Madison Boulder is thought to be the largest glacial erratic - an erratic being a boulder of a certain type of rock that was transported by glacial ice and deposited on bedrock of different type of rock - in North America.

At 23 feet high, 37 feet front-to-back, and 85 feet left-to-right Madison Boulder is estimated to weigh almost 12 million pounds, roughly the weight of 36 blue whales. It was once part of a ledge of Conway Granite and was transported about two miles south to its current location where it now sits on a deposit of Concord Granite.