The Sierra Nevada consists of an enormous mass of highland extending from the village of Padul, on the road from Granada to Motril, to the village of Gergal, on the railway line from Linares to Almería, with the high peaks of Mulhacén and Veleta at the centre of the system. A great deal of research and exploration is yet to be done in the central area, but from evidence so far accumulated it seems that there has been land in this zone since lower Permian times, with intermittent invasions of the Betic sea. In Jurassic and early Tertiary times, however, this land appears to have been less extensive, and there were large geosynclines to the north and south of the present mountains, containing seas in which were laid down sediments which were later folded and incorporated into the present systems.
The major fold movements took place in Miocene times, a little after the fold movements in the rest of Spain, and from a geological point of view the mountains have one very interesting feature. They reveal definite Alpine phenomena in the structure of the folds, particularly in the nappes which outcrop in the central areas. Such intensive folding served to emphasize the line of crustal weakness underlying the whole region, so that earthquakes, while not excessive in number, occur more frequently than in other parts of Spain. The number of seismic disturbances averages three per year, but this figure may be exceeded occasionally, as in 1911, when there were eleven. The epicentre of such movements is frequently of exceptional depth. Even while they were emerging, the mountains were subject to subaerial denudation, and huge accumulations of fluvial deposits appeared on their flanks, which now form some of the arid foothills to the north of the Sierra Nevada. These later deposits are almost undisturbed and therefore horizontal, and have been deeply dissected by modern streams.
Rocks exposed in the area show great diversity of age and substance, ranging from ancient Archaean granite to younger sedimentaries, such as limestone. The folding took place without great volcanic activity so that recent volcanic rocks are absent; there is, however, a large amount of granite and other crystalline rock in the centre (as, for example, micaceous schist) associated with the Veleta nappe, where ancient underlying rock has been forced up as an extrusion and subsequently exposed through weathering. From this Archaean core the rocks become progressively younger outwards, passing through Palaeozoic sedimentary rocks, through Mesozoic to younger sedimentaries, amongst which limestone is important, if only because it gives rise to completely arid, as opposed to almost arid, hills. Various minerals like lead, zinc and silver are associated with the older rocks, while iron occurs, though infrequently, in some of the younger rocks. Mineral waters are also found in a few places, as at Alhama, but difficulty of access prevents their being used on a large scale.
The higher region was covered by a small ice-cap during Quarternary glaciation, and ice has helped to fashion the surface features, particularly on the northern and western slopes, where insolation was less pronounced. There is much evidence of ice action; erratic blocks are sometimes found, but these are less obvious features than the mounds of glacial material, or moraine. Between the peaks of Mulhacén existed a large glacier, which has left an expanse of lateral moraine, about 1½ miles long, between Tajo de Culo de Perro and Cortijo de las Vacas; another morainic deposit may be found below the Loma de Dilar. Many corries still exist, some, like that at the head of the valley of Siete Lagunas, revealing a stepped appearance similar to the cirques á gradin, or Kartreppe found in the French or Swiss Alps. Sometimes the corries are filled with water, such as the Lagunilla de Mulhacén, and the Laguna de las Yeguas. There still exists a small remnant of the former ice in the hoyas, or hollows, of Mulhacén and Veleta; in both cases ice remains in the hollows all the year round. Some geographers refer to these patches of ice as glaciers, but since they cover an area small in comparison with Alpine glaciers, and since the ice scarcely moves, they cannot be termed glaciers in the strict sense of the word.
River action is more marked than ice action in the high zones; most geographers agree that the tremendously steep-sided valleys owe their present appearance not to ice but to the work of streams, which have great force in a region of such steep gradients. The dryness of the climate compared with that of most other high mountains has prevented lateral erosion from keeping pace with vertical erosion, and has produced valleys that are almost gorges. Routeways are possible in the valleys and gorges, but the construction of roads and railways is very difficult in the land immediately adjacent to them. On the lower foothills especially, where the rock is softer, gullying is marked, a reminder of infrequent but torrential storms. Where the rivers emerge on to the plains there is usually a large alluvial fan; such features are obvious on the west side of the Sierra Nevada, where many short streams debouch on to the plain of the Genil.
The topography of this mountain range, whether in the high ranges or the lower foothills, is imposing. An atlas map shows the general arrangement of hill masses and streams; it cannot, however, reveal the interesting local details. The central ridge contains the highest peaks and acts as a divide between the rivers, climate, cultivation and human activities of the north and south slopes. It runs almost due east-west, starting at El Caballo (10,581 feet) and passing through Veleta (11,128 feet), Mulhacén (11,420 feet) and the Alcazaba (11,185 feet) continues in the same direction, but at a lower level until the Cerros de Almirez (8,400 feet) is reached. The appearance of the actual peaks is often the result of structure rather than of erosion. Thus Mulhacén, a great granitic monolith, has a relatively rounded top; Veleta, composed partly of micaceous slates, has a very gentle slope to the south but a precipitous slope to the north, while the heights composed of younger rocks show rounded monotonous outlines if derived of softer material, but sharp, jagged peaks if of limestone.
High elevation and a low latitudinal position combine to make the climate of the Sierra Nevada a unique feature of the European mainland. The climate is basically Mediterranean, but many other factors have to be considered. In the first instance height reduces pressure; this sometimes induces slight mountain sickness. The effect of pressure, however, is not as great as that of increased insolation. Above 7,000 feet the light is strong, and some protection for the eyes is advisable, while the effect on the human skin can be dangerous. There are great extremes of temperature, both annual and diurnal. By day the summer temperature in direct sun may be well over 120° F., although on the heights shade temperatures may be as little as 45° F.; at night the temperature drops very suddenly to as little as 38° F., as a result of great radiation of heat from almost bare rock into air that contains no 'blanketing' cloud. In winter temperatures in the sun often reach over 65° F., though the whole mass of highland carries continuous snow well into spring. Much of the sun's energy at this time of year is reflected back from the snow, and for this reason springs tend to be late, particularly on the northern slopes.