Trail of Boquerón del Estena

The route follows the remains of a road that was never finished, beside the bed of the Estena river. Right at the start we cross the Chorrito stream by a small bridge, which takes us straight into Cabañeros National Park. When it rains hard it swells together with the Estena, and it is one of the reasons why the park closes the route. At the beginning of 2026 a footbridge was being built over it, paid for by the Cabañeros Tourism Sustainability Plan and the Ciudad Real provincial council; in February it was still not finished³.

We follow the path between rockrose scrub (Cistus ladanifer) and French lavender, with holm oaks and cork oaks, to the confluence of the Chorrito with the Estena, which comes down from the heights of the Rocigalgo massif. They meet in an opening of the valley that gives the route its name: the Boquerón, which means a big mouth or opening.

It is one of the most interesting routes in the park for its geology. We walk along a continuous section of the Lower Palaeozoic formations, from the start of the Cambrian to the Middle Ordovician. First we see the youngest, the Navas de Estena Slates, of the Middle Ordovician, and as we advance we reach the oldest, the Azorejo Sandstones, of the Terreneuvian, the first stage of the Cambrian⁴.

A little further on appear the famous Torres del Estena, quartzite columns born from the friction of a fault zone. The three towers are part of a tectonic breccia: movements of the earth broke the rock from within and then its fragments were cemented together again. Through the faults, which erosion no longer lets us see, hydrothermal fluids flowed and left iron and manganese oxides and hydroxides, the dark patina of the rock, sometimes in rounded shapes.

We continue and, on the right, a short path leads to a lovely viewpoint: the Risco Tirapanes, or «tirapán» (bread-thrower). It is called that because, when the river rose, the women threw bread and other provisions from here to the men working in the hills, charcoal-burning, herding or stripping cork. It was a delivery service with good aim as its only insurance.

Next we pass the Fuente del Fresno, beside a centuries-old ash tree. Very close there are some yews.

At stop 3, at the base of a quartzite layer, you can see some cruzianas, a type of trace fossil. They are the tracks trilobites left when they dragged themselves along. Trilobites (Trilobita, «three lobes» in Latin) were the most characteristic arthropods of the Palaeozoic: they appeared more than 540 million years ago and died out 250, and they lived here when all this was sea.

What we see is not the furrow but its filling: the trilobite stirred the sand on the seabed looking for food and the sediment that fell afterwards filled the track.

No, it is not the title of a film noir. After crossing the Estena by a small wooden bridge we reach stop 4: climate tracks. In the Palaeozoic storms were frequent, and you can tell from the quartzite layers, with dome-shaped structures. They are mounds of sand raised by turbulent currents less than 90 metres deep, where normal waves no longer reach the bottom.

On the ash trees and willows of the banks you can see the marks of the winter floods. At the river's edge there are also species rare at these latitudes, from damper times: yews, hollies and birches.

The yew is the most symbolic tree of the route. There are 45, some large and more than two hundred years old, in the shadiest and dampest areas, spread over two stands 300 metres apart: in the first, 4 large yews; in the second, 15 fine ones and 26 young ones, a sign that the site is regenerating well⁵.

On the slopes there are holm oaks and cork oaks, and the odd prickly juniper (Juniperus oxycedrus), scarce elsewhere in the park, which here finds the stony, steep ground it likes.

Shortly before the end, along the road that never was but left us this route, we reach one of the points of greatest interest: the giant worm, the largest and oldest marine worm on the planet. It lived here, when this was sea, 475 million years ago.

It was discovered in 2009 by a team from the Spanish National Research Council. According to its researchers, it grew to a metre long and about 20 centimetres wide, and lived in large burrows that it dug in the sand of the seabed, near the ancient continent of Gondwana. It cannot be fully confirmed, but it is thought to have come from cold waters, because of the nearness of the South Pole and its size: animals of cold climates tend to grow larger.

Almost at the end, where the private estate of El Maíllo begins, you can see ripple marks, the undulations left by waves or wind on loose sand, as on a beach or in the desert. These have symmetrical crests: they were made by waves in shallow water, near the coast.

There are also tracks of sea anemones, of the group that includes jellyfish and corals. They left circular marks in the strata when they anchored themselves with a central stalk; the mouth, ringed with tentacles with stinging cells (cnidocytes), served them for defence and for hunting.

The largest and oldest marine worm in the world: one metre long and 475 million years old.

We go back the same way. The worm is still in its place: it has not moved for 475 million years.

The Estena rises at the foot of the Rocigalgo, crosses the park and ends at the Cíjara reservoir. It is one of the best-preserved rivers in Mediterranean Spain and is declared a River Nature Reserve. To keep it that way, for about twenty years electrofishing has been carried out: small discharges that stun the fish without harming them, just enough to measure them, return the river's own to the water and remove invaders, such as the largemouth bass (Micropterus salmoides)⁶.

Of its fish, more than half of the species are threatened, and they live here because the water is so clean: the jarabugo (Anaecypris hispanica), which lives only in the Guadiana basin and is endangered, the pardilla, the colmilleja, the calandino, the cacho and the Comizo barbel. On sunny days Mediterranean pond turtles can be seen on the stones, where the otter also leaves its droppings, and it is easy to see some American crayfish and water snakes.

In the sky there are vultures, and on the cliffs and the river, birds such as the Crag Martin, the White-throated Dipper, swallows, wagtails and the Common Kingfisher. More about the birds of the rivers of Cabañeros.

On the scree slopes, crags and pedrizas grows a flora suited to that environment: Phagnalon (Phagnalon saxatile), pinks (Dianthus lusitanus), foxgloves (Digitalis purpurea subsp. toletana), docks (Rumex induratus), toadflaxes (Linaria saxatilis), ferns (Cheilanthes hispanica, Asplenium ceterach, Asplenium trichomanes) and stonecrops (Sedum spp.). They help to build soil and hold the loose stones in place. In the clearings orchids flower (Neotinea maculata, Orchis mascula, Orchis morio, Cephalanthera longifolia), Narcissus triandrus subsp. pallidulus, Crocus carpetanus and Merendera montana.

Very close to the route is the Fuente de las Chinas, a waterlogged area with plants that are rare in Cabañeros: cross-leaved heath (Erica tetralix), round-leaved sundew (Drosera rotundifolia), Lusitanian butterwort (Pinguicula lusitanica), needle greenweed (Genista anglica) and several sedges.