(David McGarvie) This sand dune in Cornwall is being formed as grains of sand are blown up from the beach down there and piled up to form a dune.

There's an awful lot of sand here.

What's it doing here, where did it come from, how did it get here, and what is sand?

I'll let you into a secret. Most of it is made up of grains of quartz.

Well, in this video we're going to go back and look at the original source of the sand, and look at some of the ways in which it could have been transported here.

And then back in the lab we're going to use some simple methods to extract more information about this sand.

But let's start at the beginning and go and find out where these sand grains began their journey.

Geology's not all sunshine and sand. I've come up here 30 kilometres from the beach and I'm 300 metres high on top of the moor. We've come up here to find where these grains started their journey, and it's right behind me here up on the top of the moor. I'm off to have a closer look.

Here we are - this is the source for much of the material that forms our sandy beaches.

This is a granite, an igneous rock that formed deep within the Earth and cooled very slowly. We know that because when we look at it we see large crystals.

Let's take a closer look at these large crystals.

I'll take my glove off to point them out to you. I can see lovely glassy quartz, white feldspar and small dark flakes of mica. But a quarter of this rock is made up of quartz. Why is this significant?

Well, physical weathering breaks up the rock, disintegrates it. Look, it's taking place here, and once it's broken up like this, chemical weathering can get to grips on the surfaces.

Now the feldspars and the micas are very susceptible to chemical weathering. Part of it goes into solution and part goes to form clay minerals, which we find as mud on our beaches as well as mud flats. The quartz is very resistant to chemical weathering and that's why we find so much quartz on our beaches.

So how does it get from here down to the beach? I'm off to find out.

There are three obvious ways of moving grains from a source down to a beach. There's glaciers, but there's no glaciers here, there's wind, and we've seen wind in action down on the beach moving grains along; and there's also water. Up here on the moor it rains for 200 days of the year, and that's 2 metres of total rainfall. So it's water like this in streams and rivers that's responsible for moving all these grains from their source up in the granite downstream to the beach.

Here's some loose grains I collected from under a bridge - let's put some in the water and see how they actually move.

It's not easy to see exactly how the grains are moving in the river, so here's an animation to help you.

The largest grains stay on the bottom and don't move - they're too big to be moved by a flow of water.

Slightly smaller grains either roll along the bottom or bounce along.

Whilst the very smallest grains are carried swiftly downstream completely suspended in the flow of water.

This illustrates one very important principle. The size of grain that will be moved is related to the current speed.

We've seen grains being moved by wind down at the beach. We've seen them being moved here by water. Which do you think would more easily move the grains? Think about it for a second.

I hope you appreciated that water is much more effective at moving grains. That's because it's a much denser medium.

Think about yourself - you can stand upright in a 40 mile an hour wind, but I'll bet you can't stand upright in a 40 mile an hour current of water.

I'm off down to the beach now, where I'm going to take a more careful look at these grains, and I hope the weather's going to be a bit better down there!

You've probably made sandcastles before, and here's two I made earlier. Let's have a look at this one.

This is a river gravel we collected up on the moor, and as you can see it's made up of larger grains and some smaller grains. Let's spread it out onto this board and have a more careful look at it.

Well when I look at this carefully I can see it's made up of some large grain sizes and some very small grains as well.

And this sort of sediment in which we have a mixture of grains, we call poorly sorted.

But, as a geologist, I'm not just interested in the size, I'm interested in what's actually here, what they're made of, and I can see rock fragments, I can see quartz, feldspar and mica, and there's so much feldspar and mica, that tells me that this sediment has not been very chemically weathered. We know that because it's come from very near its source up on the moor, where we collected it from the river.

Anyhow, let's get rid of this and look at this one here. This is much, much nicer. Spread some of it on the board. Oh, this is a much more even grain size. There's none of the large fragments that we saw in there before, and we call this sort of sediment, with a much more even range of grain sizes, well-sorted, but again it's not just size I'm interested in. I'm interested in what these grains are, and there's lots and lots of quartz. So this is much more chemical weathered than the sample we looked at before, and what that tells me is this has been in the system for longer, and that's not surprising, because this sample came from a beach just here.

