Many blockstreams occur in the Miocene volcanic area of Mont Mézenc, eastern Velay, SE Massif Central, France. They were supplied by frost-shattered exposures of lava-flow boundaries, phonolitic domes and other intrusions. These ‘rivers of rock’ are ribbons of chaotically upheaved blocks presenting an openwork structure, are 400 to 1100 m long and 20 to 200 m wide, and are today disconnected from the rock exposures that supplied the debris. Some occur on very low gradient slopes. The largest blockstreams offer evidence of compression ridges and post-depositional block subsidence. Their dynamics and age are not precisely known in the study area and they have been interpreted by default as Late Pleniglacial rock glaciers. Field observations (stratigraphic relations with other surficial deposits, topographic position, distance from bedrock source areas and from talwegs, structural context), however, indicate that the situation is more complex, with new implications for their age, origin and past dynamics. This reconnaissance study proposes a typology of blockstreams for the area. The base of the blockstreams remains inaccessible to direct observation but several ground-penetrating radar transects were carried out in order to map the topography at depth. Results show that the blocks fill gullies 5 to 15 m in depth. At the surface, a sharp textural transition with other slope deposits is clearly detected but the latter grade topographically to the surface of the blockstreams, not to the gullies beneath them. These cross-cutting relations provide a relative chronology not previously established in the case of blockstream deposits.