Physically this region consists of a Tertiary basin, once the floor of a sea between the Sierra Morena to the north, and the Betic Cordillera to the south. The Guadalquivir fault to the north forms a complete physical barrier, and is outlined by hills that rise abruptly from the plain below. To the north of the fault there is a sudden change in climate, crops, vegetation, and human response from Mediterranean to arid meseta types. To the south and east, however, Mediterranean features merge gradually from the lower llanuras of the plain to higher areas of semi-steppe, and more rugged country, until the high peaks, tremendous gorges and chasms of the high Sierra Nevada appear. Here in the south there is a zone of transition; in the north there is a line of contrast. The plain itself is not as wide as the green colouring on a map would suggest, and is confined to an area close to the river. Away from this river plain the older Tertiary beds have been dissected to form low-lying hills, minor elevations with gentle slopes, not obvious on a small-scale map, but above a level where water can be used effectively for irrigation. This is an area of gently rolling country to which such names as campiña are given. The seaward part of the valley consists of Las Marismas (the marshes), and a coastal fringe of sand-dunes, the Arenas Gordas, which owe their origin to winds and currents.
The major river of this area is the Guadalquivir, and its most important tributary is the river Genil, which flows for part of its course along a fault-line roughly parallel to the sea. Other smaller tributaries are the Guadajoz and Carbones from the south, and the Guadiato and Huelva to mention but two of the many shorter tributaries from the north. Rivers which enter the sea independently of the mouth of the Guadalquivir include the Guadalete, the Rio Tinto (so called because of the colour of its waters, stained by minerals), which, with the Odiel, enters the Atlantic in a common estuary at Huelva.
The Guadalquivir valley is an 'island' of intense heat and insolation in summer, and, with temperatures regularly reaching the nineties at midday, and on rare occasions exceeding 120° F., it is the hottest area in Spain. Temperatures are very mild in winter, which is when most rain falls. Precipitation remains slight, however; at Cádiz there is an annual total of 18 inches, at Seville 19.5 inches (according to the Oxford Home Atlas, but 23 inches according to Maria de Bolós Capdevila), and 24 inches at Córdoba. The funnelling effect of the highland on winds from the south-west helps to bring in more moisture-bearing clouds than would otherwise be the case. Precipitation on the higher areas is heavier than on the lower plains, reaching 40 inches in some years, and at least a minimum of irrigation water is assured.
Agriculture in this extremely fertile area has been noted for centuries, but it is due less to natural causes than to irrigation schemes. In wide areas where irrigation cannot be practised, extensive heathlands occur, merging into steppes, especially on poor soils. Irrigation water is stored in great reservoirs, for example at Tranco de Beas, at the head of the Guadalquivir, and there are many smaller barrages on the tributaries. In addition, many canals lead directly from the river itself, some of them being of Arab origin, and a few wells are also in use. A paucity in other sources of power has led to the generation of hydro-electricity, though it is insufficient for the demands of the region as a whole, and cannot be supplemented from power stations in the north of Spain. Although there are many small power stations in the mountains, and more are being planned, there is not enough electricity for industrial expansion, or for the comfort of all the inhabitants. With rainfall meagre and unreliable it seems doubtful whether any appreciable amelioration of this situation is possible at present.
An interesting aspect of farming in the agriculturally limited areas of Andalusia is the division of land into large estates or latifundias. In the drier parts of the south there is a great risk of crop failure, more land is needed to feed a family, and nothing grows without irrigation. Only a man with a large amount of land and capital can withstand such natural disadvantages over a long period of time. It cannot be said, however, that these large estates are run either efficiently or for the common good. The alternative seems to be cultivation by smallholders, with the State or other syndicate acting as security against hard times.
Small amounts of varied minerals are extracted in this area. There is manganese at Zalamea la Real, sulphur at Benamaurel, lead at Charches, Huétor and Monachil, copper at Grazalema, iron at Alquife and Hueneja, and a small amount of uranium in the province of Córdoba. Although minerals have not been exploited to the full, there is a greater variety in Andalusia than in any other province of Spain.
The general distribution of population well illustrates the importance of the irrigated lowlands, which absorb the bulk of the population. Town sites are interesting; the large capitals are along the river Guadalquivir, while another line of towns has arisen on the lower slopes of the Sierra Nevada, where incoming sea breezes first drop their moisture. Jaén, Cabra, Lucena and Estepa are examples of such hill towns. There are also several 'oasis' towns in the dry inland areas, the best example being Granada. Along the coast there is a line of important ports, for example, Cádiz and Huelva, both of Phoenician origin.