The coast. From the lower valleys, one naturally moves on to the coastline, which is presented to the ocean waves over a length of almost 200 km, from Biarritz to the Bilbao estuary: this indicates that it deserves to be considered as an important element of the topographical landscape. If we are careful not to take into account the fragment of coastline the mouth of the Adour and Biarritz - which is an extension of the Landes - we can see that this coastline of the eastern Atlantic Pyrenees has a great unity: it is a rocky coastline, cut by estuaries whose origin dates back to the last rise in sea level.
But this overall homogeneity hides a real diversity of details: depending on whether it is to the west or to the east of the mouth of the Bidasoa, the coast is more or less rocky, and the estuaries are more or less well preserved. From the point of Cape Higuer, which closes the bay of Bidasoa, it is possible to compare these two aspects of the coast. To the east, you can see the cliffs overlooking the ocean for only a hundred metres at the most and often for much less. The bathymetric charts attest to the existence of a continental plinth known as the "plateau de Saint-Jean-de Luz"; a submerged fragment of the last rise in sea level, of the peninsula of the fourth stage of erosion.
Its higher parts, which have become islands, have been swept away by the waves, but the lower parts have preserved the old continental modelling: the 10 m, 20 m. and 30 m. curves circumscribe the remains of an ancient ridge which stretched from Mount Jaizkibel to Biarritz, but which was already crossed by the extension of the Uhabia stream - which runs south of Bidart - and even destroyed in its southern part by the extensions of the Ugarana and Bidasoa. The Saint-Jean-de-Luz platform is joined to the coast by an inclined plane which rises very steadily to the foot of the cliffs.The estuaries are in a fairly advanced state of regularisation. This is primarily due to the existence of the aforementioned continental plinth in front of the cliffs. As a consequence of the unevenness of its relief and the shallow depth at which it is found, it forces the coastal current which, coming from the north, contours the Bay of Biscay, to deposit part of the sand it carries along the Laburdine coastline after having coasted the inexhaustible sandy area which is the Landes coast, particularly at the mouth of the coast, due to its incessant alluvial contributions. The result is that the mouths of the two eastern estuaries, the Ugarana and the Bidasoa, once living estuaries, are now on the verge of losing contact with the sea and becoming dead estuaries.
In the estuary of the Ugarana - or Saint-Jean-de-Luz - the inner part, which rises with the tide as far as Askain, is almost closed off by a thick coastal cordon on which Saint-Jean-de-Luz is built; as for the lower part, or the bay of Saint-Jean-de-Luz, it is slowly filling in.The estuary of the Bidasoa - or Endaya - where the silting up has been more rapid than in the previous one, also breaks up into two parts: in the inner part, the old estuary has become an alluvial plain, floodable at high tide, occupied in its centre by the sand-filled bed of the Bidasoa. The formation of the important coastal cordon of Endaya Playa has not contributed little to this, as it leaves only a narrow neck as an outlet to the ocean. To the north of this cordon extends the lower or "living" part of the estuary; but it is also in the process of filling in, as can be seen by the bar that has formed across the width of Endaya-Playa - the outline of a second coastal cordon - as well as when going from Fuenterrabía to the cape of Higuer. If you turn your eyes west from Cape Higuer, the spectacle is different; no small cliffs, but the mountain sinking into the ocean; no dead estuaries, but "living" estuaries penetrating far into the interior of the land.
There are several reasons for this change. Firstly, the continental plinth disappears to the west of the Bidasoa, while the mountain sinks almost directly into the Bay of Biscay trench. Here, the coastal current is much less loaded with sand than to the northwest of the Bidasoa, as it has already deposited a large part of it along the Laburdina coast; on the other hand, there is no longer a continental plinth to force it to deposit the sand it carries in suspension. It should be noted that the river alluvium is one of the most active in the eastern Atlantic Pyrenees. The accumulation of this alluvium at the mouths of these watercourses has already inaugurated the filling period for the estuaries.In Guipúzcoa, for example, the accumulation of the Urumea has resulted in the union of the old islet of Monte Urgull to the mainland: the tombola thus formed divides the old Urumea estuary into two twin bays, one the present Urumea and the other the Concha de San Sebastián. However, in Biscay, where there are so many estuaries, without generally being exempt from partial filling, they conserve living forms over a large part of their length, sometimes, as in Ondárroa and Lekeitio, over their entire length.
The Mundaca estuary deserves special mention: completely filled in Mújica and Guernica, in the process of being filled in Guernica and Murueta, it forms, to the north of Murueta, an arm of the sea 300 m wide and six km long. First of all, let us mention the Nervión - or Bilbao - estuary: filled in since ancient times Basauri and Bilbao, it describes, after this town, wide meanders upstream by the tide and widens enormously in Portugalete to form an immense triangular bay open to the northwest.
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