Chelva has had 200 million years of being on top of a sea that never really managed to leave

The Muslim castle of Chelva was built on rock. It was not a matter of chance or a defensive whim: it was a geological choice. The limestone that crops out on the central hill of the municipality holds up well under load, does not dissolve with the first heavy downpour, and provides the strength that any builder —Roman, Arab or medieval— needs to raise walls that will last. That same rock, formed at the bottom of a tropical sea approximately 180 million years ago, is today the dominant material in the subsoil of Chelva.

But beneath the limestones lies the most disturbing chapter of all: layers of gypsum and anhydrite that reveal that, before becoming a seabed, this territory was a closed lagoon that slowly evaporated under an arid, stifling climate. And even further down, in the Barranco de Alcotas, Paleozoic slates and quartzites crop out: fragments of a mountain range that disappeared more than 300 million years ago. Chelva is like that: what you walk on has more layers than the history you can read in its neighbourhoods.

A sea like the Caribbean, but where there are now pines

Today, the municipal term of Chelva is in large part covered by Aleppo pine (Pinus halepensis) woods. About 180 million years ago —in the Middle Jurassic, the period when dinosaurs dominated the continents— where there are now pines there was a warm, shallow sea, similar to the present-day Caribbean. The waters were transparent; the water temperature was around 25 °C; and on the seabed accumulated the remains of corals, molluscs and calcareous organisms which, over time and under pressure, would become the limestones and dolomites that make up today the most abundant material in Chelva’s subsoil.

This marine origin is also the reason for the municipality’s main geotechnical risk: karst. When rainwater filters through the limestone rock, it dissolves slowly over thousands of years and opens underground cavities. Chelva sits on a karst system that requires detailed verification of the subsoil before any construction. This is not a new risk: the Neanderthals themselves who occupied the Abrigo de la Quebrada, in the rambla de Ahíllas, lived within a shelter carved out of that same calcareous rock between 43,000 and more than 50,000 years ago, according to datings published by researchers from the Universitat de València and the Universitat de Barcelona.

The Abrigo de la Quebrada —a Middle Palaeolithic site studied since 2007 under the direction of Valentín Villaverde, João Zilhão and Aleix Eixea— has yielded more than 100,000 bony remains from a single archaeological level. Urochs, horses, deer and Mediterranean tortoises lived alongside groups of Neanderthals who hunted on the open plains in front of the ravine and returned to the limestone rock to process their prey. Limestone sheltered them. The same limestone that Chelva’s subsoil offers today as its most resistant material.

Before the sea, the lagoon that dried up

Below the Jurassic limestones, the stratigraphic profile of Chelva reveals a layer of gypsums, anhydrite and salts at depths of between 10 and 22 metres. These materials —called in geology the “Keuper facies”— were formed in the Upper Triassic, approximately 220 million years ago, when this area was not a sea but a closed basin with no outlet to the ocean. The climate was arid and extreme. The water that arrived evaporated before it could find an exit and, in doing so, it deposited the mineral components it carried in solution: first gypsum, then anhydrite, and finally the salts.

The result is a rock that behaves radically differently from the limestone above it. Gypsum dissolves more easily and at much greater speed. Once the groundwater level is reached or when there are water infiltrations, it can cause subsidence: slow, progressive ground collapses. In the design of foundations on these layers, the loss of support due to dissolution is the factor that conditions structural decisions. Chelva’s gypsums are also the reason why, in low-lying areas or close to watercourses, the groundwater level can be a factor to control.

The combination of karstic limestone at the surface and soluble gypsums at depth makes Chelva’s subsoil one of the most complex in the comarca of Los Serranos for construction. Geology suggests that this is also the reason why historical settlement always privileged zones of sound bedrock outcrop: the Arab neighbourhood of Benacacira, the fortress, the convents. Its builders did not have geotechnical studies, but they knew the ground.

The slates of the Barranco de Alcotas: the oldest chapter

In the Barranco de Alcotas, to the north of the municipal area, something unusual crops out: Paleozoic rocks, specifically from the Lower Ordovician —more than 470 million years ago— which, according to the IGME (Sheet 666 of the MAGNA, Chelva), are included in what geologists call the “Remedio tectonic window”, within the Chelva Anticlinorium of the south-eastern Iberian Chain. They are black and greenish slates, phyllites and whitish quartzites, deformed and metamorphosed during the Hercynian Orogeny about 300 million years ago. They are fragments of a mountain chain that no longer exists, levelled by erosion and buried beneath centuries of sediment.

Above these ancient slates, in discordant contact, appear the sandstones, conglomerates and detrital materials that are found between 22 and 37 metres depth in the municipality’s stratigraphic profile. These rocks formed in very energetic, torrential rivers —those that drained the Hercynian mountain ranges during their destruction— and today they provide an acceptable bearing capacity for direct foundation, although it is more irregular than sound limestone rock.

A quiet land, but with memory

The seismicity of Chelva is low. Between 1996 and 2026, the EMSC/IGN has recorded 15 seismic events, with a maximum magnitude of 2.3 (recorded in 2022) and a mean magnitude of 1.75. The last recorded earthquake was in 2023, with a magnitude of 1.5. For practical purposes, an earthquake of this kind is imperceptible to most residents and very rarely affects the structural design of buildings.

This seismic calm contrasts with the intense tectonic activity that the area experienced in the geological past. The Chelva Anticlinorium —that major fold structure which geologists identify in the south-eastern Iberian Chain— is the result of compressive stresses that wrinkled the rocks over millions of years, uplifting the range that dominates the landscape today. Those movements stopped long ago. What remains is not tectonic activity, but its morphological legacy: the limestone hills (muelae), the Turia’s incised gorge, the canyons that make Chelva one of the province of Valencia’s most rugged landscapes.

An earthquake of a magnitude similar to the maximum recorded (≥ M 1.8) occurs approximately every four years, always below the threshold of human perception under normal conditions.

What Chelva’s subsoil holds, layer by layer

The Arab neighbourhood of Benacacira, the irrigation channels that have continued to function for centuries, the Abrigo de la Quebrada where the Neanderthals left thousands of bones and flint tools: all of that is built, literally, on the same Jurassic sea that generated the limestones of the Chelva Anticlinorium. The municipality has been documented as inhabited at least since the Middle Palaeolithic, and throughout that time the rock has been the foundation. Not as a metaphor. As geology.

Sources consulted

· Geotechnical data: https://www.estudiogeotecnico.pro/es/estudio-geotecnico-en-chelva

· Wikipedia in Spanish: article “Chelva” and article “Abrigo de la Quebrada”

· Wikipedia in Spanish: article “Los Serranos” (comarca geology)

· IGME MAGNA, Sheet 666 (Chelva): official geological mapping. Referenced in Mi Geoblog (speedstar71.blogspot.com, 2013) and CSIC/Universitat de València publications

· Eixea, A., Villaverde, V., Zilhão, J. et al. (2011). “Aproximación al aprovisionamiento de materias primas líticas en el yacimiento del Paleolítico medio del Abrigo de la Quebrada (Chelva, Valencia)”. Trabajos de Prehistoria, 68(1), 65–78.

· Seismicity: EMSC/IGN Portal, via data incorporated in the website of estudiogeotecnico.pro.

· Gómez, J.J. (1978). “El Jurásico en la región de Chelva-Domeño (Valencia)”. ResearchGate.