here's some granite boulders similar to what (I think ) you're talking about.
http://teachit.acreekps.vic.edu.au/...2002/devilsmarbles.htm
I've seen these in person.. they are preety amazing... very smooth.
From what I remember rain falls, getts in ccracks in th rock and when the ice exapands bits of rock fall off.
THis is happening now and scientists can see the mechanisms involved.
myth busted?