In some cases, large boulders are 'picked up" by the ice and carried along. Sometimes, this includes icebergs that will carry the boulder a long way from the glacier. This is what they call 'rogue" boulders, huge rocks that ended up in odd places, many many miles away from the parent rock formation. One specific example, rogue boulders in Oregon and Washington dropped by melting icebergs from the "Great Missoula Floods."