I was surprised that when Daniels HighPark was going in, that it was all sand below: those High Park hills, it turns out, are mounded-up sand with a thin layer of top soil on the surface. Most places in Toronto, the rock is much closer to the surface, and so we see in 98% of the excavations in this city that the building nestles into the shale substrate. I'm not too surprised though that the top of the Bluffs might be like the High Park area.
In the case of DHP, when I expressed my surprise about building in sand, we were told that it's actually quite a good medium to put a building up in, that excavations go more quickly, and that foundations are actually held quite well. This type of sand (as opposed to what gets smoothed by wave action on beaches) is quite coarse and locks together well, and there's next to no shifting when a building is set into it. If you look back through the DHP front page stories, you'll find that one is about the sand situation there.