Lynds and Petrosian1 and Soucail et al. 2 have reported the discovery of circular arcs in the cores of two high-redshift (ζ~0.3) clusters. The arcs are ~100kpc in length, ~5kpc in width, and have radii of curvature ~70kpc (H0 = 75 km s-1 Mpc-1). Giant elliptical galaxies are found near the centres of both arcs. The arcs have total luminosities of ~1044ergs-1 and are unusually blue in colour. One explanation for the arcs is that they are regions of rapid star formation behind galaxy bow shocks, where our line of sight is tangent to the shock surface and approximately perpendicular to the galaxy velocity. Here we describe some of the conditions under which bow shocks can form and point out some observationally testable implications of this model.