If you arrive in Brittany and think you can do some flat cycling, think again. "Flat" is of course a relative term.
Flat: to me this term means "unchanging slope". Any side of a cube is flat, regardless of which astral bodies it is facing at any time. Yes, it may be difficult with Earth's gravity to climb some of those faces, but they are still flat. For the purpose of cycling however, we additionally expect the term flat to mean "parallel to the surface of the earth". (we assume earth's curvature to be insignificant at this scale). For the cyclist pedants "flat" might best be given thus: "land or water having constant distance from the centre of the Earth".
So the roads of Bretagne are anything but flat. The gradient is constantly changing as one rolls up and down all over the place, surprisingly perhaps at 10 or 12% more often than one might predict.
Au contraire, Les Alpes are flat. Its just that they are not parrallel to the earth's surface. Climbing L'Alpe D'Huez for example the gradient does not really drop much below 8% (except for very short sections on the hairpins) and only gets to as much as 12% - giving a tiny 4% change in slope.
This may well explain why everyman and his dog was climbing D'Huez, but the col we did in Bretagne (col de Marhalla, 284m) is obviously too difficult for the masses as we found no other climbers, or dogs, on this one!