Toward the west the tableland is separated from the Cordillera by a longitudinal depression, though the continuity of this has been exaggerated. This depression, which outlines the contact between the folded zone of the Andes and the flat zone of the tableland, is very important from the point of view of colonization. Just at the frontier of the steppe and the forest, it is the most hospitable part of Patagonia, the richest in natural resources. Amidst the glacial lacustrine deposits which are accumulated on it there rise masses of different kinds of rock which break it up into compartments, granitic ridges of laccolites exposed to view, eruptive structures that have been dismantled. In the south the sub-Andean depression forms a broad passage between Lake Maravilla and Punta Arenas, about two hundred miles long, enclosed between the basalt cliffs of the tableland on the east and the mountains of the Brunswick Peninsula and William IV Land. The bottom of it is a singular glacial landscape, sown with lagoons, punctuated by scattered hills, with an impermeable soil of drift and mud. From Lake Argentina to Lake Buenos Aires the elevated tablelands, which rise to a height of 5,000 feet, back upon the Cordillera, and the sub-Andean depression is interrupted. Similarly, between Lake Buenos Aires and Lake General Paz the contour of the Patagonian tableland is not very marked above the sub-Andean zone. The glacial alluvia at the foot of the Cordillera rise to the level of the tableland, which sinks steadily eastward toward the Genua and the Senguerr. To the north, between Carrenleufu and Lake Nahuel Huapi, the retreat of the lakes has left long narrow beds right in the Cordillera, such as the Valle Nuevo del Bolson, the bed of which has been taken over by the Futaleufu west of the Cerro Situacion. Further east the topographical features of the edge of the tableland (the valleys of the Chubut, Tecka, and Norquineo) lie from north to south. Hence within a space of little more than a hundred kilometres the sub-Andean zone has a series of parallel roads, communicating with each other by means of broad, transverse gaps, which at one time were occupied by the lower lobes of the glaciers. The sub-Andean depression does not go north of Lake Nahuel Huapi.