Glacial erratics are large pieces of bedrock that have been transported by glaciers or icebergs and deposited in a new location that typically differs from the bedrock source. Erratics can be found overlying bedrock or embedded in till in areas that were previously glaciated.
Glacial erratics may range in size from pebbles to “house-sized” boulders; however, most scientists and casual observers equate glacial erratics with boulder-sized rocks. The site of origin for an erratic is determined by its lithology, or composition, that comprises the erratic.
The massive erratic shown below is the Saratoga (Waterman) erratic, located on Whidbey Island, Washington. It was eroded and transported by glacial ice from Mt. Erie, Fidalgo Island, located approximately 60 km north of its present location. Note the person at the base of the erratic for scale.