  We report the experimental observation of the bifurcation at the origin of
localization of the deformation in a granular material submitted to uniaxial
compression. We present a quantitative characterization of the heterogeneity in
the strain field repartition allowing to evidence objectively the existence of
a bifurcation initiating the shear bands formation process. We show that this
bifurcation is supercritical and has no clear signature on the stress-strain
curve. At the bifurcation, a symmetry breaking occurs characterized by the
emergence of a well-defined orientation corresponding to the Mohr-Coulomb
angle. Yet, plasticity is still diffuse and the shear band extension is of the
order of the sample width. While loading proceeds, the shear band narrows until
it reaches, after the peak of the stress-strain curve, a stationary width.
