7/26/26 Report - Distribution of Beach Materials and Objects on a Beach. Sedimentation and Compaction.

 

Written by the TreasureGuide for the exclusive use of the Treasure Beaches Report.

I noticed that one post from many years ago getting a lot of recent views and decided it might be a good time to update that post and add some to it.  That old post is about compact versus fluffy sand, how sand is compacted and what it means to the detectorist. Compact beach sand is often a very good sign for the beach detectorist.

Did you ever walk out onto the beach and sink in the sand up to your ankles?  It is like that sometimes, but you might walk a distance and find yourself walking on a harder layer where you don't sink in at all.  I always prefer a harder area of more compact sand.  You might wonder why some sand is fluffy while other sand is firmer.

I've talked many times before about how the size of the grains helps determine the amount of water force required to erode the sand on a beach. The size of sand gains varies. The grains can be large or small and rough and irregular or smooth.  The sand can be either just recently pushed up and piled up, or it can be from an older layer that has settled over time.  Sand can be made of different materials.  Sand can be made of shell or rock.

One thing I've pointed out many times in the past is the chart below that shows sedimentation and deposition gradients for different kinds of materials.  In this case the range goes from clay to pebbles. The variable is the size of particles.  The other variable is the amount of water force.

ON the excellent chart below, the vertical axis shows the grain or particle diameter.  Going from small to larger grains, bottom to top, is clay, silt, sand and pebbles.  The chart also shows that with increasing grain size, it generally takes more water force to move larger particles, but it isn't a simple straight-line increase   As excellent as that chart is, other factors besides particle size and water force must be considered.

Sand grains are defined as being from .05 to 2 mm in diameter.  Silt grains are .0002 to .05 mm.  And clay less than .0002 mm.  

But in addition, each grain of sand is different in composition, resulting in different densities. Besides that, the shape and texture for grains varies. Silt particles are smooth, floury and greasy when wet. Clay is hard when dry and sticky when wet. Clay sticks to itself. You can see in the chart that it takes as much or more water force to move clay as it takes to move pebbles (red line on right). Pebbles drop out and are deposited at higher force levels than either sand or silt (left red line). As the force of water diminishes, transpoted pebbles will drop out and settle before the finer grained materials such as silt. The amount of force required to move sand will typically be less than the amount of force required to move a coin, not only because of the weight or density but also because of the shape.

The chart above doesn't consider all of those factor.  The surface of grains can be very different. Just like pebbles, grains of sand can be either roundish and smooth or rough and irregular in shape.

Sand that is firm is compact. There is less space between the grains. 

The amount of friction between the grains is important too. Grains that are smooth and round barely touch each other so there is little friction between the grains.  The friction between rounded grains is small.  Think of a pile of bowling balls. There is very little contact or friction between each one. 

Angular grains, on the other hand, will touch each other at multiple locations and over various areas, thus interlocking something like the stones in a stone wall. Just like river rocks, the tumbling 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.

Finer sands will naturally have smaller pore spaces between grains and angular grains are most likely to tightly interlock and cross-link. Rounded grains will always have larger pore spaces between grains. As the individual grains become more spherically shaped the grains become more incapable of cross-linking. 

If you dig a hole on a beach, you will often note distinct layers of sand. Each layer will have its own characteristics.  Bird of a feather flock together and so do different types of beach materials -and just as importantly, so do other objects such as coins. Deeper layers often have been subjected to sifting and sorting for longer and are typically more compact even though there are exceptions.

We know how builders compact earth and how sand sculpturers intentionally compact sand. They use vibration.  You may have seen compactors that vibrate on the soil and assist gravity. On the beach, the pounding surf vibrates but there are many sources of movement to jiggle and reposition the grains. The tides and surf come and goes; Water percolates down through the layers, first increasing pressure between pores and then vacating. There are also other sources of movement such wind, foot traffic and vehicles on the beach.

In recent years I've commented on finding coins on the surface of a buried layer. Of course, a lower layer won't erode until the layer above moves. When top layers erode, some items will lag behind and end up on what were previously lower layers. 

Pay attention to the feel of the sand as you walk down the beach. 

On a few occasions I've seen uncovered clay, which has often been a very good sign.  When that happens, there is a very good chance of finding older objects, including coins. .I always like to see a layer of exposed clay. When you see an exposed layer of clay, other loser materials such as sand have been removed.  One of the good things about that is when the sand is moved, many of the other lighter junk materials will be removed too.  One of the reasons I so seldom use discrimination on a beach is the sifting and sorting effect.  When seeking older things, I don't spend much time in areas with a lot of light junk.  I use a lot of that as a sign to move on to more productive areas.  

To sum it up, there are a lot of factors that affect the movement of sand and other objects on a beach and where you will find them.  If you are hunting modern jewelry or recent drops all of this doesn't matter much, but it can be a big help if you want to find older things

Here is the link to a web site on compacting sand.

http://www.sandscapes.com/sandtech.html

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No tropical weather devloping in the Atlantic or Gulf and no change in the surf or beach conditions.

Good hunting,

Treasureguide@comcast.net