Note 55 — Chapter V · PATAGONIA AND SHEEP-REARING

Most of the lacustrine depressions are continued eastward across the Patagonian tableland in the shape of distinct valleys. The eastern part of the Straits of Magellan is merely a submerged valley on the axis of Otway Water. Useless Bay also is continued eastward by the hollow which ends in the Bay of San Sebastian. Sometimes the waters of the lakes flow eastward, toward the Atlantic, along these valleys. Generally, however, the lakes of the western slope are drained on the west by means of narrow defiles across the Cordillera, or on the north and south by rivers which follow the sub-Andean depression and thread them together in the manner of a rosary. The valley which joins the lake to the Atlantic is in those cases a dead valley, and the inter-oceanic dividing line of the waters is marked by the frontal moraine of the old glacier, which confines the lake on the east. This arrangement is found, with surprising regularity, from the Alumine and the Lacar to the Neuquen, and as far as Lake Buenos Aires and the Seno de la Ultima Esperanza at Santa Cruz. The capture of the waters of the eastern slope by the rivers of the Pacific across the Cordillera is fairly ancient, and certainly pre-glacial. But during the Glacial Period the glaciers obstructed the transverse valleys of the Cordillera, and the waters of the eastern slope found their way to the Atlantic once more. With the retreat of the glaciers the valleys of the Cordillera were successively cleared. The lakes, dammed by the glaciers, were suddenly released and their level lowered. The valleys of the Patagonian tableland were finally abandoned, and the topographical accident of secondary importance, which the ancient frontal moraine of the glacier represents, came to mark the limit of the domain of the Pacific. The freshness of the contours of the dead valleys of Patagonia bears witness to the recent date of this conquest, which was too sudden or rapid to be called a "capture" in the proper sense. It has not been accomplished everywhere. From Lake San Martin to Lake Buenos Aires all the lakes of the eastern slope are drained into the Pacific by rivers which flow into the Culen fiord. But further south, Lakes Viedma and Argentino are still tributaries of the Atlantic. They correspond to the zone of the Patagonian Andes which is still covered by inland ice. To the north, in the basin of the Puelo and the Yelcho, where the trans-Andean valleys long ago ceased to be obstructed by ice, the lakes of the eastern slope which drain toward the Pacific are small in size. Their level to-day is much lower than it used to be, and a network of streams has developed east of them, on the earlier lacustrine region, which is now dry.