I heard an interesting fact at a lecture earlier this year regarding 
mill-wheels. The Roman, and later the medieval, style was to have the wheel 
upright. The Saxon style was to have it in the mill-race horizontally.
In the south and east of the country, Saxon age horizontal millwheels are 
all that are found. But in Herefordshire, recently, a Saxon mill-wheel set 
vertically was found, in a settlement where there seemed to have been no 
break in agriculture, though the 'Lord' at that  time was a Saxon.
The possible explanation was that the Justinian plagues that killed so many 
in Europe had also done so in the south and east of Britain too, and when 
the Saxons arrived there was plenty of spare land without displacing those 
who had survived. And the wheels had been disused and broken, and were 
replaced. But the plague hadn't reached as far as the Welsh borders, and 
the Saxon who took over took on a working millwheel that could be kept 
repaired.

Harriet