ABSTRACT. Field investigation of six medial moraines on glaciers near Arolla, Valais, Switzerland, has revealed that all are no urish ed by englacia l till. This is probably derived from supraglacial point-sources on the accumula tion zones of the glaciers, a nd is released in incr easing quantities on the ablation zo nes, where it emerges diffusely or by way of clearly defined transverse or longitudinal debris bands. Poss ible mechanisms of form ation of these debris bands are reviewed, and it is proposed that the transverse bands originated as crevasse fillings on the accumulation zones. Morphologically all six moraines display waxing sections (of in creasing height and breadth), related to development of a protective till cover and resulta nt differential ablation of the glacier surface. Two only display waning sections (of decreasing height), resulting from la te ral sliding of tm and " inverted" differential a blatio n r esulting from extreme attenuation of till cover. Four "moraine-types" (related to na ture of debris supply and detailed morphology) are identified in the Arolla area. It is believed that these may form the basis for a classification of m edial moraines. REsuME. Laformation des moraines medianes des glaciers alpins. Des recherches sur six moraines medianes d e glaciers pres d'Arolla, Valais, Suisse, ont revele qu'elles sont toutes nourries par la moraine interne. L es materiaux proviennent probablement de points-sources emergeant hors de la glace d a ns la zone d'accumulation d es glaciers, et sont r estitues en quantites croissantes da ns les zones d 'abla tion OU ils emergent soit d e maniere diffuse soit sous form e de rubans d e sediments clairement visibles, transversaux ou longitudinaux. Les m ecanismes possibles de formation de ces rubans sont passes en revue et on propose pour origine des rubans transversaux, le remplissage de crevasses dans la zone d'accumulation. Morphologiquement, toutes les six m oraines montrent d es sections croissantes (en hauteur et e n la rgeur), en raison d e la creation d ' un e couche protectrice de materia ux morainiques et de l'ablation differentielle de la surface du glacier. D eux sections seulement vont e n s' evanouissa nt (ha ut eur decroissante ) e n ra ison du glissement lateral de la moraine et de l'ablation differ entiell e " inverse" causee p ar l'amincissement extreme de la couverture de materiaux morainiques. Quatre "types d e moraines" (distingues d'apres la nature de leurs materiaux et le detail de leur morphologie) sont identifies dans le secteur d'Aroll a. On pense que ces types de mora ines peuvent former la base p our une classification des moraines medianes.ZUSAMMENFASSUNG. Die Bildung von Mittelmoriinen auf Alpengletschem. Felduntersu chungen an 6 Mittelmorane n a uf Gletschern b ei Arolla im Wall is, Schweiz, haben e rge ben, dass alle durch Schutt a us dem Inneren der Gletscher ern a hrt werden. Dieser stammt vermutlich von punktuelle n Quellen oberhalb d e r Gletscher und taucht in zun ehmendem M asse in den Ablationszonen auf, wo...
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