In Pleistocene Europe there were at least three warm or cold arid periods, accompanied in some phases by prevailing westerly winds,(59) in which 'loess' was widely distributed over northern Germany, covering the 'river terraces,' plateaus, and uplands bordering the Rhine and the Neckar. These 'loess' periods can be dated by the fossil remains of mammals which they contain, also by the stations of the flint quarries in different culture stages. Thus we find late Acheulean implements in drifts of 'loess' at Villejuif, south of Paris. Among the most famous stations of late Acheulean times is that of Achenheim, west of Strasburg, and not far distant is the 'loess' station of Mommenheim, of Mousterian times; both belong to the period of the fourth glaciation. An Aurignacian 'loess' station is that of Willendorf, Austria. TIMES OF THE LIMESTONE SHELTERS AND CAVERNS Beginning in the late or cold Acheulean period, the Palæolithic hunters commenced to seek the warm or sheltered side of deepened river-valleys, also the shelter afforded by overhanging cliffs and the entrances of caverns. It is quite probable that during the warm season of the year they still repaired to their open flint quarries along the rivers and on the uplands; in fact, the river Somme was a favorite resort through Acheulean into Mousterian times. In general, however, the open rivers and plateaus were abandoned, and all the regions of limestone rock favorable to the formation of shelter cliffs, grottos, and caverns were sought out by the early Palæolithic men from Mousterian times on; and thus from the beginning of the Mousterian to the close of the Upper Palæolithic their lines of migration and of residence followed the exposures of the limestones which had been laid down by the sea in bygone geologic ages from Carboniferous to Cretaceous times. The upper valleys of the Rhine and Danube traversed the white Jurassic limestones which are again exposed in a broad band along the foot-hills of the Pyrenees, extending far west to the Cantabrian Alps of modern Spain. In Dordogne the great horizontal plateau of Cretaceous limestone had been dissected by branching rivers, such as the Vézère, to a depth of two hundred feet. Under overhanging cliffs long rock shelters were formed, such as that of the Magdalenian station at La Madeleine. [Illustration: FIG. 10. Ideal section of the bluff overlying the Düssel River, near Düsseldorf, showing the mode of formation of the famous Neanderthal Cave, where the original type of the Neanderthal race was discovered in 1856. A typical resort of the 'cave man.' After Lyell. _c._ Entrance of percolating waters from above. _f._ Exit from the grotto. _a-b._ Interior of the cavern.]