Big House Rock, Weymouth

This rock ("Big House Rock") sits on top of the soil - it is not part of the bedrock, and, in fact, is of a different type of rock (Quincy granite) than the bedrock. We're several miles from Quincy. How did this rock get here? (Before you assume that people moved it, estimate the weight of this rock. The rock is roughly 12,000 cubic feet, and its density is around 170 pounds/cubic foot!)

By the way, this rock is 37 feet high.