Offa's Dyke is an entrenchment extending along the border of England and Wales from the mouth of the Dee in the north to near the mouth of the Wye in the south. In some places it is nearly obliterated by cultivation; in others it is of considerable height. Nearly parallel with it, some two miles to the east, is Watt's Dyke, which, however, seems never to have been so great a work. Offa, king of Mercia, is said to have erected Watt's Dyke in 765 to keep back the Welsh, and Offa's Dyke a few years later.