A special characteristic of the Yemen highlands is that fields and inhabited sites are found at the highest elevations, the mountain-tops forming extensive plateaux, often scarped on every side and only accessible by difficult paths cut in the cliffs which encircle them like the escarpments of a natural fortress; a remarkable example of this is Jebel Jihaf on the Aden border, 8000 ft.
More than double, to the southern, the difference being mainly due to the zone of plateaux and sierras.
The mean elevation, estimated by Elie de Beaumont at 1500 metres (4900 ft.), has been sensibly diminished by the addition of that zone to the system, and it must now be placed at only 1200 metres (3930 ft.) for the range as a whole; so important a part is played by the above-mentioned plateaux of small elevation in a chain whose highest summit reaches 11,168 ft., while the passes show a greater altitude than those of the Alps.
There are numerous large expanses of level country, the most notable of these being the plains (cameos) of the Tagus valley, and of Aviz or Benavilla, Beja and Ourique, in Alemtejo; the high plateaux (cimas) of Mogadouro in Traz-os-Montes and Ourem between the Tagus and the upper Sorraia; the highly cultivated lowlands (veigas) of Chaves and Valenta do Minho in the extreme north; and the marshy flats (baixas) along the coast of Alemtejo and the southern shore of the lower Tagus.
But we have the same phenomenon in single varieties of man, such as the American, which inhabits alike the frozen wastes of Hudson's Bay and Tierra del Fuego, and the hottest regions of the tropics, - the low equatorial valleys and the lofty plateaux of the Andes.