Dropstones: their origin and significance

Dropstones are clasts which pose a hydrodynamic paradox or which have an exotic or extra-basinal lithology with uncertain provenance. Recently, dropstones have been consistently interpreted as the product of ice rafting, and have been used to substantiate the presence of cool climatic phases in the geological record. This review illustrates that not all dropstones are reliable as palaeoclimatic indicators. Dropstones are the product of two distinct entrainment and transport mechanisms: by a rafting agent, or as a projectile. Four main processes of transport and formation are identified: biological rafting, ice rafting, floatation and projectiles. Biological rafting agents include vertebrate gastroliths and other stomach contents, vegetational rafting and anthropomorphic activity. Ice rafting can be the product of ice bergs or seasonal sea, river or lake ice. Floatation of clasts in calm waters has been recorded for stones up to 25 mm in length. Projectiles are most commonly of volcanic origin. Given the range of transport and depositional mechanisms reviewed we argue that great care is required in the interpretation of the origin of dropstones.