This line was a by no means inconsiderable obstacle. Imagine, if you please, a deep depression, twisting and turning in all directions, and from 200 to 400 metres wide, extending at least as far as Fere-en-Tardenois. It is bounded on either side by cliffs of hard limestone, 30 to 40 metres high, in which innumerable caves are scooped--the so-called _boves_, which are used as dwellings, with doors and windows flush with the face of the cliff. These _boves_ are invaluable defensive positions, out of reach of bullets and shells. The valley bottom is wet and swampy, with dense clumps of poplars mingled with alder-bushes. There are numerous villages at the foot of the cliffs,--Rozet-St.-Albain, Breny, Armentieres,--or on the slopes above, like Noroy. A frontal attack on such a position would have been too costly. The Allies turned the line of the Ourcq from the north. They crossed the river in force in the upper part of its course, where it has not yet attacked the stratum of hard limestone, and where the valley is wider, and the sides are less steep. Nevertheless they encountered terrible difficulties. [Sidenote: Strategic value of hills of Orxois.] North of the Ourcq, indeed, the last heights of the Orxois form another chain of hills, from four to six kilometres wide--the last obstacle before we come to the plateau of the Soissonnais. These hills are of the greatest possible diversity of shape and vary in height from 200 metres at the western extremity to 230 at the eastern. Their bases consist largely of sandstone and Fontainebleau sand, with clumps of forest scattered here and there; higher up is the softer limestone, the land being entirely cleared and covered with crops. Here and there we find the remains of the former covering of clay and Fontainebleau sand--wooded ridges which expand toward the east into the wood of Seringes, the forest of Nesle, and Meuniere wood. These hills, the last as we travel northward, where they command the whole of the Soissonnais, have therefore the greatest strategic value, particularly the positions of Hartennes, Plessier-Huleu, and Seringes. [Sidenote: The French approach from the west.]