On some unusual valleys in Macaronesia

Mitchell-Thomé, R. C.

http://hdl.handle.net/10400.3/5474

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Authors

Mitchell-Thomé, R. C.

Abstract(s)

A valley can be defined as: «Any hollow or low-lying land bounded by hill or mountation ranges, and usually traversed by a stream or river which receives the drainage of the surrounding heights». It is commonly a linear depression, the result usually of fluvial erosion. […]. ln all such features, valleys of volcanic islands are akin to those in headwaters areas of continental valleys, and in general it may be said that such island valleys are more 'simple', more 'straightforward' in characteristics and patterns. Thus when we find valleys in these islands which depart from the usual traits, this must arouse our interest, for certain factors have interfered with the more 'normal' evolution. Never can one ignore the climatic regime pertaining to a region. Macaronesia extends from latitudes 40°-15° N, from longitudes 31°-13° W. Our definition states that «usually» a stream traverses a valley, but this does not hold for most of these Atlantic islands, yet in dry and wet seasons, the valley is always there. Average -annual rainfalls vary from 950 mm .in the Azores to 260 mm in Cape Verde; average annual temperatures .from 17.7 °C in the Azores to 24.5 °C in Cape Verde. […].