In many places, it seems, there was. Excavation has repeatedly identified traces of cultivation under the curtain wall and its associated structures. These are often referred to as ard marks, since a proper plough (with iron-shod coulter and share) is not thought to have been introduced until the later Roman period in Britain. Such marks usually indicate the practice of cross-ploughing, where soil is ploughed first in one direction and then a second time at right-angles to the original direction. These observations can be compared with some cropmarks known from aerial photographs, where entire field systems can be seen being cut across by the Wall, as at Bleatarn, near Carlisle Airport, and at Grinsdale.