Tidal plain and Algorri cliffs

The stretch of cliffs described here runs west to east from the municipality of Deba all the way to Itzurun beach and Mariantón Point in Zumaia The rocky inclines are almost vertical and reach heights of up to 150 metres.

Geology

Remains of the cliff face

The materials which have been raised to the surface along this stretch of coast were formed during the Lower Cretaceous (around 100 million years ago) and the Tertiary Era (around 50 million years ago). During these periods, Gipuzkoa was completely submerged and subject to a prolonged process of marine sedimentation. The pressure and temperature at the bottom of the sea combined to form Rocks which, after being folded and elevated, ended up at sea level, exposed to the wind and the waves which have since moulded the magnificent landscape known as flysch. The materials belonging to the Lower Cretaceous, located between Deba and Aitzuri, are made up of sandstone, lime mudrock, limestone and marlaceous lime. From Aitzuri to Algorri, the materials date from the Upper Cretaceous and include marl and marlaceous lime. The last section, between Algorri and Mariantón Point, is made up of limestone, marlaceous lime, marl, sandstone and lime mudrock from the Tertiary Period.

Another particularity of these cliffs is the abundance of fossils (ammonites and bivalves, among others) and the marks and indications of human excavation or perforation. Each layer of sediment (now vertical) is like a page in an open book which “narrates” the history of life on our planet. Geologists and academics from all over the world come to Zumaia to try to decipher the secrets hidden in the rocks and to understand the reasons for the disappearance of numerous species which populated the planet 65 million years ago.

Reddish sediments

 
Coastal erosion in this area is particularly intense; during storms, particularly in winter, the lower parts of the cliff faces are subjected to constant battering by the waves and wind, which little by little wear them away. As a result, the upper layers of rock lose their support and collapse. The harder layers of rock manage to withstand this erosion and form points or capes, while the weaker ones are literally eaten away by the sea, giving rise to large inlets and coves.

Thus, over thousands of years the sea has gradually gained ground and has left, at the base of the cliff, evidence of what was once an old mountain, now known as the abrasion platform or tidal plain.
Small valleys are washed by short streams which, many centuries ago, flowed gently into the sea, but which today reach the coastline at a height of over 10 metres, forming waterfalls or simply spilling over the sheer walls of the cliff face.

Flora and fauna

Abrasion platform

When describing the flora and fauna of the Algorri area, it is important to distinguish between two very different habitats: the walls of the cliff face and the platform, which is subject to the daily influence of the tides.

In general, the rocky incline, with its sharp slopes and fragile materials, is not conducive to the formation of soil. Furthermore, the force of the waves often uproots and destroys any plants that might have begun to thrive. Only in small cracks and shelves do some hardy species manage to survive.

On the small shelves, the more gentle slopes and those areas more sheltered from the influence of the tides, more soil accumulates and small fields appear, scattered about with plants typical of this type of coastal region.

Spanish gorse
Genista
hispanica

 
At the top of the cliffs, where the influence of the sea lessens and the soil is more developed, you can find gorse and heather thickets (particularly of the Erica vagans species), accompanied by holm oaks and strawberry trees, which are somewhat stunted due to the need to protect themselves from the salty winds which blow in off the sea.

The flora on the abrasion platform is mainly made up of algae, along with a few lichens capable of withstanding the constant movements of the tides. In accordance with their tolerance to being exposed to the air, algae form vast belts of different species at the different inter-tidal levels.

The coastal flysch is a fairly inhospitable environment for land-based birds. Thanks to the vertical nature of the strata and their instability, it is rare to see birds in the vicinity of the cliffs. Similarly, few land-based species venture out into the tidal plain, and those that do, do so only in order to feed from the many pools that tend to form there.

For marine birds, on the other hand, along with a few waders, the abrasion platform is of incalculable value, since it offers the sustenance that is so hard to find on coastal beaches. Here, oystercatchers, turnstones, common sandpipers and phalaropes make the most of the resources offered by the shoals and pools.

Seagull

 
The marine birds most commonly found in this coastal area include divers, great cormorants, razorbills and guillemots, which wade in the coastal waters along the Bay of Biscay coast during the winter months.

The animal wildlife on the tidal plain is characterised by its diversity (there are over 200 different species) and high concentrations of certain species. It is also worth noting that the area is home to representatives of the majority of marine invertebrate groups.

Rock samphire
Crithmum maritimum

Certain filtering polychaetes establish reef structures whose hollows and cavities support a rich and varied wildlife. There are numerous populations of molluscs, limpets, winkles and octopuses. Also, special mention should be made of the nudibranchs, due to their wide range of colours and great beauty.
Crustaceans also form a significant part of the inhabitants of the plain, and it is not uncommon to find shrimps, porcelain crabs, swimcrabs, aggressive warty crabs and squat lobsters. Various species of starfish and, particularly, the common sea urchin abound in the intertidal area and the region's many rock pools. Indeed, some pools contain up to 600 sea urchins per square metre.

The communities of intertidal fish are quite unique in comparison with those found in other regions along the Bay of Biscay coast. Clingfish, little scorpionfish, gobies and blennies are all common inhabitants of the tidal plain pools.