Rig and furrow is a common sign of ancient cultivation. With origins in the medieval period, it reflects the way fields were ploughed.
Old ploughs had just a single ploughshare that invariably turned a long strip of soil to and deposited it to the right. A plough team would plough a single strip along a field, turn to the right at the end, and return ploughing an adjacent strip. The process was repeated until an area anywhere between 12 and 50 feet wide had been worked, the plough always moving clockwise and turning strips successively further apart.
Over time, this meant that soil gradually moved towards the centre of the area, forming the ridges or rigs we see today. In fact, when in use they must have been even more pronounced.
The rigs are sometimes curved. This is due to the plough team being turned slightly so that it approached the headland at the end of the strip at an angle, making turning here slightly easier. Teams with oxen were larger than horse-drawn ploughs, so an S-shape may mean oxen were used and that the filed was cultivated quite early.
One main advantage of rig and furrow was drainage. If the ditches run down a slope, this reduces the risk of waterlogging. It also means different crops could be grown on a single field – one crop on the dry ridges and another in the damper ditches.