These basins may have started even before the rock was uncovered, the soil eating away at weak points...once the Ice Age's had been and gone the rock was exposed as the soil around dissapered in a sludgy defrosting. It was then that the elements of wind and rain got to work forming the wonderful tors and cheesewrings. The basins, prone to filling with water gradually eroded further.
The fact that many of these basins are on slabs of rock perched at acute angles shows that they were formed prior to the rock falling, ie prior to the Ice age.
Mr H