The Orxois is a plateau extending north of the Marne to the Soissonnais, at a mean height of 160 metres. But it is very far from being uniform. Let us study the nature of its soil, and the relief, that we may comprehend its aspects more thoroughly. The substratum of the plateau of the Orxois is the layer of rock called "hard limestone" 30 to 40 metres in thickness, so much of which is used for building material in the towns and villages. This layer is almost horizontal, and if there were nothing superimposed upon it, the plateau would be a practically level platform. But above the hard limestone are successive layers of a far different character--layers of sand, of Beauchamp sandstone, mingled with marl, making a moist, impermeable, infertile soil; then another layer of limestone, softer and more clayey than that below. Finally, this upper limestone is covered, especially toward the east, with thin layers of marl, clay and, lastly, Fontainebleau sand, which are connected with the strata of the Tardenois. Thus, to a depth of 100 metres, we find a succession of diversified strata, hard and soft, dry and moist, which impart great variety to the landscape. The valleys which intersect this conglomeration run from east to west, toward the deep depression hollowed out by the Savieres and the Lower Ourcq. From north to south, we can count three--the Upper Ourcq, by Fere-en-Tardenois and La Ferte Milon, the Ru d'Alland, and the Clignon. Very wide where they pass through the upper strata, these valleys grow abruptly narrower and deeper when they reach the level of the hard limestone, where they are little more than deep and narrow ditches. Between these furrows, the marl, sand, and softer limestones form ridges, now steep, now rising more gently, the sandy soil bearing woods, the limestones cultivated fields. [Sidenote: The ridges run east and west.] Thus the whole plateau of the Orxois is a series of elevations and depressions, running from east to west, which form just so many obstacles to an advance from south to north like that of the Allies. Luckily they approached this locality at the same time from the west, which enabled them to outflank the obstacles simultaneously with their approach from the south. [Sidenote: Torcy, Belleau and Bouresches.]