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Location
The Avia rises in the Sierra del Suído, near Avión, in Fonte Avia and flows into the Miño River, near Ribadavia, in O Coto do Frade. It has a length of 38 km. It crosses the municipalities of Avión, Boborás, Cenlle, Leiro and Ribadavia. Its main tributaries are: Maquiáns river on the right; Viñao, Arenteiro and Varón rivers on the left.
Geomofology
The absence of not soluble rocks does not imply the absence of caves. Thus, in the granite and ganiodorite rocks of the NW and W of the Iberian Peninsula, in the provinces of A Coruña, Cáceres, Lugo, Ourense and Pontevedra, and in the Northern Region of Portugal, interesting caves have been studied, some even reaching 1562 m of topographic development. In this communication we briefly review the types of caves studied, focusing on describing the complex underground landscape that we can find in these cavities, highlighting, as part of the geological heritage, the forms sculpted in rock, the deposits, and specially the Speleothems and biospeleothems
Classification
1. Cavities with continuous limits:
a) Tafone caves, single block cavities and continuous limits
b) Lapa caves (sheet tafoni), cavities with continuous boundaries, located inside the shear bands interspersed in the slab structures.
c) Marine erosion caves, which are generally structural type caves, mostly associated with vertical or subvertical discontinuities, and developed by direct exploitation and widening of the discontinuity or by exploitation of the old deposits that fill these discontinuities.
2. Cavities with semi-continuous limits:
a) Structural caves, or cavities associated with joints, fractures or generally vertical or subvertical discontinuities.
b) Roofed canyons (rivers of blocks over canyons)
3. Cavities with discontinuous boundaries:
a) Caves associated with slab structures, and determined mainly by the foliation or lajamiento planes.
b) Caves formed by the accumulation of blocks. Three subtypes are distinguished based on the particularities of the block field and the geometry of the blocks:
1- Caves associated with rivers of blocks, that is, accumulations of blocks at the bottom of a valley, characterized by their linear extension (following the valley itself) and by the interaction between the chaos of blocks and the waters circulating through the valley itself. .
2- Caves associated with seas of blocks, that is, large accumulations of blocks.
3- Caves formed by accumulations of blocks with little movement, "in situ" produced by rocky creep or creep phenomena.
Of all of them, the caves in rivers of blocks and their structural variant, the caves in roofed canyons, are the morphological types of caves in plutonic rocks that present the largest dimensions in terms of development and volume. Both types form complex cavity systems.
CAVES IN ROOFED CANYONS.
When movements occur on the escarpments or slopes, the most common thing is that the mass of mobilized blocks reaches the bottom of the valley, burying it as it extends along it. Thus, accumulations of blocks of a certain power are formed, where the underground spaces defined between the blocks, between the blocks and the substrate, and between the blocks and the bedrock, configure the cave systems that we have called rivers of blocks.
It can also happen that the mass of mobilized blocks bury and fill a rock canyon. Even if the impact of the fallen blocks causes the canyon escarpments to break. Thus, what we have called a roofed canyon is formed. In these canyons the upper levels are morphologically similar to block rivers. And inside the canyon, there will be levels and passages formed by broken blocks, but also paleolevels characterized by their rock terraces with elevated forms of erosion with respect to the current course.
Albarellos or Susume system
Undoubtedly one of the most visually spectacular systems during the dry season of the Avia River. The records of the hydrographic basin show flows between 2,000 l/s and 212,000 l/s.
The system is defined in the contact of two convergent foliation structures. It is a large, slightly sinuous, roofed canyon, with an orientation N130ºE-N160ºE and which channels the Avia River for 295 m in length, saving a drop of 36 m. To date, 951 m of galleries and passages have been surveyed, of which a part corresponds to (paleo) levels hung during
the process of excavation-incision of the canyon. It is estimated that the total topographic development will exceed 1,100 m.
The most striking feature of this cavity are the forms of erosion, whose coexistence defines the subterranean landscape. The canyon has a U-shaped section in some sections, a Y-shaped section in others, defining a single flow channel except in filled or collapsed areas. In its main section it exceeds 11.5 m in height (bottom of channel to vault) with an average width of 5 m (see figure 10). The sink is
a waterfall 8 m high.
Associated erosion forms: Pias (Potholes)
Potholes are cylindrical holes drilled into a riverbed that vary in depth, diameter from a few centimeters to several meters. They are found in the upper course of a river where it has sufficient potential energy to erode vertically and its flow is turbulent. In the upper course of a river, its load is large and it is transported mainly by traction along the riverbed. When flowing water encounters objects on the bottom, the bottom is forced to pass over it and downdrafts form behind the bottom load in eddy shear currents. These currents erode the river bed and create small depressions in it.
As these depressions deepen, pebbles can become trapped in them. As a result of the rotating currents, the pebbles begin to cut into the depressions making them more circular, wider and deeper. Pebbles will only be able to erode a river bed if the rock of the pebbles is made of a stronger material than the bed rock of the river.
Fuentes:
Pseudokarst: un mundo en las otras cuevas Gota a gota, no 22: 56-65. Grupo de Espeleología de Villacarrillo
Cadernos Lab. Xeolóxico de Laxe Coruña. 2018. Vol. 40, pp. 159 - 186 ISSN: 0213-4497 Cuevas en ríos de bloques y cañones techados en rocas plutónicas del NW/W de la Península Ibérica
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