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

Origin

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 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 currents. Boulders the size of a man have been found in relatively recent finely laminated sediments near Jamaica,which has been a warm tropical 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, perhaps formed during periods of flooding in forests.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.

Sources:

Wikipedia

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