Valley of the Jerte River: Spanish cherries and the Jurassic Period.

28/03/2020

In the Gredos Mountains, there is a river valley that looks unnaturally straight on the map. That is no coincidence. The Jerte River flows along Spain's largest fault line, which was formed in a time when dinosaurs still roamed the planet.

© 2016 Kathelijne Bonne.

In Spain, it is a well-known fact that the best cherries of the peninsula, the cereza picota, come from Extremadura and, more specifically, from the Valle del Jerte in the province of Cáceres. Twice a year, this unique place on the southern slopes of the Gredos Mountains is flooded by visitors. In the spring the 'cerezos' (cherry trees) are in bloom, which results in beautiful fairytale-like panoramas. In the summer the trees are heavily laden with different varieties of picota cherries.

Hordes of Madrilenes then book an all-in formula: one day by bus to and fro to pick as many cherries as one can carry and to attend the 'feria de la cereza picota and cereceras', all kinds of activities and festivities related to the cherry-picking. 

But the Valley of the Jerte is more than just a hotspot for fruit.

Jerte Valley cherry trees in bloom (Gonzalo Riestra / CC BY 2.0).
Jerte Valley cherry trees in bloom (Gonzalo Riestra / CC BY 2.0).

Supercontinent

The Jerte River flows in a straight line to the city of Plasencia in the southwest, to eventually join the Tagus that flows towards Lisbon, as a part of the largest river basin of the peninsula. 

More interestingly, the Jerte runs along one of Spain's, and even Europe's largest geological fault lines, running all the way southeast to Alentejo in Portugal. 

This Alentejo-Plasencia fault formed 200 million years ago at the start of the Jurassic Period. Before, Spain, or more precisely, Iberia, was part of the supercontinent Gondwana. In this continent, Africa and South America lay next to each other, but it also comprised parts of Europe, including Iberia. In Jurassic times, a large rift zone developed between what is now South America, Northwest Africa and Iberia. In a rift zone, tectonic forces act in different directions, dragging along parts of continents. At the same time, a new ocean, in this case the Atlantic, widened between them. As such the separated continental 'blocks' became the continents that we now recognize on the world map. 

This whole continental break-up process was accompanied by enormous tensions on the Earth's crust. At some places, the crust ruptured and at such locations, fault lines formed. The Alentejo-Plasencia fault is one of the largest faults that formed during this process. 

Many of these faults filled with magma. A magmatic rock called dolerite can be found everywhere along the riverbed of the Jerte, as a silent witness to the prehistoric violence. According to geologists, the fault line is still active and could sooner or later undergo an earthquake. Or in other words, tensions in the crust can be released along this fault, as any fault always remains a weak spot. Furthermore, rivers are more likely to carve their path along a zone of weakness and fractures, which is why many rivers run along faults, like the Jerte. 

River beach near Cabezuela del Valle.
River beach near Cabezuela del Valle.

Microclimate

The Valle del Jerte is best visited in June when it's cherry season. There is a microclimate so it is warm during the day and pleasantly cool and fresh in the late hours. From Madrid, the Valle del Jerte is reached by passing past Ávila, and over the Tornavacas mountain pass, after which the track winds down along several hairpin turns. This is the border between Castilla-Leon and Extremadura.

Thanks to its mild climate and unique landscape, the valley lends itself to all kinds of natural activities. Walking along small streams and waterfalls, picnicking on beaches and swimming in the Jerte, mountain biking and wildlife watching are some of them. You can stumble upon many species such as birds of prey, otters, wild cats and even the genet. 

At Cabezuela de Valle, which hosts an information center, some larger beaches are nested in a wide bend of the Jerte. In the gorges of the "Garganta del Infierno", a tributary of the Jerte, natural plunge pools are found, carved out by loose boulders in fast-flowing streams since the last Ice Age.

To recover from active days, you can have dinner in one of the several villages along the river or on the mountain flanks. During the días gastronómicas de la cereza the most incredible combinations of cherries, used in typical Spanish dishes, are prepared. There are of course many shops where you can buy all kinds of cherry related products, e.g. jams, liqueurs, biscuits, gazpacho, tea, and of course, large boxes full of freshly picked cherries.

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Genet (Foto-Ardeidas / CC BY-SA).
Genet (Foto-Ardeidas / CC BY-SA).

Sources: 

Melendez Hevia, Ignacio, 2004, Geología de España: Una historia de seiscientos millones de años. 288 p.  

P. Villamor, R. Capote et al., 2012, Contribution of active faults in the intraplate area of Iberia to seismic hazard: The Alentejo-Plasencia Fault. Journal of Iberian Geology 38 (1) 2012: 85-111.

Marzoli A. et al., 2018, The Central Atlantic Magmatic Province (CAMP): A Review. In: Tanner L. (eds) The Late Triassic World. Topics in Geobiology, vol 46. Springer, Cham. https://doi.org/10.1007/978-3-319-68009-5_4.

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The Valle del Jerte is located between Tornavacas and Plasencia in the province of Cáceres of the region Extremadura.