  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.
