Coin Depth - A Great Debate!
For several decades a common topic for debate among metal detector hobbyists has been the subject of coin depth. Why are some older coins shallow when newer coins are deep at the same location? How can a newer clad coin be found deeper than than a old silver coin buried for 100 years? The two most commonly discussed reasons for the answers to these two questions are the “sinking ” and “displacement” theories.
The “sinking” theory has two major components. The first is soil density. The density of inorganic soil is from 2.6 to 2.8 and any object ( including coins), according to this theory, would eventually sink until the density of the soil equalled the density of that item. The second component is the sink rate. Theorists say the sink rate is determined by the difference in density. The greater the density of an object the faster the sink rate. Contributing factors are grass and leaf buildup, rain, frozen soil, amount of ground vibration, erosion and other things like placement of fill material. The biggest factor is rain. How often the ground gets soaked to the point where the dirt is suspended in the water, determines the amount of sinking that will take place. This is why many coins end up in the 5-8 inch range. It takes a real downpour to move them deeper and the difference in coin depth may be attributed to the differences in the local drainage. The coarser the soil particles, the tougher and slower the sink rate.
The finer the particles the easier they are suspended and the faster the sink rate. The density of metals like platinum (21.45), gold (19.3), silver (10.5) and copper (9.0) are much higher than junk metals like tin (7.3) and aluminum (2.7). This would say that the good stuff is sinking much faster than the junk. Yet time and time again I have dug up pencil erasers and pull tabs at greater depths than silver and gold in the same location. I have a tendency not to accept the sinking theory based on 38 years of treasure finding with a metal detector. My very best half dollar finds (1904 O XF+ , 1861 P BU 63) were both less than an inch deep in soil where I dug up pennies, nickels and dimes in the 6-8 inch range that were 50 to 100 years newer. How can this be? The “displacement” theory might be the answer. This theory does not negate or disprove the “sinking” theory but may explain the weird/strange depths of newer coins and junk items.
The “displacement” theory says that most coins are where they are, not so much a result of a specific gravity event, but because of man-made disturbances such as digging, erosion, tilling, grass cutting buildup, leaf droppage and the placement of fill materials. I found 36 mercury dimes one day after a grade school removed its sod. The dimes were still 3-5 inches deep and the supervisor of maintenance told me that that sod had been placed on top of the original sod more than 15 years earlier. A ball park yielded 240 coins after a deep tilling job churned up more than 6 inches of hard compressed clay. A 1910 era (inland) church conference ground had a 6 inch sea-shell base at about 10 inches deep. When about 10 inches of earth was removed, I dug up 119 coins in the dark by tedious digging through 6 inches of compacted sea-shell. The earliest coin was an 1892 dime but most of the coins were mercury dimes and wheat pennies from the 1920-1940 era found at more than 16 to 20 inches below the top of the flat terrain. In essence, to find these 20th century coins, 16 inches of fill material had to be removed, as there is not a metal detector made, that will find small coins in the 16 to 20 inch range.
The easiest way to see the “displacement” theory in action is to go to a salt water beach. Coins lost in the dry sand above the high tide line where they are free of moisture and heavy foot traffic will not go anywhere per se. Launch out into the water line where water action takes place and the churning of the surf moves the sand and you can experience displacement due to specific gravity. Here the erosion effect will leave coins exposed after a strong storm or deeply covered from normal surf action. More often than not, buildup will contribute more to coin depth than any sinking action. I am a meticulous record keeper of my metal detecting finds. Of the 150,000 coins retrieved, less than 1% have been deeper than eight inches. In fact, 109,000 have been less three inches deep, 10,440 have been five or more inches deep and about 30,000 have been between 3-5 inches deep. I hope these stats shed some light on the issue of coin depth and know for certainty, there is no great debate, that even a good entry- level metal detector can find some valuable coins for your coin collection.
Here’s to “diggin it”
Larry Smith
Tags: coin density, coins, metal detecting, soil density
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