Do Coins Sink?

Hello all. I am new to this forum. I think this is my kind of forum, in that the focus is often more technical than many. I have enjoyed reading the threads and have now decided to join and share something that I wrote a couple of years ago on the topic of coin depths. My research seems to fit in nicely with what Tom D. has found on pull tab depths at about 10 inches. I sent himthe article about a year ago and he urged me to post it on the forum. Due to technical difficulties I am just now getting it posted. I was not sure how to add a pdf attachment, so I have included the text below. This article was published in Lost Treasure Magazine in May 2015. Please let me know what you think.

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?
[Photo 1]
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.
[Photo 2]
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.
[Photo 3]
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”.

Sources
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 Habits. John Murray, Albemarle Street. London. October, 1881.
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.

"This article is posted with permission of Lost Treasure Magazine, www.losttreasure.com."

Soil dependent(excluding obstacles), the coin will "slither" into the ground. From a flat starting position, one side will begin to dip into the ground and it will continue and cause a "super slow motion" tumbling effect.

Welcome to the forum, Kent. That's an excellent, well-researched article you wrote and a stellar first post!

Excellent post, Kent.
Another factor that is moisture dependent is frost heave.

Kent......... HOME RUN!

Welcome!

Kent,
Welcome to the forum.

A nice piece you provided here.
Thanks for sharing.

Soil........ moisture........vegetation......coin size........direction of coin.......human action. They do both.

Dew

Good article BigSkyGuy. Welcome.

One would be surprised how deep night crawlers go. They will burrow below the frost line in winter....around here that is around 3 feet.....colder states, up to 6 feet. I have witnessed them that deep. Many species of earthworms on this planet....and they do a slow-motion churning of the soil.

Just a tiny part of Gods ecosystem.....mind blowing to say the least.

I have noticed on a couple of my coin finds that there is movement in the ground. These coins were on edge with one side against a sidewalk. The coins were sanded smooth on the face against the walk and they were actually very thin. I theorized that freezing and thawing played a part in this. I have found barber coins two inches deep in the same yard as memorial pennies over six inches deep. Very puzzling indeed.

This data validates to "organic" soil..............vs..............."inorganic" soil depth sink-rate model. The more INorganic the soil....... (the less volume of burrowing critters); with subsequent LESS sink-rate. The more organic/lively/life-sustaining soil....... the faster the sink-rate. Although I have back yard (test-garden) soil that is of regular dirt........ with nearly no critters. When it rains....... this dirt becomes mud.........,,,,,,,,,, and you can visually watch the targets (including coins) sink...... within this liquid/mud slur. And then the pine needles and scrub oak tree leaves....... add yet another 'cover' of depth to the targets.

And funny you bring up Half Dimes and Trimes (to add the U.S. $1 gold coin to the list)....... as we detectorists STILL have NO idea as to the ratio of these (most-common highly-circulated) coins were (easily) lost.

Kent...........again............ VERY good data!

Here in southern Kansas we go through periods of severe drought, right now I have deep cracks running through my yard that are up to 2 inches wide.

I would imagine that if the crack intersects a buried coin it would propel the coin to unreachable depths.

Thank you all for your feedback, input, and kind words. Ozzie and Tom bring up a good points about the earthworm burrowing depth and quality of soil. In Montana it is rare to have top soil deeper than about 12 inches. Below the topsoil is either bedrock or a hard sandy cobble-rich layer which worms cannot penetrate. This limits the burrow depth to 12 inches. Under this scenario the sink rate should not be a constant 2.5 inches per decade but will decrease over time. The sink rate should be proportional to the number of collapsing burrows beneath the coin. However, once the coin becomes buried the number of burrows beneath the coin decreases. For instance, once the coin is buried to a depth of 6 inches, there are only half the number of collapsing burrows beneath the coin compared to when the coin was at the surface. Assuming the sink rate is linearly proportional to the topsoil depth beneath the coin, the coin depth over time will follow the blue curved line shown in the figure. The orange line is the constant sink rate of 2.5 inches per decade. I think the blue line makes more sense based on my experience in Montana.

[imgur.com]

Please let me know if you are not able to access the link with the figure...this is the first time I have used a photo hosting service.

Kent



Edited 1 time(s). Last edit at 09/01/2017 04:22PM by BigSkyGuy.

Kent,
Your graph is accessible to me, just fine...
And your blue line matches very, very closely to what I see in parks here in Colorado.

thx,
mike

Darwin isn't my favorite, a naturalist with a vivid imagination but when push came to shove unmoored from science (e.g., human evolution and macro-evolution, two principle failures).

However, Darwin was on to something with earthworms (his initial findings were mostly rejected by scientists). His experiments with sink rates gave him a rate of about 0.60 cm per year (0.24"/yr) on small objects in a pasture. The flaw was in not knowing when the objects reached the depth, it was assumed to be at the time of excavation (unlikely). He assumed if he waited 29 years and the object was 18cm down it sank at 18/29 cm/yr. But it is quite possible, even likely, the object reached 18cm long before Darwin decided to dig it up.

Under recent (1955, 2005) controlled experiments, the rate of sink for small objects (metal disks) was about 3cm/year (1.18"/yr). This was for objects < 3cm (1.2") in diameter. The larger the diameter of the object, the slower the sinking rate.

Studies found if earthworms aren't present ... bones of rodent remains don't sink, remaining near the surface (applies to coins one would assume too). Generally, there are about 300-500 earthworms per square yard of habitable soil.

Here's a linear extrapolation ... assumes earthworms are present for bioturbation. Obviously, the rate of descent isn't linear, else a 1917 dime would be at 118" (9.8 feet) down. Worm activity is probably greatest in the first several feet of soil, tapering off at deeper depths and nearly ceasing much beyond 3 feet. Moisture and soil temperature determine burrowing depth--drier soil is preferred with ideal temps around 68F (much warmer and they tend to dry out).

((( John......... I feel like your graph represents/fits/depicts the Florida dirt model.... to a "T" !!! )))

This fits here I think in the overall scheme.
Good piece too.
[www.dankowskidetectors.com]

Makes sense to me.
I have found 100+ year old coins on well worn trails. One was even less than an inch. The ground was hard packed and because of the constant foot traffic no vegetation had a chance to set it.
On the same trail I have found coins 50 + year old one the side of the trail about 4 inches down.

Here's some previous discussion on the topic:
[www.dankowskidetectors.com]

I imagine a worm has no idea a coin is in front of it, until the moment it's head hits the metal, at which point it will steer off around the coin, disturbing the earth as it goes. So the buried coin is quite likely to become surrounded by void/loose soil, which will contribute to it's descent rate, in addition to the general soil disturbance below it.

This made me wonder whether object shape and size play much part in sink-rate. I can see a larger item getting in the way of more worms, ending up with more disturbed earth close to it. This would likely result in it sinking just as easily as a smaller object.

But finger rings may be a slow-sink target. The small amount of metal in the band means they're less-often encountered by worms, and easily manoevred around. A worm could burrow straight through the centre of a ring without encountering it, and without contributing to it's sink. Only when there's been disturbance around a significant part of the periphery will it move.

I've detected a woodland site here in the U.K with some near-zero-sink conditions. Heavy clay / stony soil, with a thin layer of leaf-litter on top. Musket balls are often lying on the surface or one inch down. This is curious in itself, the real surprise came when one of those shallow musket balls turned out to be a Roman coin. 1700 years without sinking?

I am not able to see John's graph. I just have a little black box with an "x" in the middle.

Here's a link to Johnny's graph, you may have more success with it:
[i1155.photobucket.com]