Much of the Cotswolds lie in the county of Gloucestershire, one of the counties where traces of the ridge and furrow farming system is much in evidence. When you see a field of grass growing out of a corrugated surface you are almost certainly looking at the remains of medieval ploughing typical of the open field system (very large fields owned by the local lord of the manor, farmed in strips by individual families). In Britain the earliest examples date to the immediate post-Roman period, continuing in use in in some areas until the 17th century. Surviving ridges are parallel, ranging from 3 to 22 yards (3 to 20 m) apart and up to 24 inches (61cm) tall – but were up to six feet tall when in use. Older examples are often curved. The effect was created as the result of ploughing with non-reversible ploughs on the same strip of land each year.