Now we've learnt quite a lot about these sands by just looking at them here. But we can learn more by taking them back to the lab, so let's go.

Fieldwork's a very important part of geology, but we can't do everything in the field. So we've come back here to the lab to look at the sand in more detail. We're going to use two very simple techniques, one using sieves where we can get an idea of the grain size distribution of the sand, and secondly a microscope, where we can see more clearly exactly what the sand's made of. Let's start with the microscope.

Here's some sand I bought back from the beach.

Right, this is a lovely sand - lots of large, quartz crystals, which are fairly angular and have got nice reflecting surfaces on them.

There's also some, white, creamy feldspars, and there's some lovely little dark mica flakes, and also some clear mica flakes, but I'm particularly interested in the shapes and surface textures of these quartz grains, because they can tell me a lot about what's happened to the quartz grains in this sand.

If I look at the shapes first of all, I can see that they're very angular.

In fact they're remarkably similar to the angular quartz grains we saw in the granite up on the moor, and that suggests that these quartz grains haven't gone very far.

If I look now at the surface textures, they're glassy and reflecting, and that tells me that these quartz grains have been transported by water, because water has 'cushioned' the impacts between the grains

Already we've got two vital pieces of evidence from these quartz grains, first of all shape - they're angular - and that tells us that they've not been processed for very long, otherwise they would be more rounded. Secondly, we have surface texture. These quartz grains have glassy reflective surfaces, and that tells us that main transporting medium was water.

I have a river sand here. Let's have a more careful look at that one. I'll get rid of this one first.

This is quite, quite different. What about this reddish surface colour we've got here? Let's take a more careful look at that.

It certainly appears to be just a surface coating with the red stuff around it, so this appears just to be an iron oxide coating on some of these grains. But again, I'm very interested in the surface texture and the shape. Let's have a look at the shape first.

Oh it's really striking, these grains are beautifully rounded. There's hardly an angular surface to be seen anywhere. I have to think of some way in which I'm going to get very rounded grains with all the angular comers knocked off. But let's have a look at texture now.

Look at this one here. You can see its surface is covered with tiny little pits and scratches. That's what's giving this grain its surface frosted appearance.

Putting these two pieces of evidence together- we've got well rounded grains, and we've got frosted surfaces - we need to think of an environment where grains are colliding violently with each other, causing all the pits and scratches which give rise to a frosting, and also they get the corners knocked off them to give you the rounded shape. Can you think of a modem day environment where you might find this sort of sand?

I hope you thought of a desert - There's no water there cushioning impacts between the grains. They can collide violently as the wind blows them around on the desert.

So, microscopes provide us with three key pieces of evidence. First of all, the presence or absence of other rock fragments or mineral grains can tell us much about the original source of the sand. Secondly, the shapes of the quartz grains tell us how much they've been processed, and thirdly, the surface texture of the quartz grains tells us a great deal about the main transporting medium - frosted if it's wind, and glassy and reflecting if it's by water.

We've finished with the microscope now. What we'll go and do now is look at the grain size distribution of the sand using these sieves.

As you'll see, we have the largest mesh size in the very top, and gradually we go down smaller and smaller mesh sizes until we get right to the bottom, and this is where the very smallest grains pass through every mesh and end up in the pan at the bottom.

Now how are we going to use these? Well, down on the beach we describe sand as well sorted and as poorly sorted. Now these sieves allow us to quantify exactly how poorly sorted or well sorted the sands are. So I'm going to put them back together now and we'll sieve the sands and we'll look at the results later.

All we do is shake gently for 5 minutes. OK, so let's have a look and see what's in each of these sieves.

There's a couple of grains in the largest one, quite a few more in there, a lot in that one, a good swag in there, a fair amount in there, a smallish amount in there, a tiny amount there, just a few grains in the pan.

Immediately we get a clear, visual impression of the amount of sand in each sieve. We can take the sand in each sieve and weigh it to obtain its mass, and from that mass we can construct a histogram which gives us a very clear impression of the degree of sorting of this sand.

The horizontal axis shows the grain size, the largest on the left, and smallest on the right.

