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Geology in Sobrarbe

Glacial and periglacial landforms

An alpine glacier is a thick ice layer formed in high mountains as a consequence of snow compactation and recrystallization, flowing through valleys which originally had been occupied by river flows. Glaciers are formed where snow and ice accumulation exceed snow and ice melt.

The erosive power of moving ice is huge. Glaciers erode the bedrock by pulling up fragments of rocks of different sizes; they also grind and scrape (abrasion) the surface below like sandpaper. Erosive forms are easily recognizable, for example: glacial valleys, typically U-shaped, hanging valleys, glacial lakes (known as ibones in Aragon), cirques, edges, etc.

The most interesting sedimentary glacial formations are moraines, placed in different positions along the main glacier (lateral moraines, medial, ground and terminal). They are formed by unconsolidated sediments, a non-sorted mixture of rock, gravel and boulders within a matrix of a fine powdery material. Terminal moraines are arch-shaped and they can give us valuable information about the maximum size of the glacier, its thickness and the stages of its reversal until it retreated.

60.000 years ago, during the last maximum Pleistocene Glacial in the Pyrenees (Sancho et al., 2003), glaciers flowed downhill along the Sobrarbe valleys reaching Planduviar and Las Devotas. The landscape at the north of the Geopark shows several shapes left by the Ice Ages. Since then, those glaciers have retreated to their current and reduced form, except for short periods when cold weather favours their brief redevelopment.

Glaciers in northern Aragon are the last ones remaining in the Iberian Peninsula and the southernmost Glaciers in Europe. Although they are fast retreating due to global warming, they still constitute singular ecosystems with unique species, and studying their evolution, we can obtain valuable data which can help us to understand climatic changes.

For those very reasons, Aragonese Pyrenean glaciers were declared Natural Monuments by the Bill 2/1990 passed by the Aragon Congress Later, the protected aea was enlarged and modified. 2007 enlargement and modification of Protection Planp>

Glacial valley in Cotatuero, in the Ordesa and Monte Perdido National Park.

In those places previously occupied by glaciers, where cold and freezing temperatures are still present, ice and water mix thereby sculpting the landscape.

When water penetrates fractures in the rock, it then freezes, expands and creates pressure against the walls of the crevices. If this process is repeated, crevices widen until the rock is fragmented into angular debris (gelifracts or crioclasts). Repeated freezing and thawing can also affect subsoil (cryoturbation). Currently, we can say that the periglacial domain prevails in the Pyrenees from 1900m above sea level..

If water soaking the soil freezes and melts, it can cause the soil to move irregularly downhill. Landforms associated with periglacial environments show patterned grounds and terrassettes, also known as "cow tracks".

When the flow of melt water moves sediments deposited by a glacier, it can form layers of sorted debris, known as stratified scree, (grèzes and groizes litées) very common on south-oriented slopes.

Periglacial moulding. Stratified scree between Silves and Aguilar.

The most breathtaking periglacial landscapes are stone rivers, formed by angular boulders. Their origin is associated with ancient glaciers and snowfields covered by the moraine of a glacial cirque. A well preserved example can be found on the north face of the Peña las Onze, in the Cotiella Massif.

Scree also has a periglacial origin. These are unconsolidated deposits of debris, covering many slopes. These deposits are easily developed from limestone, granite and metamorphic rocks, which are easy to fracture. At the foot of steep mountains, we can find debris cones. We can find this debris in lower areas, as ancient witness to a colder climate in the past.