The 27–45 m high bluffs along the north shore of Lake Erie in Ontario are retreating inland at a 150 year average rate of 0.5 to 3.0 m/year. The retreat mechanisms and rates are cyclic and consist of complex interrelationships of toe erosion by wave attack, cliff steepening, sheet sloughing, and landsliding associated with softening in response to changes in effective stress. Four cyclic failure types are presented.1. Slowly retreating (0.6 m/year) unvegetated cliff fronts of 27 m height and nearly constant profile associated with low energy wave attack (0.5 kW/m) at the toe and cyclic sheet sloughing caused by desiccation and wetting at monthly intervals. Landsliding very rare and surficial.2. Cliffs of 34 m height retreating at intermediate rates (1.4 m/year) showing cyclic changes in profile in response to major, cyclic landslides. Retreat mechanisms involve toe erosion by intermediate energy wave attack (1.0 kW/m) and large toe failures at 10–20 year intervals in response to softening and changes in effective stress.3. Cliffs of 42 m height retreating at 2.8 m/year showing cyclic changes in profile in response to major, high velocity, cyclic landslides that deposit offshore fans of slide debris that provide erosion protection but no counterbalancing moment protection. Toe sands play a significant role, enabling undercutting by high energy wave attack (~2 kW/m). Size and frequency of landslides too variable to quantify at present.4. Cliffs of 38–45 m height retreating at 2.0 m/year in response to toe erosion under high energy wave attack (2 kW/m). Thick water bearing surface sands cause sloughing and shallow failure of the upper slopes at 1.5 year intervals above near vertical wave cut toe cliffs. Huge, deep seated failures may develop during periods of cyclic low water level at roughly estimated intervals of 30 years.
