@Steve 41oo,
Somebody wrote a book called South Riding, did they not.
But there were of course only three, because "riding" was once a term for the third part of a whole, or so I was told.
and EDIT
The word riding is descended from late Old English *þriðing or *þriding (recorded only in Latin contexts or forms, e.g., trehing, treding, trithing, with Latin initial t here representing the Old English letter thorn). It came into Old English as a loanword from Old Norse þriðjungr, meaning a third part (especially of a county), cf. farthing. The modern form riding was the result of initial th being absorbed in the final th or t of the words north, south, east and west, by which it was normally preceded.[2][3]
A common misconception holds that the term arose from some association between the size of the district and the distance that can be covered on horseback in a certain amount of time.