Why were mouldboard ploughs more efficient in the middle age?

Medieval ploughs were made in many different ways, according to local soil conditions. The mouldboard plough was used in heavy soils such as clays and its main benefit was in turning the soil aside, producing distinctive ridges and furrows - the crops were then grown on the ridges and water could drain away into the furrows.

A drawback in the design was that the wooden mouldboard could become clogged with soil, meaning that the plough was less efficient and much harder to move. Many ploughmen took along a carpenter's axe (also known as a T-axe) and hung this on the beam of the plough, just behind the oxen. It was used to clean off the mouldboard whenever it became clogged.

I have a number of manuscript illustrations from the 12th century showing this axe hung on the plough-beam (in one case the ploughman carries it in his hand).