What is "dropstone"

n. (context geology English) An isolated fragment of rock found within a sedimentary rock (that had fallen, rather than being carried along)

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.