As the sun sets over Plentzia and Gorliz Bay(20 minutes from the city of Bilbao), the coast seems to transform. The tides reveal a rocky landscape filled with sloping lines, like the open pages of an ancient book of stone. They are the flysch strata, a geological formation that preserves millions of years of history beneath the feet of anyone exploring this coast of Uribe Kosta, in the heart of Bizkaia.
And beneath those same waters, in this privileged setting, Crusoe Treasure it preserves another kind of legacy: underwater wines that mature in silence inside an artificial reef winery, where the sea, science and nature come together.
In this article, we invite you to discover what flysch is, how it formed, what makes the flysch of Plentzia-Gorliz Bay unique, and how you can experience it through our maritime and wine-tourism experiences.
What is flysch?
The word flysch comes from German and means something like “to flow” or “to slide”. It describes a type of sedimentary formation made up of alternating layers of hard and soft materials —sandstones, limestones, marls and shales—deposited on the seabed over millions of years.
On the Basque coast, these layers were uplifted, tilted and eroded until they were exposed to the air and sea, forming cliffs of parallel lines so straight and sharply defined that they appear knife-cut.
The result is a fascinating landscape, a “geological mille-feuille” where each layer tells an episode from Earth’s past: ancient ocean floors, periods of calm and storm, tectonic movements, continental collisions… it is all written there, in the same setting where the waves of the Cantábrico break today.
How the flysch of the Uribe Kosta coast was formed
More than 100 million years ago, the area now occupied by Barrika, Plentzia and Gorliz was covered by a deep sea. Sediments gradually settled on its floor: sand, silt, shells and clay carried by underwater currents.
Over millions of years, these particles were compressed and transformed into rock, forming successive layers that piled up one on top of another.
Later, the movement of the tectonic plates—especially the collision between the Iberian and Eurasian plates— folded and uplifted the land. The sedimentary layers that had once been horizontal tilted and emerged, forming the spectacular sloping cliffs we see today along the Bizkaia coast.
Over time, the force of the sea shaped these structures: the softer layers eroded first, while the harder ones endured. This created the stepped forms, fissures and reliefs that so strongly characterise flysch.
Every visible line on the cliff represents thousands of years of history. Observing it is, quite literally, looking into the planet’s geological past.
The flysch of Plentzia-Gorliz: a discreet and accessible treasure
Although the Basque Country’s most famous flysch formations are found along the coast of Zumaia and Mutriku, the one at Plentzia-Gorliz has a very special charm: it is more intimate, more accessible and closer to visitors.
In this bay, the flysch appears in elegant fragments, visible from the coastal promenade or directly from the beach, especially around Astondo and the cliffs leading towards Barrika.
The layers slope gently towards the sea, and their colour ranges from slate grey to ochre, depending on the light and the tide. When the tide goes out, small rocky platforms are revealed, allowing visitors to literally walk across millions of years of history.
In Uribe Kosta, flysch is not merely a geological phenomenon: it is an defining feature of the landscape. In few places in the world can one contemplate, so close to an urban beach and a harbour, such a revealing natural cross-section of the prehistoric seabed.
How to experience it: routes and viewpoints
For those who want to explore the bay’s flysch, there are several ways to do so:
-
On foot, following the coastal promenade linking Plentzia and Gorliz. From the harbour to Gorliz lighthouse, the route offers natural viewpoints from which the stratification lines can be clearly distinguished.
-
From the sea, by boarding one of our boat experiences from Crusoe Treasure. Sailing in front of the cliffs makes it possible to observe the inclination of the layers and how the sea has sculpted the rocks over millions of years.
-
By kayak or paddleboard, for the more adventurous, who can approach the lower formations and see up close the folds, fissures and fossils scattered through the rock.
The best time to see it is at low tide, when the rocks are exposed and the evening light brings out their colours. It is a natural spectacle that changes every day, like a canvas constantly retouched by the sea.
Where sea and rock meet: the Crusoe Treasure reef winery
Right in the heart of this same bay, where the flysch emerges, Crusoe Treasure it has created a pioneering project: an underwater winery that also functions as an artificial reef.
There, several metres below the surface, our wines mature surrounded by marine life inside infrastructure built more than 15 years ago from concrete and steel, designed to become a natural habitat for the species and our wines. The sea’s currents, pressure and constant temperature enhance and distinguish the wine’s evolution, giving it unique nuances that cannot be achieved on land.
Over time, the concrete structures that make up the reef and winery housing the bottles have become a refuge for hundreds of species: more than five hundred forms of marine life have been recorded on our reef, from sponges and anemones to small fish and crustaceans, as well as numerous types of algae.
The bay floor is now a natural laboratory of biodiversity, and the sea that shapes the flysch is the same sea that gives our wines their shape and character.
This connection between geology, sea and wine defines the philosophy of Crusoe Treasure: respect the environment, learn from it and share it through authentic experiences.
Our experiences: flysch from the sea and wine from the heart
One of the most special ways to discover the flysch of Plentzia-Gorliz is through our guided tours.
During the boat trip across the bay, our guides explain how the rock layers formed, how the underwater relief influences the currents, and how this entire setting relates to the ecosystem that protects our winery.
From the deck, visitors can admire the cliffs, the flysch and the horizon while tasting some of our underwater wines. It is a complete sensory experience: history is seen in the rocks, the sound of the sea is heard, and the wine that matures beneath those same waters is savoured.
We also offer onshore tastingoptions, combined with an introduction to local geology, cultural visits around Uribe Kosta and activities in collaboration with sustainable tourism companies in the area.
Each experience is designed to connect science, nature and emotion, so that every visitor feels part of the landscape.
A shared story between stone and wine
The flysch and our reef winery share something essential: both are the result of the passage of time and the action of the sea.
While the rock layers slowly formed beneath prehistoric oceans, our bottles now rest beneath the waves, maturing at their own pace in an equally unique setting.
Both hold stories that only the sea knows how to tell.
Walking alongside the flysch is a way of looking back through time. Tasting an underwater wine is, in a sense, tasting the present of the very sea that gave rise to those rocks. At Crusoe Treasure, we believe that this connection—between geology, sea, life and wine—is what makes this corner of the Basque coast truly exceptional.
Plan your visit
📍 Location: Plentzia-Gorliz Bay, Uribe Kosta, Bizkaia, Spain. You can travel from Bilbao by Metro and get off at the final stop, Plentzia; from there, it is a beautiful walk.
🚤 Experiences: Boat tours, onshore tastings and personalised experiences.
🌊 What to see: Flysch cliffs, Gorliz beach, Plentzia harbour, the lighthouse viewpoint and the Crusoe Treasure winery-reef.
🍷 What to experience: The meeting of ancient geology and underwater winemaking innovation.
We invite you to discover it for yourself: come and experience the flysch, the sea and the wines that the Cantábrico keeps secret.
Because in Plentzia and Gorliz Bay, the history of the Earth and a passion for wine merge into a single wave.
👉 Book your experience at www.crusoetreasure.com