OK...said I would reply.
I am not a geologist but have always been led to believe that the basins were formed/eroded by water and acidity. Granite, although a very hard rock, is prone to decomposition, hence we get china clay. 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. See the Devil's Chair on Carn Brea http://www.themodernantiquarian.com/site/757/carn_brea.html
and the stone on Carey Tor http://www.themodernantiquarian.com/site/7028/carey_tor.html
There are countless other examples in Cornwall and I am sure further afield.
The gullys I am not sure about...they are probably natural but whats to say they have not been cut to allow man to extract water from the basins.
As for the Merry Making Stone, the holes are only around 4 to 5 cms across, very much manmade and only date back to the 19th century.

Don't quote me!..it's all off the top of my head!

have a great weekend

Mr H