Sunday, March 8, 2015

3/8/15 Report - Discussion Of How Coins Sink In Soil. North Wind Saturday But Little Erosion.

Written by the TreasureGuide for the exclusive use of

Treasure Coast Beach Saturday Morning After High Tide

We had some nice North winds Saturday morning, but it turned before noon.  Around noon it was coming from the East.  The result was no erosion to speak of.

Here are two pictures.  I didn't get the best pictures, but I think you can see some decent waves but no erosion.


A few days ago I asked people to send their ideas about how coins sink in rich soil.  I received a good number of thoughtful responses.  Instead of simply summarizing those emails I decided to post many of them as they were sent.  You might benefit from any or all of them.  There was a very consistent theme.

Beach Saturday Morning After High Tide

Troy T.  sent the following email.

Thanks for the great daily blog. I live on the other coast in Tampa but I like to know what's going on over there regardless. I especially love reading your thoughts on target depth and movement and the other technical aspects of metal detecting.
My puzzling experience was when I found an 80's quarter close to a river under about 8" of dirt laying directly on top of a thick layer of clay.  I cut an unnecessarily large plug and the moist soil and the clay separated nicely with the coin in between. I was dismayed to see that a modern coin would be so deep but then I realized that when the river rises to that point the ground becomes a slurry and due to the clay the water has no where to go. A heavier coin like a quarter could, in the right circumstances, work its way down to the clay layer over a relatively short period of time.
Thanks again for the great reading and I would love to see some guest reports featured from my side of the state.

I will post reports from the West Coast of Florida, but since I'm not over there very often will depend upon such reports being sent to me via email.

Chris S. said,

I've had experiences such as Joe D. In my case, I was in an area close to a river that floods during hurricanes and such. I was digging state quarters at 9 inches and no silver to be found that should be there. Attributed that to the silt that builds up over time. Of course the opposite could become true under the right circumstances. Let's see what some others have to say.

I should have stated that the coin may not be sinking as much as soil being placed on top. Rich soil as Joe is speaking of would be a common byproduct of a flood.


Robert H. said

I mostly beach hunt and rarely park or land hunt. One thing I've considered why coins sink so quickly is in parks or land sites is when we get a nice down pour and the ground is very saturated or flooded with standing water. I've come across very deep modern coins 5 to 7 inches down and was always thinking how could modern coins sink that fast? Well my main thought is hard down pouring rain and flooding turns the dirt into muddy soup. Guess where the coins will settle? At the bottom of that soupy mess. That's my thought.


William M. had the following to say.

The first thing that comes to my mind is why is the soil so rich? How was the lot cleared originally? Was it root raked? box bladed?.. was the tree and other wood material mulched on site?... I believe there's something different going on there and it's not coins sinking fast because of earthworms.. sounds like really bad drainage at very least it may have been a marsh of sorts at one time.


Russ P. said, Here are my thoughts:
I've certainly spent a lot of time thinking on this issue.  It is, I believe, a complex one with many factors at play, similar to thinking about where a coin might be found on the beach.

Instead of thinking about a coin "sinking", I come at the problem from a  slightly different perspective and that is the position of the coin relative to the surface.  This is an important distinction because often there are two major forces at play:  those factors that cause a coin to actually sink and those factors that lead to additional soil or debris on top of the coin.  There are many factors contributing to each of those forces, but I'd like to briefly mention a general observation.

In my opinion, in rich soil the factors leading to soil creation, thereby adding soil to the top of the coin which causes relative "sinking", is the predominant factor at play.  Several observations have supported this hypothesis through the years.  First, if one looks at sidewalks in old areas or leading up to old houses, they often look like they were dug out, almost a slight valley between berms on either side.  Years of mowing and grass clipping has increased the height of the ground on either side of the sidewalks.  As one moves a few feet to either edge of the sidewalk the difference can be easily 4 inches or more.  Human activity accelerates this phenomenon.   In well-manicured lawns with St. Augustine grass the difference is even more pronounced.   Second, finds discovered after concrete pavement is removed are often very near the surface, even those those finds may be extremely old.  This suggests to me that those finds remained in a very stable position from the moment the pavement was poured.

