dropstones

Dropstones are solitary rocks that appear in otherwise fine-grained sediment layers on the sea floor or lake bed. They are formed when rocks embedded in moving ice are released as the ice melts, or as icebergs calve and drift away. When the ice becomes grounded or thins sufficiently, the incorporated clasts settle to the bottom, creating a discrete rock distinct from the surrounding sediment. In lacustrine settings, dropstones can form from icebergs that enter a lake or from debris carried by glaciers into glacial lakes.

These stones are a key indicator of past glacial activity. Their presence, especially when found within laminated or fine-grained sediments, points to episodes of iceberg rafting and proximity to glacial sources. Dropstones help reconstruct past climates and ice-sheet extent, complementing other proxies such as varves, radiometric dates, and sedimentology. They occur in both marine and freshwater settings and across a wide range of geological time, from ancient shoreface deposits to late-Quaternary sequences.

Identification and interpretation rely on their contrasting lithology and isolation within the sediment matrix. The clasts can vary in size from pebbles to boulders and often differ lithologically from the surrounding sediment. Their distribution, size, and the sedimentary context aid in distinguishing dropstones from other non-glacial rock fragments. Analyses frequently involve petrographic comparisons with potential source rocks and integration with other paleoclimate evidence.