, formed by the Foret des Ardennes or French Ardennes, roughly coincides with the Franco-Belgian frontier and the Semois River. The Ardennes has three recognized subdepartments: the High Ardennes in the south, the Famenne Depression in the middle, and the Low Ardennes in the north. The Low Ardennes tends to be open and rolling, but includes two plateaus: that of Herve, between Liege and Aachen, and Condroz, between the lower Ourthe and the Meuse in the vicinity of Dinant. This sector is more readily traversed than is the High Ardennes, but it is relatively narrow, maneuver is constricted by the flanking line of the Meuse River, and entrance from the east presupposes that the invader has possession of Aachen and the roads circling north or south of the Hohes Venn.
The Famenne Depression is only a thin sliver of the Ardennes wedge. The Famenne is free from tree cover except for the characteristic buttes which dot the depression. Scooped out of the Ardennes massif, this long, narrow depression originates at the upper Ourthe and extends westward through Marche and Rochefort. It reaches the Meuse between Givet and Dinant, offering a good crossing site which often has been employed by European armies operating on the Meuse. But an invader from the German frontier must traverse much difficult terrain before debouching into this "march through" depression.
The High Ardennes is often called the "True Ardennes." It is not properly