A very large deep-seated landslide (DSL) in the northern Pyrenees with over ~1.4×109m3 was mapped and dated based on sedimentation rates and 10Be terrestrial cosmogenic radionuclide surface exposure (CRE) dating. Our analysis on the landslide reveals the role of inherited structures in the landslide process, and highlighted typical gravitational morpho-structures and a small lake trapped at the toe of the landslide head scarp. The rate of lake sedimentation (0.86±0.57mmyr-1) also provided us with the approximate age of the landslide: 1106±540yr. The CRE dating result highlights two main slope destabilization phases. Then we discussed the history of DSL activity and its controlling factors. Information related to historic markers and the absence of particular climate markers and changes during the Medieval Dark Ages point to a single event in AD 1380 due to a major seismic event (Lavedan earthquake). © 2013 Elsevier B.V.