Exhumation de sols polygonaux et dégradation de la pelouse d'altitude sur le Pla de Gorra Blanc (2.450 m). Massif du Puigmal - Pyrénées méditerranéennes

Abstract

Summary. Under the high altitude grass of « Gorra blanc » Pla, polygonal soils dating from the cold phases of the Quaternary period can be observed. This ancient configuration is revealed by the presence of apertures in the argillaceous central part and of traces of alpine clover on the bigstoned divisions. Tufts of plants and straggling grass have superseded the dense, boreo-alpine grass implanted during the Atlantic Optimum. This degradation has allowed, deep down, the starting of movements of geliturbation. The risings of clay are revealed on the surface by the presence of argillaceous or humiferous apertures. Trifolium alpinum can only survive away from those elevatory apparatus. A complete transformation of the ancient polygonal structure corresponds to a more thorough stage of exhumation.

Geomorphic Processes above Timberline in the Spanish Pyrenees

“…They contain a large proportion of periglacial dasts within a fine-grained matrix. Llobet (1975) place s them on schist substrates, and Soutade (1976) has found them on the Puigmal Massif(Eastern Pyrenees), at 2,300 m (on south and west-facing slopes), and at 2,400 m (on north-facing slopes) with no interference of vegetation at this altitude. Apart from mass wasting, which is the most evident transport system where deep soils occur, a significant propor- tiQn of sediment transfer is achieved by runoff waters.…”

“…They pro pose that, after the extinction of the glaciers, stone stripes and polygons were formed on the floors of cirques unless rock glaciers occurred instead. Soutade (1970a) also has discovered recent polygons on the Gorra Blanc high plateau in the Eastern Pyrcnees, which are at present undergoing degrading processes; thcir sizes are somewhere between mesopolygons and tundra macropolygons. Gonzalez-Martin (1986) has observed that most references to non-active periglacial phenomena are centered on the Eastern Pyrenees, so far thc most closely studied area.…”

“…Sorne authors use an altitude criterium (which ii:> actually a thermal criterium) to diffcrentiate sections within periglacial systems. Hence, Soutade (1976) notes that in the Puigmal massif (Eastern Pyrenees) there is a threshold at about 1,500 m aboye which cryonival processes prevail oyer surface-wash processes; that is, a limit that separates purely periglacial processes from those which include processes of humid and less eold c1imates. Hollermann (1985) distinguishes two belts aboye the: timberline: a) an active solifluction belt, with its lower limit irnrncdiately aboye the tirnberline, formed mainly by solifluction lobes and terracettes and generally occupied by pastures; b) an upper belt of solifluction and patterned ground, where blockfields, screes, and stone pavements (dallages) are predominant, and with extensive areas of bare ground.…”