Banks, unfaced boundaries with height on both sides (cf lynchets, boundaries with height on one side only) can be divided into stony and grassed over (94473). At Bosigran stony banks are, without exception, either prehistoric or medieval. Most of the surviving unfossilised boundaries of the MBA co-axial field system (94440) are either stony banks or stony lynchets (94474) and many of the unfossilised R-B field boundaries on the northern cliffs also are. The dimensions of these prehistoric boundaries are such (1.0 to 2.3m wide, 0.4 to 0.7m high) that it seems probable that they are collapsed stock-proof boundaries, perhaps dry stone walls or Cornish hedges. They are therefore banks by taphonomy, not design. Medieval stony banks, on the other hand, may never have been structured. Throughout highland Britain subdividing boundaries within furlongs or other cropping units have traditionally been low stony banks, or balks (see Herring 1986A, Vol 2, 8). The extremely low banks within Bosigrans outfield strip groups (94406 and 94426) (0.1 to 0.3m high, 0.7 to 1.2m wide) fit into this category; their chief function was to mark lines, with a secondary one being to consume stones cleared from the field.