A dropstone is an isolated fragment of rock that has been transported by floating ice, such as an iceberg, and deposited in marine or lacustrine sediments. The presence of dropstones in sedimentary layers is an indicator of glacial activity and is important in paleoclimatology and the study of ancient ice ages.
Reference: Andrews, J. T. (2000). “Icebergs and the Greenland Ice Sheet: The Evidence from Radiocarbon Dating.” Quaternary Science Reviews, 19(10), 983-1014.