The Geomorphology of the surroundings of the Ría de Arosa (Galicia, NW Spain)

This paper is intended to provide a geomorphological introduction to a series of papers on the sedimentology and weathering phenomena of the Ría de Arosa area. The area contains a large mass of coarse-grained porphyritic granite surrounded by other crystalline rocks. The granite is deeply weathered and displays typical features such as spheroidal weathering and tors. The main relief elements are the following. (a) Low-angle slopes, many of which are foot-slopes (glacis) developed mainly on deeply weathered rock, and covered in many parts by colluvium and bedded slope-deposits (Nonn 1964). The latter locally continue below the present sea-level as kaolinite deposits. The low-angle slopes occupy a large area on the intrusive granite around the Ría de Arosa. (b) Rounded residual hills. (c) Mountain massifs in metamorphic rocks and migmatites, with steep slopes and flatter top-surfaces which are perhaps remains of a peneplain. The distribution of these features is given on the geomorphological map. The drainage pattern has been strongly influenced by some of the fracture directions of the basement rocks and by late-Tertiary faults. The latter broke up a Tertiary peneplain and created a long N-S rift. It may also have affected the coast and influenced the bottom topography around the Galicia Bank (Black et al. 1964). The main rivers maintain their original SW directions at the points where they cross the rift. Because these points had been lowered by subsidence of the rift zone, the downstream parts of the valleys, now occupied by the rias, became antecedent valleys in the rising block W of the rift. It is not necessary to assume individual subsidence of each of the rias. The valleys may be merely the result of denudation and erosion which processes must have continued on the present ria bottoms during the glacials when the sea-level was lowered. The rias must have existed in at least the last two interglacials, and probably much earlier (Mensching 1961); there are no indications of differential tectonic movements since then.