UDC: 551.44:001.4 Jean Nicod: A little contribution to the karst terminology : special or aberrant cases of poljes? A usual definition of polje states that it is "great closed karst basin with flat bottom, karstic drainage and steep peripheral slopes". But the Dinaric karst shows a wide range of poljes. The article discusses the main criteria of polje definition and the different degrees of evolution of the polje are emphasised. The essentials are gathered in the table with new tentatives on classification of poljes and comparing the Dinaric karst with other Mediterranean and Alpine countries.
In account of many studies (particularly geotechnical for dams build), these canyons are now defined in their relations with the karst hydrosystem : in their hydrological functions, and by the relations with neotectonics, change of base-level, and development of the endokarst. Exceptionally some anse from a collapse processes (breakdown in the karst passages). These news studies give data up the hydromecanical (evorsion) and hydrochemical processes and speed of the incision (maximum m proglacial canyons). The profiles of streams lead from chaotic to graded ; however in some mediterranean canyons travertines dams have blocked the incision. In most cases a turn or a sinuosity is in connection to fracturation and some incised meanders only in table like structures. The cutting of the Pont d'Arc (Ardèche) make a good model of karstic short cut. Many processes infere in the evolution of the cliffs and sides : unloading and gravity (mass-movements, collapses), cut off of the karst tubes and caves ("baumes-porches"), cryoclastism (inherited penglacial forms in North-mediterranean land, particularly frost-pockets and graded sides), weathering, calcrusts (case-hardening) and travertines deposits. On that account, the comparison between Sumidero of Grijalva (Chiapas, Mexico) and the mediterranean canyons is very useful.
Résumé. L'essai de synthèse que nous présentons ici s'appuie sur les nouvelles recherches effectuées sur la sédimentation des évaporites et sur l'examen de leurs propriétés (solubilité et plasticité). Certaines parties des grands bassins sédimentaires présentent des régions karstiques très étendues dans les gypses : Texas-Oklahoma, Nouveau Mexique, zone de contact au SO du Harz, région de Perm et Podolie. Moins étendus, les karsts des gypses triasiques des Alpes présentent des formes spectaculaires, particulièrement en Beaufortin et près du Col du Mont-Cenis. En altitude, les dépressions sont particulièrement fraîches et évolutives ; elles se sont créées depuis les derniers stades de déglaciation tardiglaciaires. Dans les régions méditer ranéennes, les karsts du gypse sont nombreux et variés, en fonction des conditions structurales et de la durée plus ou moins grande d'évolution : ainsi, en Basse-Provence coexistent des formes anciennes, poljés néogènes plus ou moins oblitérés, et des effondrements, actifs mais dispersés.
Surface features reflect not only the importance of cave collapse, but also the action of present and inherited zonal processes. The rates of geomorphological evolution are compared (main data in table 1). The particularities of hydrological circulation are studied, as well as the ivorkings of some karstic lakes, particularly the. Banyoles lake (Catalonia). The various processes generate numerous hazards enumerated in the paper and evident in urban and suburban areas and in the civil engineering works such as tunnels, dams, etc...
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