Mountain biking is self explanatory really.
It must take place in the mountains. It must involve the ascent of at least one significant peak. It must involve at least one section of hike-a-bike. It must also involve at least one section of carry-a-bike.
Today, Clare & Di went mountain biking. It definitely took place in the mountains; it took in 2 significant peaks; it involved numerous sections of hike-a-bike; and it also involved a rather steep carry-a-bike section.
It was probably the first [and probably the last] time that Clare will traverse a mountain pushing a mountain bike both in ascent in descent.