Tierra Mayor is a wide valley of glacial origin, traversed by the Lasifashaj River that forms a great number of meanders, which flows into the vicinity of Estancia Harberton. The beginning of the valley coincides with the point where the Carbajal valley turns to the south, then runs straight between mountains, within the Fuegian Andes range from west to east. The dividing lines of waters mark its northern and southern limits. In the north, the Alvear mountain range separates it from Lake Fagnano, and to the south another line of mountains, the Sierra de Sorondo, separate it from the Beagle channel. The valley is largely occupied by Sphagnum bogs. The slopes of the mountains are covered by the characteristic Fuegian forest, where the trees of the genus Nothofagus dominate. Above the forest boundary, we find a high Andean ecosystem, with some peat bogs.

Part of the valley is framed, next to part of the Carbajal valley, within the Valle Tierra Mayor Natural Reserve. The reserve was conceived as a tourist corridor so its location is determined along the National Route No. 3. It has an extension from west to east from the eastern boundary of the Tierra del Fuego National Park to the confluence of the Lasifashaj and Hungry, with an approximate area of ​​370 km2, and a length of about 40 km and an average width of approximately 8 km.

Valle Tierra Mayor Natural and Landscape Reserve was created in 1994 by Provincial Decree Nº 2256/1994,