To the north of the Pyrenees lies a deep trough extending from the Bay of Biscay to the Corbières that links the Pyrenees to the Cevennes, and which at the present day forms the watershed between the Mediterranean and the Atlantic. This gulf was gradually silted up by the torrents from the Pyrenees. Masses of rubble may be seen backing and capping isolated hills of sandstone, and forming long ridges, as that of the Park at Pau. The drift was from east to west. All the low hills are crowned with rolled stones. The boulders vary in size in proportion to the distance they have travelled. At Pamiers, Tarbes, and Pau they are of the size of a child’s head, but farther north dwindle to pebbles and gravel, and finally we enter on a region of clay and sand, which heavy rains convert into quagmires. Indeed, those of Armagnac, between the Garonne and the Upper Adour, have hardly their equal in France. These are not glacier deposits, for the stones and pebbles have been rolled, and the clay or mud is the chewed or mumbled remains of boulders. At a later period the entire basin thus choked was lifted high above its original level. That there was a glacial period in the south of France is certain, and the glaciers have left their moraines behind them. The glacier of Argelez extended in one stream to Lourdes, and then fanned out towards Tarbes. At Argelez it filled the valley to the height of 4430 feet. To morraine is due the desolate plain of detritus of Lannemezan. Separated from the mountain spurs by the profound depression in which flows the Neste, it is attached to the main chain solely by the isthmus that runs out from the Pic d’Arneille towards the plains. The true watershed, between the Atlantic and the Mediterranean, lies far to the east, on the frontier between Ariège and the Pyrénées Orientales. All the streams and rivers to the west of the insignificant chain there flow into the Bay of Biscay. The rivers to the east are comparatively unimportant, the Aude alone being of a respectable size; and this does not derive its waters from the main chain of the Pyrenees, its sources are in the spur that acts as the watershed. [Illustration: THE CASCADE, GAVARNIE] The lakes of the Pyrenees are nothing more than mountain tarns; the largest is the Lac Lanoux, in Pyrénées Orientales, lying below an irregular cirque, commanded by the Pic Pédroux. It stands 6500 feet above the sea-level, and is about two and a half miles long. It is usually frozen over from September to the end of June.