According to various studies, the degradation of marly lithologies that constitute heterogeneous rock masses such as flysch, can trigger and condition many slope instabilities. However, not all marly lithologies degrade in the same way, exhibiting considerable differences in the rates and ways of degradation. Therefore, it is very important to know the degradation behaviour of different lithotypes against environmental conditions, characterizing their weathering patterns and quantifying their rates of deterioration. This issue has been widely studied trough different approaches: a) through laboratory tests, mainly based on the slake durability test (SDT); b) through the study of monitored natural slaking behaviour; and c) by means of the characterization of in situ observed patterns and natural degradation rates. The main aim of this work is to link these different approaches used to perform this characterization. For this purpose, an experimental test, in which various samples have been exposed under monitored natural climatic conditions for 12 months and have been characterized through the study of the fragment size distribution curves of the degraded particles, has been developed. Additionally, these samples have been also classified based on the Potential Degradation Index (PDI) determined through laboratory tests. Finally, the in situ weathering behaviour profiles have been also logged. Preliminary results of this research suggest the existence of a clear relation between the different classes of degradation stated by the Potential Degradation Index (PDI), the behaviour of the fragments retained in the drum along the SDT cycles, the slaking behaviour under natural climatic conditions and the weathering behaviour patterns and rates observed at weathered profiles in the field.