It is 420 million years ago and the fusion between the two continents which would form Ireland's land mass is almost complete.

Through a series of individual collisions, north-west and south-east Ireland are finally welded together along a 'suture', which runs from here in Clogherhead in County Louth to the Shannon Estuary in the south-west of Ireland.

There is no agreement among experts on the path that the suture follows across the Midlands of Ireland as it is concealed by much younger rocks.

And there is nothing left of the enormous ocean that once separated those continents as the evidence has been totally destroyed.

So although some geologists refer to the join as the 'Iapetus suture' (after the ancient ocean which separated the continents), rather than a scar across the landscape it is best to think of the point of closure as a zone comprising a number of kilometres to the north and south of the join.

What is amazing though is to imagine that when you stand at Clogherhead the lands to the North and South were once thousands of miles apart.

The great bonding marked by the almost vertically folded, greywacke rocks at Clogherhead is reflected in the mix of fossils found in and around this headland.

Those on one side fit North American patterns, and those further south are more European in make-up.

It is the existence of both 'American fossils' and 'European fossils' on one land mass which allows geologists to identify that this join in the land exists.