Throughout the whole of its length, the Wall is accompanied on its northern margin by a broad and deep Fosse, which, by increasing the comparative height of the Wall, would add greatly to its strength. This portion of the Barrier may yet be traced, with trifling interruptions, from sea to sea. Even in places where the Wall has quite disappeared, its more lowly companion, the fosse, remains. In some fertile districts the plough has been carried over it in vain; owing to the moisture of the site, the corn sown upon it springs up with undue luxuriance, and is almost uniformly laid prostrate before it can ripen. From this circumstance the ground is frequently retained in grass, while the neighbouring parts are under tillage.[[19]] The fosse thus more readily catches the eye, and is likely longer to retain its groove-like form than if subjected to the ordinary process of cultivation.
When the ditch traverses a flat or exposed country, a portion of the materials taken out of it has frequently been thrown upon its northern margin, so as to present to the enemy an additional rampart. In those positions, on the other hand, where its assistance could be of no avail, as along the edge of a cliff, the fosse does not appear.
No small amount of labour has been expended in the excavation of the ditch; it has been drawn indifferently through alluvial soil, and rocks of sandstone, limestone, and basalt. The patient exertion which this involved is well seen on Tepper Moor, where enormous blocks of whin lie just as they have been lifted out of the fosse. The fosse never leaves the Wall to avoid a mechanical difficulty.
The size of the ditch in several places is still considerable. To the east of Heddon-on-the-Wall, it measures thirty four feet across the top, and is nearly nine feet deep; as it descends the hill from Carvoran to Thirlwall, it measures forty feet across the top, fourteen across the bottom, and is ten feet deep. Westward of Tepper Moor is a portion which, reckoning from the top of the mound on its northern margin, has a depth of twenty feet.
The dimensions of the fosse were probably not uniform throughout the line; but these examples prepare us to receive, as tolerably correct, Hutton’s estimate of its average size. ‘The ditch to the north,’ he says 'was as near as convenient, thirty-six feet wide and fifteen feet deep.'[[20]]
The care with which the fosse was dressed, has varied with the taste of the overseer and the forbearance of the enemy. In some tracts, the work presents as smooth and trim an aspect as a modern railway cutting; in others, marks of haste, carelessness, or sudden surprise, appear. The curious circumstance which Hodgson describes in the following paragraph may be seen in more than one locality:—
'A little west of Portgate, the appearance of the fosse is still, to the eye that loves and understands antiquity, very imposing and grand. The earth taken out of it lies spread abroad to the north, in lines just as the workmen wheeled it out and left it. The tracks of their barrows, with a slight mound on each side remain unaltered in form.'[[21]]
The works near the 18th mile-stone West of Newcastle.
