Tree on a rock

How can a tree grow out of a rock? It is a common sight in the Highlands.

This valley feels rough and wild. The hills around are craggy with just thinnest of skins of turf and heather here and there. The slopes are also strewn with dozens of big boulders, all sharing the same trait. The erratics (and I'm not sure if they are) have clear bed planes running through and it looks like an angry giant has stormed through the glen with a huge axe and split them in two taking advantage of the obvious weakness along the planes.
Some look like loaves with the first couple of slices cut off and still leaning at impossible angles. 
The -on end- boulder in the extra is about 6 metres tall and is a good example of the wee layer of turf I was talking about earlier.