QUATERNARY CLIMATE CHANGES IN THE O COUREL MOUNTAINS.
THE TRACES OF THE LAST GLACIATION.
The action of glacial ice during the Pleistocene, a geological process that began 2.58 million years ago and ended about 10,000 years ago, is manifested in the glacial remains of the Sierra de O Courel, which are interpreted here taking into account the materials present in the soil and the traces left in the landscape produced by the action of ice.
GLACIATIONS
Each ice age is a long period in which the global temperature of the Earth’s climate drops. As a result, there is a southward expansion of the ice caps and the glaciers grow. We are now living in a warm interglacial period that has already lasted 12,000 years. The first ice age was between 2.7 and 2.3 billion years ago.
During cold periods, the ice masses became 3,000 m thick and the sea level dropped 120 m, when much of the ocean froze.
Further south of the ice line, ice ages also affect mountainous regions.
PARTS OF A GLACIER
A glacier is a river of ice that runs from the highest parts of the mountains to the bottom of a valley, to reach lower areas with higher temperatures where it ends up melting. It works like a giant bulldozer that drags, wears down, files and compacts everything in its path.
In the figure:
-Accumulation area.
-Clean rocky.
-Crevasses.
-Ablation area.
-Bottom moraine.
-Front moraine.
-Lateral moraine.
-Circus.
-Glacier tongue.
-Central moraine.
-Seracs.
-Glacier tonque.
GLACIER TYPES
Glaciers, depending on their distribution in the different glacial areas of the world, according to the relief of the mountains in which they were formed or the local climatic conditions, take different forms to classify them and explain their different origins or behaviors.
In Galicia they are of two types. All Galician glaciers are of the ice cap type except those of O Courel, which were of the circus type.
Those of the ice cap type are characterized by forming in areas where the ice was always below its melting temperature, without alternating with periods of thaw. Permafrost clung firmly to the ground, so it could cover the entire crest line of a mountain range or range, right down to the most vertical peaks or walls.
In the image:
* Extreme cold most of the year, with little thaw. Compacted snow and ice stick to rock even on steep slopes and near-vertical ridges. They completely cover a ridge, mountain range, or ridge.
Cirque glaciers originated from the accumulation of snow, later frozen and compacted to form ice, on the concave wings of the mountains. This formation of less severe local climates depended to a large extent on the orientation of the slopes and the prevailing winds, which accumulated snowfall and storms. This ice could be subjected to periodic melting processes, so its fixing power on the rock was less. In this way, it took refuge in the less vertical concave slopes and left the highest and steepest peaks and walls bare. In a mountain range like that of O Courel, cirque glaciers could cover their summits but leave their crests and peaks free, separating the bare rock from the lower ice layer.
In the image:
* Extreme cold located on faces facing north or prevailing winds. Annual thaw periods. The snow is only preserved and compacted on slopes exposed to the cold and supported by concave slopes with a moderate inclination.
THE GLACIATION IN COUREL MOUNTAINS
About 50,000 years ago and up to 11,000 years ago, the A Seara and Vieiros glaciers, here in Quiroga, began at about 1,500 m altitude and melted at about 900 m. In Galicia there is evidence of other glacial phenomena, such as the O Xistral, Os Ancares, O Eixe, O Invernadeiro, Queixa, O Xurés, Manzaneda and Pena Trevinca mountain ranges.
The A Seara-Vieiros glacier was formed by three ice tongues that converged downstream from the A Lucenza Lagoon: Porto Mourelo, A Lucenza and Formigueiros. It is believed that there could be a fourth, that of Vieiros (Alto dos Castros Sur and Pallosa Sur), but it has not been sufficiently studied. The main glacier reached its maximum recognized advance about 6 km long and 200 m thick.
In other areas of the O Courel mountain range, such as the Visuña valley, the presence of glacial ice also played an important role on the north and east faces of Formigueiros mountains.
IN OTHER AREAS OF GALICIA
Traces of fossil glacialism in Galicia are found at certain points in the Sierra de Os Ancares, O Courel, Queixa-O Invernadeiro, Larouco-O Xurés, O Xistral, O Eixe, Queixa, Manzaneda and Pena Trevinca.
During the phase of maximum expansion, the glacial tongues descended to altitudes of 700-800 m, both in massifs with peaks close to 2,000 m (Os Ancares, Trevinca) and in others of lower altitude (O Xistral, 1,062 m).
Legende 1:
-Glacial cirques and edges.
-Lateral moraine.
-Medium moraine.
-Glacier polished surface.
-Deposit.
-Village.
Legende 2:
-Glacial cirque.
-Reconstruction of valleys and glacier dynamics.
-Direction of ice tongues.
-Difluence. Part of the glacier goes up the pass towards the Soldón valley.
A SEARA AND VIEIROS GLACIERS
Image 2:
-Transfluence pass.
