The Cliffs of Moher are a line of sheer cliffs some 8km in length, extending from Luogh Point in the north to Cancregga in the south. The cliffs attain their maximum height of 200m at Knockardakin just northeast of O'Briens's Tower. Knockardin is the highest point of a range of hills that the cliffs cut obliquely across. To the north the cliffs are cut into a northwest-facing slope, whereas to the south the top of the cliffs are the highest point, with the ground falling away immediately to the southeast.
The cliffs are formed of layers of siltstone, shale and sandstone, and represent a slice through geological time, with the oldest rocks at the bottom of the cliff. The layers are tilted very slightly towards the southwest, so that layers which are at the bottom of the cliff below O'Brien's Tower appear at the cliff edge 2km north of the tower. The cliff-top walk towards Doolin is a journey back through geological time.
The sandstones are more resistant to erosion than the siltstones, and thus stick out from the cliff-face compared with the intervening layers of siltstone. One such sandstone forms the platform below the path at the viewing point.
The sediments which now form the rocks of the cliffs were originally deposited on an ancient sea-bed some 320 million years ago. The shore of the sea lay some distance away to the north, and rivers flowing off the land dumped their sediment into the sea in a series of deltas....
As you walk from the visitor centre towards the viewing point notice the flagstones lining the path. The squiggly marks on them are the impressions left behind some 320 million years ago by soft-bodied animals which burrowed through what was once mud and sand looking for food.
These and similar flagstones were quarried in an area southwest of the visitor centre (see map). One abandoned quarry is set perilously at the cliff edge..
An audiovisual exhibition of the former quarrying industry of the area is at the Liscannor Stone and Rock Shop, beside the road to Liscannor.
While O'Brien's Tower provides a fine viewpoint of the cliffs to the south, the sure-footed will find it worthwhile following the cliff-path (the Burren Way walk) to the north. (This is not a good idea in windy weather, as there is little space between the path and the abyss). The crowds will soon be left behind, and you will have only the company of seabirds such as fulmars, and, if you're lucky, choughs.
A few hundred metres to the north the path drops down to the level of a prominent sandstone bed. If the weather is calm it's worth proceeding very cautiously back along the sandstone ledge (see picture to right). Above the sandstone are black shales, in the lowest part of which you may see the fossilised remains of goniatites (circular or spiral markings up to 2cm across). Goniatites, a now extinct group, were similar to the present-day Chambered Nautilus, in that they were free-swimming in the ancient oceans.