Note 123 — PART III. LOCAL DESCRIPTION OF THE WORKS. (continued 3)

Thirl, from the Saxon thirlian, signifies to pierce, to bore. It is generally supposed, that this stronghold derived its name from the Scots having broken through the Wall here. It may, however, have taken it from the sluice or bridge where the river passed through the Wall; thirl, says Hutchinson, being frequently applied to the opening left in moor fences for sheep to pass through.