  This paper is devoted to the basic question of what factors determine the
strain field in a quasi static granular flow. It is shown that using stress -
strain rate relations is not the proper way to understand quasi static
rheology. An alternative approach is discussed where the local deformation is
regarded as the cause of deformation in the vicinity. We suggest a continuum
model where the local shear strain is proportional to its Laplacian and the
proportionality factor is determined by the local stress. The predicted
behavior is tested in a three dimensional shear cell by means of computer
simulations. The simplicity of our setup makes it ideal to demonstrate and
examine the fundamental open questions of collective granular flows. The
observed shear profile is interpreted in the framework of the suggested model.
