This paper aims to define the control exerted by geological setting on the evolution and segmentation of the Montaguto earth flow in Southern Italy. A detailed geological survey and multi-temporal maps were used to reconstruct the geological model of La Montagna Mt. and to quantify the evolution of the flow in terms of sediment pulse formation, movement direction and spatial change in earth-flow area. The spatial distribution of structural fabric of the mountainside was compared with (1) planar geometry of earth-flow source area, (2) orientation of strike-slip faults bounding the earth-flow moving core (i.e. movement direction) and (3) kinematic-zone position and extent. We infer that the evolution of the earth flow is controlled by geological structures and lithology. Moreover, the entire basal-slip surface is composed of newly formed basal-slip surface in the source area, and corresponds to pre-earth-flow topography downslope from the neck. Implications of our work are that (1) a detailed geological model of an unstable mountainside, combined with information about earth-flow spatial evolution, might be considered as a tool that allows the localization of areas of possible earth-flow enlargement and (2) geological structures constrain earth-flow movement, inducing a predictable seasonal behaviour in terms of movement direction.