In some regions the clay banks of the St. Lawrence River along the Montréal to Lac St. Pierre reach have recession rates of up to 1–3 m/year. The banks are formed of structured marine clays of the Champlain Sea (Leda clay). In this laboratory study, undisturbed samples of this high-plasticity inorganic clay taken at Îles de Verchères were subjected to a unidirectional current and a constant wave climate to investigate the mechanisms of erosion and the factors influencing erosion rates. Initially, surface erosion resulted in the formation and enlargement of cracks and the smoothing of competent surfaces. The dominant erosion process was a mass erosion of the blocks of clay delineated by the cracks. Desiccation or weathering significantly increased erosion rates, as tension cracks formed due to drying, and upon rewetting, the formation of microfissures resulted in disintegration into small, easily erodable flakes. The estimated critical shear stress of the samples was 6–20 Pa. For the St. Lawrence River, these results suggest that waves are the dominant erosion mechanism, with shipping contributing significantly to the erosion of banks close to the navigation channel. Weathering caused by wetting and drying from changing water levels or wave runup greatly increases erosion rates.Key words: erosion, Leda clay, undisturbed clay, natural clay structure, St. Lawrence River, waves, weathering, desiccation, vegetation.