Dropstones are isolated fragments of rock found within finer-grained water-deposited sedimentary rock

Sedimentary rock

Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....

s. They range in size from small pebbles to boulders. The critical distinguishing feature is that there is evidence that they were not transported by normal water currents, but rather dropped in vertically through the water column.

Background

When deposited into fine layered mud, such evidence includes an impact depression beneath the dropstone, and indication that the mud has been squeezed up around the edges of the falling rock. Subsequent deposits of mud drape over the dropstone and its crater. Glacial dropstones, involving rocks falling out of iceberg

Iceberg

An iceberg is a large piece of ice from freshwater that has broken off from a snow-formed glacier or ice shelf and is floating in open water. It may subsequently become frozen into pack ice...

s, are one of the most common types of dropstone preserved in the geological record, particularly when deposited in low-energy deep sea or lake environments. Dropstones differ from erratics

Glacial erratic

A glacial erratic is a piece of rock that differs from the size and type of rock native to the area in which it rests. "Erratics" take their name from the Latin word errare, and are carried by glacial ice, often over distances of hundreds of kilometres...

 found in glacial till

Till

thumb|right|Closeup of glacial till. Note that the larger grains in the till are completely surrounded by the matrix of finer material , and this characteristic, known as matrix support, is diagnostic of till....

 in that they are deposited in a lake

Lake

A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...

 or marine environments. Dropstones may also be deposited by a variety of non-glacial means.

Origin of dropstones

There are four natural mechanisms that produce dropstones:

Glaciers

As glaciers move across a surface, they pluck rocks from it, and incorporate them into their mass. At the coast, fragments of glacier detach and float away as icebergs, which are often transported (ice rafted) many miles into the ocean, where they melt and deposit their load. When entrained rocks sink to the ocean floor, they can be incorporated into the oceanic sediments, which are typically fine grained.

Volcanoes

Whilst dropstones were once thought to be diagnostic of glaciers, it has since been realised that they can also be formed via volcanic eruptions. Volcanic 'bombs' are large fragments of rock, projected many miles by the force of an eruption. If these land in fine sediments, they can form dropstones. Dropstones originating in this fashion are relatively rare in the geological record as most will invariably land on high ground, which has a poor preservation potential as it is in an erosive

Erosion

Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

 environment. However, a large blast may spread bombs far enough for them to end up in a marine setting of fine enough sediment for them to be recognised.

Turbidity currents

Dropstones can also be deposited through the action of strong ocean-floor turbidity current

Turbidity current

A turbidity current is a current of rapidly moving, sediment-laden water moving down a slope through water, or another fluid. The current moves because it has a higher density and turbidity than the fluid through which it flows...

s. Boulders the size of a man have been found in relatively recent

Eocene

The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...

 finely laminated sediments near Jamaica

Jamaica

Jamaica is an island nation of the Greater Antilles, in length, up to in width and 10,990 square kilometres in area. It is situated in the Caribbean Sea, about south of Cuba, and west of Hispaniola, the island harbouring the nation-states Haiti and the Dominican Republic...

, which has been a warm tropical

Tropics

The tropics is a region of the Earth surrounding the Equator. It is limited in latitude by the Tropic of Cancer in the northern hemisphere at approximately  N and the Tropic of Capricorn in the southern hemisphere at  S; these latitudes correspond to the axial tilt of the Earth...

 island entirely devoid of glaciers since it came into existence. Whilst turbidity currents are cited as the origin of the boulders, they are not found in association with deposits formed by them.

Biological rafts

Stones can also be transported large distances by becoming bound in a raft of floating plant material or in the roots of floating trees. When such a raft disintegrates due to waterlogging and sinking of its constituents, the transported rocks would also sink. Dropstones formed in this manner are typically associated with organic matter, especially logs - the fossilised remains of the raft that caused its transport.
Vertebrates may also act as "rafts" by ingesting gastroliths

Gastrolith

A gastrolith, also called a stomach stone or gizzard stones, is a rock held inside a gastrointestinal tract. Gastroliths are retained in the muscular gizzard and used to grind food in animals lacking suitable grinding teeth. The grain size depends upon the size of the animal and the gastrolith's...

and depositing them in standing bodies of water by regurgitation or when the organism dies.