The paper describes a slope failure involving an unreinforced geosynthetic clay liner (GCL) in a municipal solid waste (MSW) landfill liner system. The precise mechanism for the shear movement of the interim slope is not known. However, the significant damage observed in the upper components of the composite liner system suggests that the failure was translatory primarily along the interface between the recompacted soil liner and the overlying hydrated bentonite of the geomembrane - backed GCL.