This Geosite has primary geomorphological significance and features a coastline with vertical-strata morphostructures parallel to the shore of a synclinal flank, representing the internationally significant geological context of the Coasts of the Iberian Peninsula.

From east to west, the first area is the Covachos complex, dominated by El Castro Island, which results from coastal erosion of the surrounding area by wave action. In addition to the characteristic leafy appearance of the cliffs, composed of marl-limestone rhythmites with subvertical dip and large blocks formed from the tops of the strata on the southern cliffs, one can observe the effects of coastal cliff retreat in lithologies with dips favorable for the sliding of large plates along bedding planes. The island’s summit consists of residual relief, formed from the dismantling of the gentle hill landscape prior to marine action, where soils and vegetation have been preserved. The most notable features, however, are the depositional forms on this platform, particularly the tombolo connected to the island.

Next is the La Arnía geomorphological complex, a particularly rich aggregation of geomorphological features, as well as complementary stratigraphic and paleontological elements. Broadly, it consists of a cove with a small beach at its base and an elliptical intertidal abrasion platform, separated by residual relief from the fluvial valley that ran parallel to the coast prior to dismantling. Resistant lithologies formed by Cenomanian limestones close the complex to the north, forming a row of spires and rocky outcrops of various sizes. The base of this beach is occupied by the El Castruco promontory, a residual landform protected from erosion by the Cenomanian cliffs, preserving the characteristic profile of the former fluvial valley. Additionally, the cliffs exhibit excellent examples of differential erosion on marl-limestone sequences.

Immediately west of the La Arnía platform is the Pedrondo hollow, a relatively large collapse depression measuring 985 m² in area, 42 m in depth, with a voided volume of approximately 18,000 m³. Its formation is due to the opening of a pathway through the Altamira Formation calcarenites at the cliff base by wave action. This opening consists of a series of blocks defined by the orthogonal joint network, typical of these Cenomanian materials. Wave action effectively penetrates and erodes the Turonian marly materials previously protected by the resistant limestone. About 50 meters west of the hollow is the La Baselga complex, which follows the same erosive pattern as the La Arnía platform.

El Portío cove lies immediately to the west. This wide, elliptical cove, measuring about 40,000 m², formed through the advance of marine erosion across a 250 m-long breach in the Cenomanian limestone crest. The dismantling of the pre-existing relief is more advanced here than in the rest of the coastal stretch. Sandy beaches form on the eastern and western margins over the abrasion platform. Behind the eastern beach of El Portío is one of the most striking outcrops in the area, due to the subvertical dip of the Turonian marly series combined with differential erosion, controlled by alternating layers of varying wave resistance and networks of joints with different patterns and dimensions in each layer.

Immediately following the cove is the Somocuevas Peninsula, composed of Aptian limestones, the oldest rocks in the Geosite. It forms a set with the line of islets of the same nature called the Urros de Liencres, which extends eastward. The western end is occupied by the depositional complex of the Liencres dunes, formed by the Pas River’s entry into the Cantabrian Sea. The extensive beach (excluding its extension at Canallave) measures 1,500 m in total, half of which forms a NE-SW oriented coastal spit. The beach front slope varies longitudinally: the northeastern end has low slopes and is clearly dissipative, while the spit end presents a markedly reflective character. From the beach, in the direction of prevailing NW winds, a large climbing dune ascends the slopes of nearby Monte Tolío. The system includes a primary dune of variable height between 10 and 15 m, with secondary and tertiary dunes progressively increasing in height towards the SE, alternating with interdunal depressions that, in some areas, form wetlands due to the proximity of the water table.

ACCESS TO THE GEOSITE

The main access points to the Geosite are: on the western end, via the A67 motorway towards Santander, taking the Boo de Piélagos exit, then following the CC231 towards Liencres, entering through the Las Dunas de Liencres Natural Park and Costa Quebrada. On the eastern end, access is via the S20 from Santander towards Torrelavega, exiting at Corbán and following the CC231 towards Soto de La Marina, where signs guide visitors to the Covachos and La Arnía beaches. Internal accessibility within the Geosite is good, with most points of interest reachable by paths and roads, and the entire coastal section walkable on foot.