Pyrenees High Route 5 - Karst Landscapes

This is stage five of the Pyrenees High Route through the far east of the Basque Country and into the high mountains of the Pyrenees. The trail runs along the French/Spanish border for much of the way before entering a magical limestone landscape that is very beautiful but challenging to navigate.

Limestone of the Pyrenees


Tectonic plate movements over the period 20-75 million years ago pushed the Iberian crust (the land we know as Spain and Portugal today) north colliding with the larger European crust. This movement caused the Pyreenes mountains we see today to be created. Under huge pressure and heat, layers of rock were squeezed together, old layers pushed under and over younger layers. These bands of mostly granite and limestone are visible today across the mountain range.

Limestone is a sedimentary rock that is composed of the skeletal fragments of marine organisms such as coral, foraminifera, and molluscs. It contains calcium carbonate and therefore is susceptible to water erosion.

The karst landscapes we see today have been formed by the erosion of soluble rocks like limestone over millions of years by rainwater. As water seeps into the rock, it slowly erodes. Karst landscapes can be worn away from the top or dissolved from a weak point inside the rock. They are characterised by caves, underground streams and sinkholes on the surface. It is common for the caves to contain stalactites and stalagmites, which are formed by the collection of mineral deposits from water dripping into caverns.

Bedding Planes: horizontal layers are formed as the rocks are compressed under deposits formed above.

Limestone Pavements: glacier movement removes the topsoil exposing the beds of limestone rock. Weathering (rain erosion, freezing water, thawing) further erodes the rock. Carbonation and freeze thaw weathering expose cracks in the limestone - called joints and bedding planes. As the cracks widen a series of rectangular blocks are created. The blocks are called clints and the joints are grykes.

Sinkhole (aka pothole and swallow hole): surface and rain water do not flow far on exposed limestone, but infiltrate rapidly into the rock and soil. Where a joint or intersection of joints has been greatly weathered or dissolved water can pass down through the limestone. A stream travelling over an impermeable rock will very quickly disappear when it has to travel over limestone. These sinkholes can be many metres deep leading down to a series of subsurface features.

Shakehole: a depression in the limestone landscape. In some limestone areas there is a covering of boulder clay about two or three metres thick. Shakeholes are formed where surface water washes the boulder clay down into cracks or fissures in the limestone under the boulder clay. They are usually found in groups. Cavers have to dig the boulder clay out to see if there is a pothole of any size underneath.

Spring (Resurgence): when a stream disappears it travels underground through a complex series of caves and eventually works its way down to a level of impermeable rock or until it reaches the top of the water table. The stream travels along the surface of the impermeable rock until it reaches the surface as a spring. Where limestone lies on top of impermeable rock along a valley there can be several springs formed along the intersection of the two rocks. This is a spring line.

Dry Valley: during the last ice age the limestone was frozen to great depths. When the ice melted it carved out valleys over the frozen rock. When the limestone thawed out the surface water was able to infiltrate down through the rocks and the dry valleys were left with no surface water.

Gorge: during the meltwater phase of the last ice age, underground streams eroded vast caverns as huge volumes of water travelled through the limestone. Sometimes the roofs of these caverns can collapse exposing the "underground stream" in a very steep sided gorge.

Scars: during the last ice age huge ice sheets scraped away the soil covered spurs in many valleys. Steep cliffs of bare rock were exposed. Because these scars are more liable to frost shattering and other forms of erosion they usually have scree slopes of broken rock below them.

Erratic Bolder: the ice sheet which covered the area carried with it huge boulders. When the ice melted these huge boulders were left sitting on the surface. They are called erratics because they are totally different from the bed rock of the local area.


Sources


en.wikipedia.org
www.nationalgeographic.org
www.britannica.com

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