Monday, September 26, 2011

9/26/11 Report - Analyzing a Beach

Written by the TreassureGuide exclusively for

House of Refuge This Weekend.

Thanks to Bernie C. for the photo.

What do you do when you first step onto a treasure beach? Your answer to that question will have more to do with your rate of success than what detector you use.

The first thing a lot of people do is turn the detector on and start walking to a favorite spot. Some, who perhaps don't have a favorite spot yet, will start wandering around on the beach without any particular plan or direction. They just go where they feel like going. And others are guided by a general feeling that one area or another might be a good place to look.

What I'd recommend is first finding a good place to view as much of the beach as possible - someplace where you can see the back dunes and down in the water and also north and south as far as possible. Slowly scan the entire beach. Look for any dips and get a general feel for what the beach is doing.

Sketch Showing Cross Section of Theoretical Beach.

A beach has four dimensions that are important to me - length, width, depth, and time.

The area of a beach that changes the most is obviously the front beach because of all the water action there. The middle and back beach changes much less frequently and mostly when the waves and tides are high.

Since the back dunes change less often and the movement of sand is most affected by the water, first check out what the water is doing to the front of the beach.

You can tell a lot just by looking at a beach, but to really know what is going on, it is helpful to have a mental model of the beach as it has changed over time - days, months, and even years.

The sketch above represents a model of a theoretical beach showing a cross section and the different levels.

It is helpful to know where the bedrock is. On the sketch it is shown near the bottom. Often you won't know where the bedrock is.

At different times the beach will be cut back and down, sometimes very deeply exposing layers that have not been exposed for a very long time, and at other times only few inches or feet of sand are moved from the front beach. Those smaller cuts will quickly filled again, usually within a few days.

The result of all of that movement is some layers of sand that are deep and that have been exposed only a few times over the centuries and on top of that are layers of sand that are exposed and moved frequently.

You will seldom see sand on a front beach that has not washed in, and it is rare to see so much erosion that you see layers that have not been exposed for a long time. Usually you will only see sand that was deposited relatively recently.

When the back dunes are low and far back, you will seldom find coins on a front beach that have washed down from the dunes rather than washing in from the ocean. Old coins will occasionally wash out of the dunes and down onto a front beach, but that doesn't happen very often, especially on beaches where the dunes are low and far back from the front beach.

Returning to the layers. The more you know about the various layers of a beach the better off you will be.

The above sketch illustrates how I conceptualize a beach. The layers are shown from layer A, the bedrock, and going up to the top layers of the dunes.

You can learn where the different layers are and can learn to identify some of them by sight.

You can't get a good mental model of a beach unless you carefully inspect it a number of times. Landmarks will help you immensely. It is difficult to simply look and tell how much sand has moved without measuring the change relative to some stationary points.

Those landmarks can be dunes that haven't moved for a long time, trees, rocks or anything that will not move much. Caution: some things, like tree stumps, will move even though you might think they are firmly anchored.

The type of sand or beach material will also provide indications of movement.

When you dig a hole, notice any layers that you see. Notice layers of shell, black sand, clay, or different colors or sand of different textures. When you dig a hole, you'll normally only see some of the top layers that move a lot.

Deeper layers towards the back of the beach will change much less often.

Try to get a fix on which way the sand is moving. Is it building on the beach front? Is the cut moving farther back towards the dunes? Is the sand being washed down the slope and into the surf?

Most people know to look for cuts, but the position of the cut and the depth of the cut are both important. Cuts can be high or low on the beach, deep or shall, long or short, cut into new or old sand or other materials. When cuts occur far back on the beach and are deep, you are getting into layers that probably haven't been exposed for a long time and may contain old objects.

You need to make a number of careful observations before you can construct a useful mental model of a particular beach. After you know a beach well, then you don't need to observe it as frequently to understand what is going on.

It also helps to know where past cuts have been at particular times. How far back and how deep has a particular beach cut at different times? That information will help you a lot.

Lets say that a beach eroded down to very near the water line and back almost all the way to the dunes after a hurricane. That means that that all of the sand there now has accumulated since that time. That's obvious enough, but what are the implications?

That depends to some extent upon how the area filled. Was the new sand washed in from the ocean, dragged down from the dunes, or both, or moved in from the north or south of the cut? Sometimes you'll have a layer that was dragged down from the dunes and then covered by sand that washed in from the ocean.

Were there coins or artifacts at the level of the cut before it was refilled, or in the sand that filled the cut? If so, some might still be waiting there.

These are all important questions.

If you've worked a particular beach a lot, you will be able to identify different levels. You might know what the different layers look like that have accumulated at different times.

Sand will vary is texture, color, and composition. It is sometimes helpful to know where the various layers are and when they were deposited. You might see them appear from time to time and perhaps be move or covered again.

In the back dunes it is a bit easier to identify layers where items of different ages of items are found. Those layers will mix when the dunes erode and the sand falls down or is dragged onto the beach.

One of the most desirable situations is when layers are exposed that haven't been uncovered for a long time. There are levels at some beach locations, particularly back in the dunes, that have probably not been uncovered for a few hundred years. There are probably also layers on the middle beach, although much deeper that probably have not been uncovered for a very long time.

Well, I better wrap that up for today. I could go on for a long time about this stuff. I'll have to continue with this some other time.

Treasure Coast Beach Forecast and Conditions.

There is one tropical disturbance just north of the leeward islands. It has a chance of developing. And there is Philippe, which is headed into the middle of the Atlantic.

The wind is from the south. There are a lot of rain showers just off shore this morning.

Seas are running less than two feet and will remain relatively calm for the next few days.

Conditions for finding cobs on the Treasure Coast remain poor.

Bernie reports about 4 feet of sand was recently removed down at Bathtub Beach and tree stumps were newly exposed. A good amount of clad coins were found there.

That's it for today.

Happy hunting,