Wednesday, June 29, 2016

6/29/16 Report - Compact and Loose Beach Sand. How It Happens and Why It Is Important To The Detectorist.

Written by the TreasureGuide for the exclusive use of

Did you ever walk out onto the beach and sink in the sand up to your ankles?  It is like that sometimes.  Sometimes the sand is soft and fluffy and other times it is packed and firm.  Sand is sand, so what makes the difference?

Actually there are a variety of types of sand.  There is sand made of rocks, shells or coral for example.  And each of those an be worn and polished to different degrees.

But what makes it fluffy or firm?  Sand that is firm has little space between the grains, but that isn't the only important thing.  The amount of friction between the grains is very important.

Grains that are smooth and round barely touch each other, so there is little friction.

Angular grains, on the other hand, will settle, touching each other at multiple locations and over various areas, interlocking something like stones in a wall.

Here are two illustrations from the SandScapes website showing smooth grains compared to angular grains.

Smooth Grains (left) and Angular Grains (right).

The surf smooths sand over time, and like everything else on a beach, sand gets sifted and sorted, as do shells and lost objects such as coins and rings.

Irregular grains can be compacted, as shown in the next illustration from the same site.

Here is what the web site says.  Uncompacted sand has relatively large pore spaces between the grains but compacted sand shrinks these spaces increasing points of contact between the individual grains and thereby increasing the friction between them. The more friction there is, the more resistant the grains are to separation.

One other important dynamic is "cross-linking", a term from soils engineering. Forcing randomly shaped grains tightly together causes many of them to naturally cross-link. Cross-linking is a common technique in masonry work where vertical joints between bricks, stones and block joints are intentionally staggered thereby vastly increasing the strength of the structure.

...Finer sands will naturally have smaller pore spaces and angular grains are most likely to tightly interlock and cross-link. Rounded grains will always have larger pore spaces between grains no matter how well compacted, and a naturally smooth surface further reduces friction. Beyond being merely rounded as the individual grains become more spherically shaped the grains also become incapable of cross-linking. Try to imagine stacking a pile of bowling balls...

I am talking about sand here, not clay or silt.  Clay has a particle size ranging from .002 inches down, and silt has a particle size ranging from .003 inches to .002 inches.  Clay and silt are cohesive and tightly bound together by molecular attraction.

I always like to see clay or silt exposed on a beach, but it does not happen very often.

Sand is not compacted by downward pressure so much as vibration.  A compactor, maybe like one used to compact sand for paving bricks on a driveway, vibrates.  The vibrations penetrate down through the sand.

Both construction workers and sand sculptors add water to sand before compacting, and they generally do it in layers.

So how does sand get compacted on the beach?  Well, first off you want relatively small grain sand. Then water.  Then the third thing is vibration.

The water is easy enough to come by, but what causes the vibration.  My thought is that the pounding surf does an excellent job of pushing pressure waves down through saturated or semi-saturated sand. You can hear the crashing surf miles away if you are someplace where it is not too noisy.

Enough of the how, now we'll get to why it is important.

If you are hunting recent drops in dry sand all of this doesn't matter much, but if you want to find older things, it is more important.

First off, I think almost everybody agrees that deep fluffy sand is not a good sign for the detectorist that wants to something other than recent drops.

Some people might say that things sink in soft fluffy sand.  When you drop something having weight, that certainly happens, but once it sets there a while it is not going to sink any deeper without some sort of agitation.  There is agitation of various kinds, though.  In the dry sand, wind moves sand, and so does rain, as do people walking and running.  But most important to me is the agitation caused by the surf flowing and pounding, and in some cases washing up more sand to cover what ever was there before.

Compacted sand, will not move as easily as loose sand.  Loose sand will wash away, while the more compacted sand remains.   Of course that compact sand could eventually wash away too, but it will take more to do it.  To use terminology I recently used, the trigger point will have to be higher before the compacted sand will start to move.

The vast majority of cobs that I've found have not been deeply buried in the sand.  Some were visible on the surface.  Almost all have been within a couple inches of the surface.  They are seldom found in soft fluffy sand, but sometimes they are in a slab of sand that just came off the front of an eroded cliff or flipped up over the face of a cut.

Compact sand is a good sign.  I often go by feel as well as sight.

I should stop there for now.

I have fascinating information about counterfeiting colonial coinage that I'll get into some day soon.

Here is the link to the web site I referenced a lot today.

Nothing much has changed on the beach.  We still have a small surf.

Happy hunting,