n the basic mouldboard plough the depth of the cut is adjusted by lifting against the runner in the furrow, which limited the weight of the plough to what the ploughman could easily lift. These ploughs were fairly fragile, and were unsuitable for breaking up the heavier soils of northern Europe. The introduction of wheels to replace the runner allowed the weight of the plough to increase, and in turn allowed the use of a much larger mouldboard that was faced with metal. These heavy ploughs led to greater food production and eventually a significant population increase around 600 AD.