(upbeat music) We're on Pen-y-Fan in the heart of the Brecon Beacons National Park. Like most of Wales, 23,000 years ago the valleys of the Brecon Beacons were filled with ice. Massive glaciers that completely dominated this landscape. Over the next 12,000 years, the glaciers came and went as global temperature increased, and the last glaciers were still here just 11,000 years ago. We only have to look around to see the marks that these glaciers have left on the landscape. Look at this magnificent valley. It's not a V-shaped valley created by a river cutting down into the landscape, but it's a U-shaped valley that has been carved out by a glacier. With a little help from his freezer, Tim has been trying to better understand how glaciers have shaped this landscape. So to understand how glaciers turn V-shaped valleys into U-shaped valleys, we need to do a little bit of kitchen geography. So the first thing that we need is a bottle of water. And we're going to put this in the freezer. Then we need another bottle of water, but in this bottle of water, we're going to put some rocks. We're going to shake these rocks so they're just lying along the bottom of the bottle, and we're going to put that flat into the freezer. Now we're going to wait. After a few hours, we're going to cut away the plastic. This is what we've got. Two model glaciers, one with no rocks at the bottom and one with rocks at the bottom. So we've got two halves of a watermelon here. We're going to use these to show the importance of glacial abrasion. If I take the glacier with no rocks at the bottom, you can see I could be rubbing this over the watermelon for days on end and nothing's going to happen to the skin of this at all. Whereas if we take the one with the rocks at the bottom, let's see what happens. So if we rub this over here, we can see the contrast is massive. The glacier with the rocks at the bottom of it has scoured away at the skin of the watermelon, and this is very similar to a real glacier. (upbeat music) Now, as the glacier moves down the hill due to the force of gravity, it picks up or plucks material from the valley bottom. This material is then attached to the glacier, and in a process called abrasion it scours away the landscape. The rocks literally act like sandpaper. So we've got another watermelon here. And into the watermelon, we've cut a V-shaped valley just like you would get from a river. And what we're going to do is see what happens and how that valley changes when we send a glacier down it. So, if we take our glacier away, we can see how those processes of plucking and abrasion over thousands of years have eroded the valley sides and the base of the valley, to turn that V-shaped valley formed by a river, into a U-shaped valley, or glacial trough. And here back in the field, we can see this glacial trough. Along the valley sides are spurs or hills, which would have once reached right into the centre of the valley. These have been chopped off, or truncated by the glacier moving down the valley. We call these truncated spurs. And that, is kitchen geography. So after a few hours, we'll take them out of the freezer cut away the plastic, and then we've got two fantastic glaciers. (ice breaks)