Jason in Enid made me do it! He used his coin in a jar example again, so I am going to repost my original post from the aforementioned thread to explain why coins do, in fact, sink. So here it goes....
If someone were to drop a penny into a glass of water what happens? It drops through the water and to the bottom of the glass. The reason this occurs is because the penny has mass, and gravity takes an effect on it. In a simplified explanation, as gravity pulls down on the coin, the molecules of water under it move out of the way, or displace, allowing the penny to sink to the bottom. This is a fairly simple concept.
Now drop a penny onto the ground. Of course it doesn't disappear into the ground right away because the dirt doesn't move out of its way. However, over time it does. Some folks theorize that rather than sinking, the coin gets buried as dirt and organic matter is deposited on top of it. Although this contributes to the coin disappearing into the ground, the more influential factor is that the ground beneath the coin does actually move, allowing the coin to sink.
So taking the same principles as the water example, in order for a coin to sink the molecules beneath it (in this case the particles of dirt), have to move out of the way. This does occur by a variety of means.
First of all, and the most powerful, is water. Put a coin on a patch of dry dirt. Now wait for it to rain. After the rain stops, pick up the coin. Obviously the dirt will be wet underneath it. This is because the water flowed under the coin. What happens when water flows? It erodes the ground and moves the particles of dirt. Whether it be rain or sprinklers, we have now begun the slow process of moving the dirt particles out from under the coin. The sinking process has begun.
Now enter grass. When grass or other plants grow, they produce roots. Roots extend into the ground and under the coin. The roots move (displace) the dirt so they can take up the space they need as they grow. Any mildly experienced detectorist has dug up a coin that had been encased in grass roots. Well eventually those roots under the coin die. Because plant material is mostly made of water, when that organic material dies it decomposes and reduces in volume. What is left is a void, or space under the coin, where this small root once was. So what happens? This minuscule hole collapses, allowing the coin to ever so slightly fall (aka sink).
Let's add some earth worms into the mix. Worms essentially eat dirt. They take in dirt, digest the organic material in it, and discard the rest. So when a worm burrows under a coin, it leaves yet another void (hole) in the ground. Eventually the mass on top of this void, be it the coin or otherwise, is pulled down by gravity and the hole collapses and fills in the void. Well the coin has now sunk just a bit more.
Now let's add rain and snow melt into the equation. The water seeps into the ground and by simple physics, it will flow into areas of lesser concentrations of dirt. Basically it will fill in those voids left by dead roots, worms, etc. And when water flows, it causes erosion. Dirt is moved which allows the coins to sink further into the ground.
This is why most coins, even the oldest of colonial ones, are typically found no more than ten inches deep. This first few inches is where there is the most activity in the ground occurs. The roots of grass and other foliage only grow so deep. When it rains, water only soaks so far into the ground. Worms and bugs eat organic material so they burrow only as far down as they need to find food. For these reasons, coins don't sink farther and farther into the ground over time. They do reach a limit because at some point there is very little occurring under the coin to cause the dirt to move. But they absolutely DO sink some over the years.
I mean no disrespect, but the jar example that Jason In Enid gave is flawed. Of course a coin dropped into a jar dirt won't sink. But that jar and its contents aren't exposed to any environmental factors, such as weather, plant growth, etc.. A closed jar is a static environment. The real world is not.
If you still think that coins don't sink and instead are covered up over time, think about this. My family owns a house that was built around the turn of the century. I have hunted the property and have found many old coins along the cement walkway that leads to the house, as well as along the cement sidewalk in front of the house. These coins are routinely five or six inches deep, below the level of the sidewalk and walkway. So if you subscribe to the theory that coins don't sink, then it would have to be concluded that the sidewalk and walkway have raised up five or six inches over the years. And since the walkway to the house still meets the foundation of the house at the same place it did 100 years ago, then it would have to be concluded that the whole house has also raised up five or six inches over the years. And this was all occurring while the coin was staying in the same spot on the ground and just being covered up. Obviously this isn't what happened.
Water, plants and worms aren't the only factors that contribute to coins and other objects being able to sink into the ground. The freezing and thawing of the ground during the seasons can have an effect, as well as other things. But the bottom line is this: over periods of time and with the right conditions, coins do sink into the ground. The rate by which they do and the depth that they get is determined by a number of factors.
