Disharmoniske dalformer på Gran Canaria

Abstract

Schou, A. 1973: Disharmoniske dalformer på Gran Canaria. En analyse ved hjælp af blokdiagrammer. Geografisk Tidsskrift, 72: 1–11. København, September 30, 1973.

In Gran Canaria there are valleys of two heterogeneous types. I. Deep canyons bear witness to fluvial erosion of largest dimensions in a humid period of the past. 2. Flat bottom valleys with smoothly sloping sides like present days sheet wash depressions. There is a significant discrepancy between these valleys and the present fluvial dynamics of the subtropical semiarid environment.

SUMMARY

The surface relief of Gran Canaria (1550 km²) is dominated by a variety of volcanic landforms intensively dissected by normal erosion. There are valleys of two quite heterogeneous types.

A. Deep canyons (local: “Barranco”) radiating from the high central part (1950 m) of the circular shaped island in all directions constitute typical landscape elements. They bear witness to a fluvial erosion of largest dimensions in a humid temperate climatic period of the past.

B. Flat bottom valleys with smoothly sloping valley sides (local: “Valle”) may be met with mostly but not exclusively in the higher levels. These valleys are formed in old weathering masses of tropical character (red soils) by sheet wash in rainy periods of a savannah environment, judging from their similarity to present-day depressions (Kehltäler, H. Louis 1964) in the outher tropical zones. The “Valles” are fossil remnants of the Tertiary surface relief secured by a lava cover.

“Barranco” and “Valle” thus are relics of the past, respectively of a Holocene Pluvial period and of a Tropical one of Miocene age. There is a significant discrepancy between these valley types and the fluvial dynamics, qualitative as well as quantitative, of the subtropical semiarid climatic environment of to day's Gran Canaria.

Uplifted shorelines occur in many places. Especially along the North coast of Gran Canaria wave cut terraces and dead cliffs constitute characteristic landscape features. The longitudinal profile curve of the “Valle” South of Arucas seems to continue asymptotic to the terrace indicating a Tertiary sea level (150 m). The various lower terrace levels correspond approximately to Holocene ocean highwater situations of the Interglacial periods observed in other regions for example of the Mediterranean area. The deviations could be explained as results of isostatic movements.

A geomorphological analysis of these valleys is given by means of block diagrams. The morphogenetic development is illustrated by series of valley cross sections.

The author's preliminary work, execution of field sketches, photos of details etc. was carried out in December 1970. Heinz Klug's fundamental research results, published 1968 in his elaborate representation of the geomorphology of Gran Canaria, has been a most valuable and inspiring source of reliable data, for which I am indebted to my colleague from the summer 1969, when I worked as guest-professor in the University of Kiel.

This paper has been presented to The Royal Danish Academy of Sciences, 8th December 1972.