The vertical axis shows the mass of sand in percent. The mass of sand in each sieve is represented by a column. The histogram for this beach sand shows that almost all the grains have been captured by just 3 sieves.

Okay, so you've seen the histogram. From this we can construct a cumulative frequency curve. It sounds a bit complicated, but in fact it's very straightforward to construct. Here's how we do it.

The vertical and horizontal axes are the same as the histogram. The tricky bit is that we want to plot a total percentage of sand that fails to get beyond a given sieve size. But in fact this is quite easily done by simply moving the columns successively to the right on top of each other.

The top of each column is marked by a point. And then we draw a smooth curve through all the points to give us our cumulative frequency curve for the beach sand.

Okay, you've seen the histogram and the cumulative frequency curve for the beach sand we collected. Let's now look at the histogram and the cumulative frequency curve for the desert sand.

With this desert sand, nearly all the sand grains have been collected in just one sieve. This gives a very much steeper cumulative frequency curve. And here's a beach sand curve for comparison.

Here's another diagram. It's not one that we constructed from information we collected from our 2 sands. It's a very important diagram, but it's also a very complex one.

What it does show is the current speeds at which grains of a certain diameter will be transported in either wind or water.

We'll start with water and then look at wind.

The vertical axis gives current speed, and this is a powers of ten scale, not a linear scale. The horizontal axis gives grain diameter. This time the largest grains are on the right, which is the reverse of what we saw on the histograms and the cumulative frequency curve. We've shown typical grains at the bottom drawn to scale to help you with this.

Let's see how this graph works by going on a little journey. Imagine that you're a sand grain 1,000 microns in diameter - that's 1 millimetre. You're sitting on the bottom of a stream where there's just a sluggish flow of water. This is point A on the graph. It rains upstream and the extra water in the stream causes the water speed to rise. The water speed continues to rise, but the grain remains in the bottom until at point B the grain begins to move and is transported.

Let's change things. It stops raining and the water speed slows down. Now here's a surprising bit. The grain does not stop moving at speed B but keeps moving until speed C, which is a slower water speed than speed B. Below speed C the grain will not move. This is a crucial finding. The current speed needed to start a grain moving is higher than that at which the same grain stops moving and is deposited.

These sorts of measurements can be carried out in laboratories, and using different grain sizes and current speeds, a pattern develops.

We call the shaded zone on the graph the transportation zone. Above this zone grains cannot be deposited, and below it grains cannot be moved.

But what about transportation by wind? Here we have a different part of the graph and note that the current speeds on the vertical axis are much higher than those for transport by water. The same pattern emerges. For a given grain diameter, there's a speed at which the grain will start to move and a lesser speed at which it will cease to move and be deposited.

If we put both parts of a graph together now, you'll see quite clearly that the transportation zone for wind is well above that for water. In other words, a grain of a given diameter can be moved at lower current speeds by water; much higher wind speeds are needed to move the same grain.

So, we've been to the source of our beach sand, in this case a granite up on the moor - an igneous rock which consists of interlocking crystals of the 3 minerals: quartz, feldspar and mica. And we've seen how physical and chemical weathering combine to break up these interlocking crystals into small rock fragments and mineral grains. And we've also seen how quartz, the most resistant of these mineral grains provides us with information about how the grains have been transported, because the shapes and surface textures of quartz grains provide important clues about transportation by wind or by water.

We can even use the sizes of grains to gain an appreciation of the speeds of currents that transported them.

So, careful visual examination of a sand can reveal a great deal about the environment in which it was deposited, and this means we can study modern day sedimentary environments and use our observations of these to help us interpret ancient sedimentary environments.

In this video we've used simple observational techniques, combined those with experimental work in the lab, to give us an understanding of how modern day sediments have been formed. If we look behind me here you can see a slightly older sand dune. It's well on its way to being a proper sandstone, and you can even see layers in it. And they tell us that the wind was depositing the sand grains at different times.

Now we've looked at modern dunes earlier on in the video. We're seeing a slightly older dune here, and we see exactly the same features in ancient sand dunes which are millions of years old; therefore we've got a link between the past and the present. By looking at the present we can begin to see what happened in the past.