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. [Illustration: FIG. 11. Formation of the typical limestone cavern. After Gaudry. _V._ Vertical section of limestone cliff showing (_S_) waters percolating from above; (_A-O_) interior of the cavern; and (_G_) grotto entrance, original exit of the cavern waters. _H._ Horizontal section of the same cavern showing the (_G_) grotto entrance and (_A_, _G_, _O_, _B_) the ramifications of the cavern.]