In this paper a hybrid particle–fluid simulation code is used to examine the large‐scale (&lgr;∼c/&ohgr;pi) low‐frequency (&ohgr;∼&ohgr;ci) formation and evolution of a collisonless shock in two dimensions where the shock is caused by a plasma slug of finite thickness traveling perpendicular to the ambient magnetic field in the presence of a uniform background plasma. During the evolution of the shock structure, background ions are energized in the perpendicular direction and then isotropized. The isotropization process is associated with the presence of large‐amplitude, primarily electromagnetic, waves propagating parallel to the ambient magnetic field that are created by the self‐consistent plasma interaction. High Mach number shocks are found to be inherently two‐dimensional objects.