In the highly erosive marly catchments of Draix (Southern Alps, France), downstream fining of sediments has been observed and can not be explained by selective sorting. Moreover, high concentrations of suspended fine sediment (up to 800 g/L) are measured during flood events in these basins. These observations lead to the hypothesis that abrasion and fragmentation of marly sediments during transport play an important role in the production of fine sediments. In order to quantify these processes, experiments are realized in two different devices and provide abrasion rates ranging from 2 to 20%/km. Field measurements show a clear decrease in marl proportion and grain size. A simple model allows us to extract field integrated abrasion rates which appear to be higher than the experimental ones. This might be the evidence of the role of long term in situ degradation processes.