We reckon that this cloud formation is the catchily named Stratocumulus undulatus or Stratocumulus billows, needless to say we were being dragged along by the two Jack Russells at the time. It goes without saying that they have no interest in clouds, just scents when they're out foraging and hunting.
Why the straight line along the edge of the formation and what causes that phenomenon, is it the wind or temperature?
The sky seems to be full of questions.