This study presents a new geologic map of the Internal Prealps between the Arve and the Rhône rivers and compiles current knowledge about this complicated region of the Alpine chain. Several chaotic units, or mélanges, have been identified on the basis of their structural position and the nature of their included elements. The Suprahelvetic mélanges, comprising a lower, a middle and an upper sub-unit, directly overlay the Helvetic or the Autochtonous basement, and include blocks from the outermost basins of the Alpine domain. The Infrabrèche mélange appears exclusively below the Brèche nappe and regroups lenses from the central parts of the Alpine domain (briançonnais and piémontais realms). The Transverse mélange bodies contain elements from all other units but show a distinct oblique orientation to regional structural trends. Fragmentation, mixing and emplacement of all but one mélange seem to result mainly from tectonic processes: the Transverse mélanges are associated with the local strike-slip faults; the Infrabrèche mélange was formed during thrusting of the Brèche nappe over the Briançonnais domain; the middle and upper Suprahelvetic mélanges were propably generated during the deformation of the Helvetic domain. No biostratigraphic or sedimentological arguments support a possible sedimentary origin for the lower Suprahelvetic mélange. Moreover, its basal contact has been overprinted by several deformation phases. Consequently, caution is required when applying sedimentary models to this mélange.