The central focus of this work is to study the processes acting well below the surface of a moving rock or debris avalanche during travel over stationary substrate material. Small-scale physical models at a linear scale of 1:104 used coal as avalanche analogue material and different granular material simulating sedimentary substrates varying in frictional resistance, thickness and relative basal boundary roughness, as well as inerodible, non-deformable runout path conditions. Substrate materials with the least frictional resistance showed the greatest response to granular flow overriding, becoming entirely mobilized beneath and ahead of the moving mass and producing the longest runout observed with a unique deposit profile shape. With a smooth substrate basal contact, failure occurred along this plane and avalanche and substrate became coupled during runout. With a rough base, however, temporary force chains of grain contacts in the substrate prevailed longer, imparted their resistance to motion/shear into the granular flow, and the flow rear section consequently halted earlier than when moving over substrates with a weak base. Reducing substrate thickness diminished the effect of basal contact roughness on granular flow runout and deposit length. Inerodible, non-deformable substrate conditions caused changes in granular flow behaviour from essentially en masse sliding on low-friction surfaces to increasing granular agitation over rougher paths. Copyright © 2012 John Wiley & Sons, Ltd.