  We investigate the effective friction encountered by a mass sliding on a
granular layer as a function of bed thickness and boundary roughness
conditions. The observed friction has minima for a small number of layers
before it increases and saturates to a value which depends on the roughness of
the sliding surface. We use an index-matched interstitial liquid to probe the
internal motion of the grains with fluorescence imaging in a regime where the
liquid has no significant effect on the measured friction. The shear profiles
obtained as a function of depth show decrease in slip near the sliding surface
as the layer thickness is increased. We propose that the friction depends on
the degree of grain confinement relative to the sliding surfaces.
