Despite that, they were successful in reaching the Aisne in advance of the 6th French army. The latter, it ought, however, to be said, had to operate through a difficult area. From the Ourcq to the Aisne there is a succession of forests. Of these the great forest of Villers-Cotterets extends northwards from the Ourcq to within six miles of Soissons. East of the stretch of forests the country is more open. Given these facts of topography, it is evident that on following the line of the Ourcq, with the object of barring its passage to the enemy, the French had in the forest belt a formidable obstacle. Perceiving that in this lay his chance, General von Kluck hurried as large a part of his force as possible across the Ourcq in order to bar the advance of the French by the forest roads through Villers, and by the comparatively narrow break in the forest belt between Crepy and Pierrefond. He was thus able, notwithstanding that the British were hanging on to and harrying his rear, to hold the outlets against the troops of General Maunoury until he slipped past them.[18] And once on the Aisne and in touch with his Amiens rearguard, now on the Oise above Compiègne, he was in a position to initiate a complete change in tactics, and, his force being comparatively secure, the other German armies could again fall into line. Before dealing with those new German tactics, it is advisable briefly to sketch the defence works thrown up by the Germans along their line, because both these defence works and the character of the country are intimately related to the tactics. As already stated, the highlands of Champagne extend north-west nearly as far as Peronne. They are chalk hills and uplands cut by deep valleys. The most northerly of the valleys is that out of which flows the Somme. Then comes the much wider valley of the Oise. Still farther south is the valley of the Aisne. Between the Oise and the Aisne is a roughly triangular tract of country, its apex at the point where the Oise and the Aisne join. Across the broad end or base of this triangle run the open downs. Towards the narrower end of the area the country becomes broken and hilly, and is covered with great patches of wood and forest.