We present deep, panoramic multi-color imaging of the distant rich cluster A851 (Cl0939+4713, z=0.41) using Suprime-Cam on Subaru. These images cover a 27' field of view, ~11 Mpc at z=0.41, and by exploiting photometric redshifts estimated from our BVRI imaging we can isolate galaxies in a narrow redshift slice at the cluster redshift. Using a sample of ~2700 probable cluster members brighter than 0.02 Lv*, 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 < 0.1 Lv* galaxies at a local density of 100 gal. per sq. Mpc, with the population in lower density regions being predominantly blue, while those in higher density regions are red. 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. Comment: 4 pages, 4 figures, uses emulateapj.sty, accepted for publication in ApJ Letters