We present deep, panoramic multi-color imaging of the distant rich cluster A851 (z=0.41) using Suprime-Cam on Subaru. These images cover a 27' field of view (~11 h_{50}^{-1}Mpc), and by exploiting photometric redshifts, we can isolate galaxies in a narrow redshift slice at the cluster redshift. Using a sample of ~2700 probable cluster members (<M*+4), we trace the network of filaments and subclumps around the cluster core. The depth of our observations, combined with the identification of filamentary structure, gives us an unprecedented opportunity to test the influence of the environment on the properties of low luminosity galaxies. We find an abrupt change in the colors of faint galaxies (>M*+1) at a local density of 100 gal. Mpc^{-2}. The transition in the color-local density behavior occurs at densities corresponding to subclumps within the filaments surrounding the cluster. Identifying the sites where the transition occurs brings us much closer to understanding the mechanisms which are responsible for establishing the present-day relationship between environment and galaxy characteristics.