Just been looking at Dave Wheeler's site, which has reminded me that I
totally forgot to mention spotting not just a snow roller, but a couple of
fields full of them.
This was in feb and the fields in question, on the road to Stamford Bridge,
are used for producing turf (there's a lot of them in Yorkshire).
Consequently, they are flat and even, which no doubt aided the formation.

--
David Mitchell, 70m amsl, Langtoft, East Riding of Yorkshire.