The cove of Bréhec halfway between Paimpol and St Quay Portrieux is a very interesting geological site. The formations that outcrop on this stretch of the coast belong to the red series of the North Armorican domain and lie unconformably on the local Brieurian basement.
A cliff is an escarpment of the littoral (a big unevenness) in edge of sea. The cliff is created and maintained by the action of erosion of breaking waves on the beach. The wall of a cliff is not or very little covered with plants but is mostly rocky.
The existence of a cliff depends on the nature of the rock and the terrain attacked.
Several types of cliffs exist: the high cliffs (exceeding 10 meters), the medium cliffs (between 2 and 10 meters) and finally the low cliffs (whose height is less than 2 meters).
At the edge of tidal seas, the sea water, especially when it is agitated by a storm, has a considerable force (it can reach 30 tons in M2). The water that rushes into the cracks of the submerged rocks violently pushes the air filling the hollows; this compression ends up cracking the rocks. Wave after wave, the work of crack continues. At one point pieces of rock break off and fall.
The materials torn from the rock also help the destruction. Being thrown by the waves on the rock, it ends up digging it. These shocks over long periods down the cliff eventually create a kind of notch or hollow down the rock face. The rocks above are without support underneath.
In the upper part, the infiltration of rainwater, long before the land is at the seaside, has already cracked the rock. When the rock is exposed, the succession of freeze / thaw can also cause erosion, it is cryoclasty.
With the steep slope and erosion at the base, some of these rocks come off and collapse at the foot of the cliff. The next tides then take care of breaking the rocky debris into smaller and smaller pieces, to the point where they can be washed away.
This attack of the relief by the marine waters is done around the level of the surface of the water (altitude zero); the action of the waves is very small or nil with the depth. Note that the sea level has varied over time, one can find sunken cliffs or cliffs at altitude compared to the current level of the sea.
At the bottom of the sea, in front of the cliff, there is a platform of abrasion, the rocks are identical to those of the coastal region. This platform is traversed by the tides and it shows that the relief has receded at this place.