Fluid-mechanical erosion of solid material occurs across many scales, from massive geological structures down to tiny granular constituents. Here we examine the erosion of a granular medium in Stokes flow - the typical flow regime of groundwater - using numerical simulations. We combine a highly-accurate boundary-integral formulation (for the Stokes flow) with stable interface-evolution methods (to treat the eroding bodies). A single eroding body tends toward a slender, for-aft-symmetric morphology which can be described analytical. Supplementing the Stokes solver with the Fast Multiple Method allows us to simulate 10-100 bodies. We find that the erosion of many bodies naturally leads to the formation of channels, as well as anisotropy in the medium conductivity. This latter feature we connect to the single-body limiting morphology.