  Shock parameters at Earth's bow shock in rare instances can approach the Mach
numbers predicted at supernova remnants. We present our analysis of a high
Alfv\'en Mach number ($M_A= 27$) shock utilizing multipoint measurements from
the Magnetospheric Multiscale (MMS) spacecraft during a crossing of Earth's
quasi-perpendicular bow shock. We find that the shock dynamics are mostly
driven by reflected ions, perturbations that they generate, and nonlinear
amplification of the perturbations. Our analyses show that reflected ions
create modest magnetic enhancements upstream of the shock which evolve in a
nonlinear manner as they traverse the shock foot. They can transform into
proto-shocks that propagate at small angles to the magnetic field and towards
the bow shock. The nonstationary bow shock shows signatures of both reformation
and surface ripples. Our observations indicate that although shock reformation
occurs, the main shock layer never disappears. These observations are at high
plasma $\beta$, a parameter regime which has not been well explored by
numerical models.
