Sunken lane

A sunken lane is a road or track that is significantly lower than the land on either side, not formed by the engineering of a road cutting but possibly of much greater age.

Various mechanisms have been proposed for how holloways may have been formed, including erosion by water or traffic; the digging of embankments to assist with the herding of livestock; and the digging of double banks to mark the boundaries of estates. These mechanisms are all possible and could apply in different cases.

Source: Wikipedia