Here is an article that I wrote for Lost Treasure magazine before it went under that you guys (and gals) may be interested in. Not sure how to bring in the photos, but here is the text.
Do Coins Really Sink?
Charles Darwin’s Contribution to Metal Detecting
By Kent Whiting
During my 30+ years metal detecting I have often wondered how coins become buried in the ground and what determines how deeply these coins are buried. There are two main theories; the first advocating that coins sink through the soil and the second that coins become covered by plant matter that decays and forms layers of soil on top of the coins.
Over the years I have observed huge differences in coin depths from one site to the next or even within the same park. In 2011 I found an 1894 Barber quarter in good condition laying right on the surface of the ground at a site that for many years has been a dry poorly vegetated field. The following year I found, an 1894-O Barber quarter in good condition at a depth of over 10 inches in a well-watered park lawn. Both coins were probably lost in the 1930s, so why was one on the surface and the other over 10 inches deep?
One possible explanation is that the coin lost in the park was buried by years of accumulation of lawn clippings. One problem with this theory is the soil above the coin was not pure organic humus, but also contained sand and silt grains. Could it be that the coin in the park was able to sink because the soil was moist? I have seen theories that attribute coin sinking to density differences between the coins and the soil. True, such processes can and do operate within streams. However, such density segregation does not occur within a soil. The frictional forces between soil grains are far too strong to permit sinking of a coin. So, if coins do not sink due to density differences and they are not covered by grass clippings how do they get so deep? Further complicating the situation is the fact that within the same park beneath a spruce tree I found a 1905 barber dime, at a depth of only about 4 inches.
Fortunately, these observations can be explained by the work of one of the greatest scientists in history, Charles Darwin. Darwin is best known for his theory of natural selection to explain the diversity of life. In October 1837, about a year after he returned from his famous voyage on the Beagle, Darwin made a trip to visit his uncle, Josiah Wedgewood, at his country estate at Staffordshire, England. Wedgewood related that 12 years previously the surface of a certain pasture had been covered by a layer of lime, but the layer had since become buried by the action of earthworms. Darwin found the white layer of lime at a depth of 3 inches. According to Darwin, the burial of items by earthworms is related to their burrowing activity, which is intimately related to the feeding habits of certain species. Deep burrowing earthworms, such as the common night crawler, feed on both organic material within the soil, and on surface material, such as leaves and grass clippings. They drag their food down their burrows and deposit their waste, called “casts”, at the surface. They ingest organic-containing soil, including small grains of silt and sand, which become part of the casts. Darwin proposed that an item on the surface becomes buried due to the collapse of the subsurface burrows beneath the item, causing subsidence. The resulting surface depression, then becomes filled in by the granular casts.
Darwin calculated the burial rate of the lime based on the application date and on the depth of the layer. He came up with a rate of 2.5 inches per decade. Darwin presented his findings in a paper read before the Geological Society of London in 1837. He concluded his presentation with the remark “it is probable that every particle of earth in old pasture land has passed through the intestines of worms”. Unfortunately, the paper was poorly received by Darwin’s geologist colleagues, who had expected great things from the man who, even then, was considered a celebrity following his voyage on the Beagle.
Darwin soon moved on to other interests, but never completely abandoned his earthworm research. He published a few more papers on the subject, as well as beginning a few long term experiments. He spread a layer of lime over one of the pastures at his country estate in 1842 with the view of monitoring the rate of burial over time.
In 1871 Darwin checked on the status of his lime layer, finding that it had become buried to a depth of about 7 inches over the 29 year period, for a burial rate of 2.4 inches per decade. He calculated the mass of soil material brought to the surface by carefully monitoring the burrows over time and found that the mass of casts brought to the surface matched reasonably well with the burial rate of surface material. The burial rate of an object due to worm activity is related to the population of deep burrowing worms for a given area. Darwin found that worm populations were considerably lower beneath beech trees compared to other areas. In 1881 he published his findings in the form of a 326 page volume which he referred to as his “little book”. The book was a commercial success, but was never embraced by the scientific community and was soon forgotten. Darwin’s work was rediscovered by scientists when the book was reprinted in 1945, and by the 1950s a few archaeologists began to recognize the importance of his work. The concepts described by Darwin are now well accepted by many archaeologists. Within the last few years new scientific terms have been coined to describe these processes. The action of earthworms on the soil is referred to as “bioturbation”, and the animals responsible for the bioturbation are the “ecological engineers”, which include not only earthworms, but any burrowing animal that deposits dirt on the surface.
So how can Darwin’s work explain the depth differences between the two Barber quarters? The dry field was an inhospitable environment for earthworms. There was little or no organic material in the soil and the sparse vegetation provided little or no surface litter for them to eat. On the other hand, the park provided an ideal environment for earthworms. The soil was moist, and rich in organics, and abundant food was provided by grass clippings, and leaves. The area under the spruce tree was a relatively unfavorable environment. As mentioned previously, Darwin found that the areas beneath beech trees were unfavorable for earthworms. More recent studies have found the same to be true for conifers such as spruce and pine, and for larch trees, but not for oak or maple trees.
So what does all of this mean for coinshooting? First, the idea that denser or larger items should become buried faster than smaller or less dense items can be discarded. How many times have you dug a can at over a foot deep? If cans are this deep, coins of a similar age will be just as deep. The number of remaining deeply buried half dimes and trimes in many “worked out” city parks is probably mind boggling.
Another important take away is that under ideal conditions for deep burrowing earthworms, most all of the old coins could be out of range of most metal detectors. Earthworms are most active when the soil is warm and moist, so most of the burrowing and coin burial, occurs in the spring and fall. Clearly, some areas of the country are more favorable to earthworm activity than others. Warm and wet climates are particularly well suited to year round earthworm activity and potentially very deep coin burial.
So, do coins sink or are they covered up? Clearly the answer is BOTH; with the help of a few “ecological engineers”.
Atkinson, R.J.C. 1957. Worms and weathering. Antiquity, v. 31, p. 219-233.
Butt, K.R., Lowe, C.N., Beasley, T, Hanson, I, and R. Keynes. 2008. Darwin’s earthworms revisited. European Journal of Soil Biology, v. 44 p. 255-259.
Darwin, C. R. 1837. On the formation of mould. Proceedings of the Geological Society of London, read November 1, 1837, v. 2 p. 574-576.
Darwin, C. R. 1881. The Formation of Vegetable Mould, Through the Action of Worms, with Observations on Their
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Desmond, A. and J. Moore. 1991. Darwin. Michael Joseph, Penguin Group, London.
Feller, C., Brown, G.G., Blanchart, E., Deleporte, P. and S.S. Chernyanskii. 2003. Charles Darwin, earthworms and the natural sciences: various lessons from past to future. Agriculture, Ecosystems & Environment, v. 99, p. 29–49.