Rhythmically layered rock recording deepwater turbidites. Magnetic polarity stratigraphy dates layers to 90 million years ago when Africa collided with Europe, creating the mountains that shed these sediments.
Torpe, CC BY 2.0, via Wikimedia Commons
A Record of Tectonic Collision
The cliffs at Islares are a physical record of immense geological forces, written in stone around 90 million years ago during the Late Cretaceous period. At that time, this part of Spain was a deep marine environment known as the Basque-Cantabrian Basin. The geological story began as the Iberian microplate, pushed by the larger African plate, started to collide with the Eurasian plate. This slow-motion collision initiated the Alpine Orogeny, the mountain-building event that would eventually form the Pyrenees and the Alps.
As mountains rose, erosion immediately began to wear them down. Rivers carried vast quantities of sand, silt, and mud to the edge of the continental shelf. This sediment accumulated until it became unstable, often dislodged by earthquakes related to the ongoing tectonic collision. The result was a series of massive underwater avalanches, known as turbidity currents, that cascaded down the continental slope into the deep basin. Each avalanche deposited a single layer of sediment. Over millions of years, thousands of these events stacked one on top of another, creating a sequence of rocks more than 3,000 meters thick in some parts of the basin. The subsequent mountain-building squeezed and tilted these once-horizontal layers, lifting them from the seabed to form the vertical cliffs seen today.
Reading the Rhythms of Flysch
The distinctive, striped appearance of the cliffs is characteristic of a geological deposit called flysch. The term, from a Swiss-German word for "to flow," describes these repeating sequences of sedimentary rock formed during active mountain building. Each stripe represents a two-part cycle. The thick, hard layers are sandstone, formed from the coarse material that settled first from a fast-moving turbidity current. The darker, softer layers are shales and marls, made of fine mud that slowly settled from the water in the long, quiet periods between avalanche events.
This rhythmic alternation of sandstone and shale records ancient submarine landslides. Geologists can study these layers, called turbidites, in detail. A complete turbidite often shows a graded sequence known as a Bouma sequence, with the coarsest grains at the bottom and the finest at the top, reflecting the waning energy of the turbidity current as it came to rest. The Islares cliffs expose this pattern, allowing shows geological processes that occurred in a deep ocean environment tens of millions of years ago. The cliffs also contain trace fossils, or ichnofossils, which are not the remains of organisms themselves, but the preserved tracks and burrows of deep-sea creatures that lived on and within the seafloor sediment between depositional events.
💡Fun Facts
The term "flysch" was first used by Swiss geologist Bernhard Studer in 1827 to describe similar-looking rocks in the Alps.
Each pair of sandstone and shale layers represents a single major event—like an ancient earthquake—and the centuries of calm that followed.
The cliffs are part of a larger flysch formation that extends along the Basque coast, including the famous strata at Zumaia.
Scientists use magnetic minerals within the rock layers as a geological clock. As the sediments settled, minerals like magnetite aligned with Earth's magnetic field, locking in a record of its polarity at the time of deposition.
Accessible 24/7. Viewing is best during daylight and at low tide to safely explore the wave-cut platform at the base of the cliffs.
Admission
Free.
Accessibility
Viewing the cliffs involves walking on natural coastal paths that can be uneven. Accessing the beach area requires care, as paths can be steep and rocks slippery, especially at high tide.
