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Erratics
Erratics are rocks that are carried from one area and deposited in another by hitching a ride on or in a glacier. These rocks range in size from small pebbles to very large boulders. When a glacier retreats the rocks are then left behind, sometimes miles from the present day glacier, or in the case of ice age glaciers, they can be thousands of miles from any glacier.
Erratics end up in glaciers by many different means. The large ones are often broken from the bedrock by the movement of the glacier in a process known as "plucking". The smaller ones are often produced by an abrasion process.
This is caused by the glacier's constant movement over the bedrock, polishing and scouring off little pieces. Both large and small erratics can also fall onto the ice during a rock avalanche on a slope above the glacier. A famous large erratic is Big Rock located in Alberta, Canada. It is about 30 feet high and weighs approximately 16,500 tons!