Water against limestone: how the Serranía was formed

A wetland from 125 million years ago

The Serranía’s rock formed in water. About 125 million years ago, in the Early Cretaceous, where Las Hoyas stands today, in the municipality of La Cierva, there was a subtropical wetland: lakes and pools of lime-rich water, with a rainy season and a dry one. The bottom was so still that it preserved its dead with unusual delicacy: a fern, insects, shrimps, a frog, the bird Eoalulavis, the dinosaur Pelecanimimus, the mammal Spinolestes and shoals of fish fry that died suddenly. A team from the Universidad Autónoma de Madrid has been studying it for more than thirty years, and since 2016 it has been a Bien de Interés Cultural (Spain’s highest heritage listing).

The rivers of the last dinosaurs

At the end of the Cretaceous, between what is now Arcas and Fuentes, rivers flowed through waterlogged areas that left coloured marls and mudstones. In May 2007, when a small hill was lowered for work on the high-speed railway to Levante, Lo Hueco emerged: titanosaurs, theropods and ornithopods, crocodiles, turtles, fish and ferns, more than 8,500 large fossils. It was fully excavated and moved. Fossils from both sites are in Cuenca’s Museo de Paleontología.

Fractured limestone

The seas and lakes of the Mesozoic left layers of limestone and dolostone. Earth movements then folded and fractured them, and rainwater, which dissolves limestone, entered the cracks. Since then the Serranía has been a karst: a landscape made more of what is missing than of what remains.

In the Ciudad Encantada, the “Enchanted City” rock formations, water has dissolved the stone unevenly and left shaped blocks between 1,340 and 1,410 metres; the IGME delineates 95 hectares within a karst landscape of about 20 km². It has been a Natural Site of National Interest since 1929. A few kilometres away, in the Torcas de los Palancares, the same process can be seen downwards: the ground collapsed over dissolved rock and opened round hollows with vertical walls. The Torca Larga is about 450 metres in diameter; the Torca Honda goes down about 50. Almost all have names: Del Lobo, De la Novia, Del Tío Joaquín, Las Mellizas.

Lakes at the bottom of the torcas

When a torca, a collapse sinkhole, reaches groundwater, it fills. That is how the lakes of Cañada del Hoyo formed. One of them, La Parra, keeps a climate diary in the mud on its bottom. A team of geologists extracted it in cores and dated it by radiocarbon: before Roman times the doline was dry, the lake formed around the 4th and 5th centuries, was shallow and turbid between 980 and 1350 and deeper between 1350 and 1950, during the Little Ice Age.

Rock that grows

The water that dissolves also builds. At the source of the Cuervo, beside Vega del Codorno, the river emerges from the limestone at about 1,490 metres, loaded with lime, and deposits tufa as it releases it. With this it has built a barrier 300 metres long and up to 25 high that dams the water and makes the waterfalls cascade.

The Cuervo is not an isolated case. Between July and September 2021, a team travelled around the province looking for tufa systems and counted 119 sites; 56.3% had not appeared in any previous inventory. The authors estimated that 25.2% were in optimal condition, 31.1% in suboptimal condition and 1.7% in poor condition; the remaining 42% could not be assessed. The highest densities by area were in Fuertescusa and the Escabas basin. The figures cover the whole province, not just the Natural Park. It was a preliminary assessment, made by eye and by category. To check the mosses, 302 photographs were sent to specialists at the University of Valencia, and the initial identification, made by non-specialists, had been wrong in 18.1% of cases. The authors proposed reviewing a sample of the systems at least every six years.

More about this era: Geology and dinosaurs.