Page last updated at 11:05 GMT, Friday, 15 July 2005 12:05 UK

Surprises lie around coastline

By Mike McKimm
BBC Northern Ireland environment correspondent

If people go down to the beaches in Northern Ireland and look very carefully, they could be in for some big surprises.

Most people wander onto the beach more intent on finding that perfect spot away from the screaming kids on the rug next door.

Carrick-a-Rede rope-bridge
Carrick-a-Rede rope-bridge actually joins two parts of an extinct volcano
The next expedition is usually to find the toilets or the ice cream van. But there is plenty more to spot - often staring you in the face.

Take the Carrick-a-Rede rope bridge on the north Antrim coast.

It started life as an essential tool for local salmon fishermen but has now become one of Ireland's top tourist attractions.

Each year, thousands of visitors nervously cross the bridge, frightened even to look down into the swirling water far below.

But the moment they step onto the wooden slats, there is an astonishing secret right below them.

They are walking over some of what was Ireland's most violent geological history.

The bridge is attached, not to the solid black basalt or limestone of the area but to prehistoric compacted volcanic ash.

Volcanic scar

The bridge spans two parts of a volcano, long extinct. If you look down at the cliff face just below the bridge, huge boulders known as volcanic bombs lie studded in the compacted fine ash.

These were blasted out of the volcano along with the ash. The explosion blew away the surrounding limestone as well, leaving Carrick-a-Rede as a black volcanic scar on the long run of limestone cliffs that make up this part of the coast.

It certainly makes the crossing a lot more exciting, especially as you have to look down to see the bombs - for those who dare.

Keeping your feet firmly on the ground, you can enjoy the delights of the little harbour of Ballintoy. It's a picture book scene, snuggled up against the limestone cliffs. But just to one side of it is the Ballintoy of thousands of years ago.

The hummocks at Ballintoy were formed by sea erosion

Small curious steep hills lie between the cliffs and the sea. A dry arch that was once hewn out by the waves stands isolated with the water barely lapping at its base in a high tide. But how did they get there?

When ice, a kilometre high, retreated after the last ice age, the land began to rise and tide didn't come in as far as it used to. The feature is known as a raised beach. But this one had the extra attraction of small rocky outcrops eroded by the seas of 8,000 years ago now high and dry.

Grass has invaded the fractured rock producing a strange landscape of hummocks. It's well worth the short walk from the harbour to explore the phenomenon.

Further afield, just north of the small Donegal village of Rathmullan, on the shores of Lough Swilly, is the White Strand.

It's an unremarkable beach and most people drive past it without a second glance. But a few months ago, a local man was walking his dog on the beach and was startled to discover the remains of a previously unknown ancient forest.

Ancient tree roots on Donegal beach
Not rocks on this beach but ancient tree roots

Storms had eroded much of the sand away and revealed a feature of the past that had been preserved for thousands of years.

Today you can walk along the beach and see the huge roots of trees. It's real wood, not fossilised rock. But how did the trees get there?

One possible theory is that there was a forest of large trees that extended far out into what is now the sea. About 8,000 years ago the sea overwhelmed the forest and the trees died off.

At the same time the beach moved inland forming high sand dunes and burying the remains of the trees - until today.

Sadly the feature won't be there for ever. Every high tide is working at the wood, breaking off fragments. Marine erosion will eventually mean any exposed timber will disappear.

Northern Ireland may not have the miles of sun-kissed Mediterranean-style beaches but is acknowledged to have some of the most diverse coast and geology in the world.

Even a casual exploration of the shores will throw up everything from fossils to coral sand and it's all absolutely free.

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