  We consider a stochastic flow in which individual particles follow skew
Brownian motions, with each one of these processes driven by the same Brownian
motion. One does not have uniqueness for the solutions of the corresponding
stochastic differential equation simultaneously for all real initial
conditions. Due to this lack of the simultaneous strong uniqueness for the
whole system of stochastic differential equations, the flow contains lenses,
that is, pairs of skew Brownian motions which start at the same point,
bifurcate, and then coalesce in a finite time. The paper contains qualitative
and quantitative (distributional) results on the geometry of the flow and
lenses.