-Cu de Galo Glacier.
-O Valecín Glacier and As Forgas.
-Murelos Glacier and O Fócaro.
-A Lucenza Glacier.
-Formigueiros Glacier.
-A Campa da Retorta Glacier.
-Vieiros Glacier.
Image 3:
-Transfluence pass.
-Cu de Galo Glacier.
-O Valecín Glacier and As Forgas.
-Murelos Glacier and O Fócaro.
-A Lucenza Glacier.
-Formigueiros Glacier.
-A Campa da Retorta Glacier.
-Vieiros Glacier.
Inside the image 3:
-Outer edge.
-Lateral moraine.
-Inner edge.
THE TRACES OF THE GLACIERS. The passage of rivers of ice shaped the landscape and left behind remains of crushed rock.
*GLACIER CIRQUES
Small in size, in O Courel they are oriented to the north and northeast and are around 1,200 and 1,500 meters high. The best example is at the head of the A Lucenza valley, in Quiroga.
*ROCK THRESHOLDS
Most of the O Courel glacial cirques have associated rocky thresholds, originated in most cases both by glacial activity and by the disposition of the rocks over which the ice has moved.
*GLACIER SHOULDERS
Unlike what happens in other mountains that have suffered from glacial activity, in those of O Courel these forms hardly appear, which are abrupt changes in the profile of the terrain. The most striking example is in A Seara.
* HANGING GLACIER VALLEYS
The A Lucenza glacier joined the Porto Mourelo glacier, forming a spectacular ice balcony right where the village of A Seara is now located.
*SHAPED AND POLISHED ROCKS
The appearance of rounded rocks, without edges, is a good symptom of the presence of glaciations in the past. However, the low hardness of the rocks at O Courel did not facilitate this phenomenon: ice could do more than rock. Still, beautiful examples are preserved.
*TRANSFLUENCE PASS
They occurred when the ice on the sides of the glacier, pushed by the one in the center, was able to climb the slopes of the valley to, leaving its basin, descend through a parallel valley. Once again we have a good example in A Seara.
Photo 1:
-Cu de Galo glacier cirque.
-As Forgas glacier cirque.
-O Valecín and O Fócaro glacier cirque.
Photo 6:
-North glacier of O Carballón.
-O Porto Mourelo Glacier cirque.
Picture 8:
-Difluence towards the Formigueiros Glacier.
-Cirqie of the head of the valley of A Lucenza.
-A Lucenza Lake
Photo 13:
-North glacier of O Carballón.
-Transfluence pass.
-Porto Mourelo glacier.
A LUCENZA LAKE
It is a lake that is heavily filled with sediments.
It is located at an altitude of 1,440 m and is about 70 m in diameter.
It was formed after the disappearance of a glacier. The deposits it left behind in its retreat, aided by the presence of a rocky outcrop, caused meltwater to accumulate.
Dating by the carbon 14 method indicates an age of the lagoon of approximately 13,400 years.
At the end of the summer it dries up completely, while in the winter it is so surrounded by snow that it is almost inaccessible.
Photo 2: Morainic ridges.
Photo 3:
-Morainic archs.
-A Lucenza Lake (summer time).
-Rocky ledge.
GLACIAL DEPOSITS
Glaciers are true sediment-carrying machines both at the front and at the sides, at the base and inside.
When the ice disappears, there are very diverse accumulations in the territory, which currently help us to detect its presence in the past.
*A SEARA SUBGLACIAL DEPOSIT
-The cutting of the land caused by the construction of the road from Cruz de Outeiro to A Seara allows us to discover the interior of a moraine.
-Very fine and very compact sediments that show different moments in the life of the glacier and the enormous pressure they suffered due to the weight of the ice.
-Among the different songs that we find in the moraine, the difference in color indicates their different nature and origin. The dark ones are slate rocks from the substrate itself. The clearings are diabases coming from the upper part of the valley, dragged by the glacier.
-These photos, taken under a microscope, allow us to see in detail what glacial sediments and quartz grains with glacial pressure morphology look like. Arrangement and shape of grains in sediments from subglacial deposits.
Microscopic photos 1 and 2: Arrangement and shape of the grains in the sediments of the subglacial deposit.
Microscopic photos 3 and 4: Superficial microtextures of the grains produced by glacial erosion.
A LUCENZA MORAINIC RIDGES
In the area where the A Lucenza lagoon sits, there are a series of morainic ridges that record the phases of the glacier’s retreat. It is in circuses like this that ice tongues begin at the beginning of the glaciation and where they later disappear.
Blocks of glacial modeling from the morainic ridges of A Lucenza.
-They usually have a shape with a profile similar to an ironing board. Flattened, fairly smooth, narrower at one end and broader at the other, and with rounded edges, no sharp edges.
-They present a multitude of grooves as a result of friction caused by the advance / retreat of the ice.