Subject: How did this rock get to be the shape that it is?

I work on a construction site in northern Alberta. At the beginning of the 
construction job during the initial site excavation and grading we unearthed a 
large boulder. It is approximately 1.2 m in diameter with a mass of about 2200 
kg, according to our crane operator. The interesting thing was that it was 
almost a perfect sphere. How did it get to be that shape? 

The landscape around the site is very flat and forested with lots of muskeg. We 
had to excavate down several metres in some locations to get to the underlying 
clay. The ground under the bog is mostly clay with very little gravel or stones.

Re: How did this rock get to be the shape that it is?