The Picos de Europa are a geological unit composed almost purely of limestone, and are the largest single mass of mountain limestone in Europe. The limestone was caused by the compression of sediments laid down beneath the ocean during the Carboniferous period, 345 to 280 million years ago. Later periods of uplift thrust the limestone high above sea-level, fracturing and folding it due to the great stresses involved, until three huge lumps of limestone remained - the three massifs. More recently, glaciations during the ice ages widened the fractures into narrow but deep gorges, such as the Cares Gorge. The actions of ice and rain continue to erode and dissolve the rock, forming the characteristic landscapes we see today.
Limestone dissolves in rainwater by a process known as karstification, the most visible sign of which is the vertical fluting of rock-faces. Also notable, and typical of karst landscapes, are the sink-holes or "jous" (or "hous" in Asturian), formed by lakes having drained through underground passages. The largest of these seemingly-lifeless jous are over 1km across, and are just as dramatic as the peaks surrounding them. The passages through which they drained have formed an extensive and complex cave system.
The high dry karst landscape is typical for its lack of surface water, but infrequent springs do arrive at the surface, though not for long, and they are notorious to locate. There are few lakes or tarns in the mountains (lakes Enol and Ercina being the obvious exceptions), few streams or rivers (except in the gorge bottoms), and not a single waterfall of note. Mining for zinc and lead ore used to be an important economic activity, mainly centred in Ándara in the eastern massif, and Áliva in the central massif, but all operations were abandoned towards the end of the twentieth century.
To read more about the Picos de Europa, click on the chapter headings at the top of this page.