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.
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.
Many caverns were formed, some of them in early Pleistocene times, by water percolating from above and (Fig. 11) resulting in subterranean streams which issued at the entrance; this formed the expanded grotto, sometimes a chamber of vast dimensions, such as the Grotte de Gargas. Outside of this, again, may be an abri or shelter of overhanging rock. In other cases the rock shelter is found quite independent of any cave.
Where the glaciers or ice-caps passed over the summits of the hills the subglacial streams penetrated the limestone of the mountain and formed vast caverns, such as that of Niaux, near the river Ariège. Here a nearly horizontal cavern was formed, extending half a mile into the heart of the mountain. The material with which the floors of the caverns are covered is either a fine cave loam or the insoluble remainder of the limestone forming a brown or gray clayey substance. The Magdalenian artists produced drawings on these soft clays and, in rare instances, used them for modelling purposes, as in the Tuc d'Audoubert. The sands and gravels were also swept in from the streams above and carried by strong currents along the wall surfaces, smoothing and polishing the limestone in preparation for the higher forms of Upper Palæolithic draughtsmanship and painting.