There is a question that I don't have an answer to yet that I would like others opinion.  In typical Florida conditions, do coins or other objects eventually reach a stable point where they don't sink any further?  It would seem to me that, with soil creation, objects would continue to "sink" indefinitely, but now, based on some experience, wonder if there is a typical depth that essentially is a steady state, above which soil creation and possible soil destruction (if it exists) is balanced.  I've done some recent sifting which seems to suggest that possibility.

Hope this helps.  It would take quite a long blog entry to tackle this issue in a comprehensive manner!


Bill H. said,

In Michigan where I normally hunt in summer, it is generally thought that coins drop at least an inch for every ten years time.  This takes into consideration the creation of new 'dirt" in the form of rotting leaves and grass clippings and the gradual drop in soft dirt.

There are so many factors,however, that I  don't think there can be any one rule. Tree roots can bury or lift up old coins. Adding top soil can bury them deeper for sure. Roto-tilling a garden/lawn certainly can either raise up or lower coins. I have found 1980s coins at 4 inches and 1800s coins an inch down. The clay/stone base also obviously plays huge.

But if you give me an old farm house without any of the above, I'd expect an inch every ten years down to about 4-5 inches, then after that  perhaps an inch in 20 years. The drop seems to slow down the deeper they go. I used to keep accurate records and on virgin land this was the rule. Then I decided that hunting was much more fun than record keeping and I just go for it.

Thanks for the great site- I have hardly missed a day in three years.


When I originally received an email asking how fast coins sink in rich soil I responded that it might not be so much a matter of coins sinking as it is a matter of the coins getting covered or buried.  That is a theme you see in many of the emails I received too.  The person who asked the question wrote back and said that seemed like it could be the case.   He then mentioned that the area in question was under a banyan tree.

Much of what you read would seem to suggest that coins simply sink in dirt over time.  It often sounds like it is a very passive process that requires little or nothing more than gravity.  I believe that that when coins sink in soil it is a more active process. 

In this blog I've reported on experiments that show how coins sink in sand.  I've shown that coins simply laying on sand that does not move do not sink.  Gravity alone isn't enough.  The sand has to be moved.  I'm convinced that the situation is similar for coins in soil.

I'm not saying that coins do not sink in soil.  I'm only saying that much or all of what appears to be sinking is actually a process of covering or burying.  I know that may be nothing more than semantics to some, but I am making a distinction.   In other words, when things sink (work their way down through existing material), there is something at work besides gravity - something that agitates the soil in one way or another.  The emails posted above mentioned various processes such as rain, floods, root growth, worms, etc.

William M. asked an important question when he asked what made the soil so rich.  In this case it appears that one thing is rotting leaves.  William also listed a variety of other things that could happen to the soil.

I was surprised that people emphasized the burying process so overwhelmingly. 

The observations of Russ relative to the sidewalks were helpful.  One of those, the observation that items under concrete do not seem to sink much, is thought provoking.  The soil under a sidewalk would be prepared before the sidewalk was poured.  A sidewalk would be poured on a packed hard base.  Besides that, the sidewalk itself, would protect the ground under it against other forces such as rain.

The idea of a slowing rate or limit on sinking (if that is what you call it) is also interesting.   Even at one inch per year, it is what I would call a slow process - too slow for me to conduct good field experiments.

On a beach coins can disappear very quickly.   The tides come and go daily and the sand moves a lot.  It can happen fast - even while you watch.

As you know, I occasionally do detect up north.  I've told about a few of those hunts in this blog.  One place I hunt is heavily wooded and is very hilly.  A lot of leaves, limbs, bark, etc. falls and decays, continually producing new soil.  In depressed areas where decaying matter accumulates I find very few coins.  I've even tried raking the top layers.   Inches can accumulate in a single year there.  I tend to focus on areas where the wind or rain moves the loose material and does no allow new soil to form and accumulate.  An 1829 large cent that I found there was probably less than two inches deep,

I appreciate all of the responses I received.  They were all helpful and thought-provoking.  I hope you now have a more complete understanding of the factors involved and gained some new insight from the discussion.  I did.

Thanks to all who sent responses as well as that one person who  initiated the discussion by asking the question.


We'll have a fairly rough surf on the Treasure Coast today (somewhere around four or five feet), but I'm not expecting much improvement in beach conditions at all.

Happy hunting,