by Michael J. Oard
Dropstones are rocks whose diameter is larger (outsized) than the thickness of the sediment beds within which they are found. Sometimes large ‘rocks’ within fine-grained, massive sediments are considered dropstones. Dropstones have commonly been interpreted as being dropped into the sediment by icebergs floating in a lake or the ocean. This interpretation is considered one of the three main diagnostic features for ancient ice ages, which occurred hundreds of millions to billions of years ago within the uniformitarian timescale.1 The ice age interpretation is considered proven, or at least well founded, when boulders are found within thin beds, especially if those beds are couplets of silt and clay or sand and silt, assumed to be varvites, the consolidated equivalent of a varve (figure 1). A varve is a couplet of different sublayers laid down in one year.
Non-glacial occurrences of dropstones
It is also well known that dropstones can occur as a result of non-glacial mechanisms.2 Dropstones can also end up on the bottom of a lake or ocean due to sea ice rafting, floating kelp, tree stumps, swimming animals with stomach stones, sinking projectiles and even waterspouts that pick up boulders on the beach and carry them over the water.3 Because of the variety of emplacement mechanisms, dropstones should be equivocal paleoclimatic indicators. They certainly should not be diagnostic of an ancient ice age.
Although the sediments that contain the dropstones are commonly assumed to have settled over many years, sometimes these sediments are actually products of mass flow, and the rocks are actually transported laterally. Many presumed dropstones in fine-grained sediments have been reinterpreted as stones emplaced laterally by turbidity currents, a fast-moving, bottom-hugging flow of sediment. One example is the rocks within thin bands from the famous 2.2 billion-year-old Gowganda Formation, Ontario, Canada (figure 1). This formation was considered a classic dropstone varvite until reinterpreted as rocks within a distal turbidite,4 which is the far travelled product of a turbidity current. Distal turbidites can mimic varves….
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