Fernando Escartín commentary
Mid-mountain. The San Martín mountain pass will await the peloton 16.6 kilometres from the finish-line. A short ascent with very tough slopes that will result in important differences to the general classification. If the leaders lose their concentration, even for a second, we could see some surprises.
Geological asturias: 500 million years of history
Campo de Caso - Gijón
This stage crosses the entire Principality of Asturias from South to North, cutting across rocks of varying ages and origins, which allows us to briefly review the history of the peninsula. Between the departure and Nava, the route goes through slates, sandstones, conglomerate rocks and limestone rocks from the Carboniferous Period (359 - 300 million years ago, dark grey colours). After the town of Nava, and up until we reach Pola de Siero, we cross a significant tectonic accident that fragments the terrain and puts extremely diverse rocks in contact with each other.
Geologically, we start the route in what is known as the Región de Mantos, where we can see a series of ancient rocks that are arranged on top of much more modern ones, forming what we, in geology, call an overthrust formation. In this case, we have rocks that are 520 million years old (Cambrian Period) on top of rocks that are 315 million years old (Carboniferous Period).
Carboniferous, unsurprisingly, gets its name from carbon (or coal), as it was a time when forests became enormously developed and these large accumulations of plants resulted in layers of coal that have historically been exploited in the region. As an example, we have the Coto Bello Mines, in Aller, currently the object of landscape restoration.
Further along, after overcoming one of the stage's various ramps, we arrive at Nava, leaving behind the Paleozoic rocks and moving on to more modern ones, from 100-120 million years ago (Cretaceous). From Nava, the route heads West, leaving the Meso-Tertiary depression, a large valley where even more modern sediments flourish (Eocene, 56 million years ago) to our right. These sediments provide a better ground for human settlements, which explains why Oviedo, Pola de Siero and Nava are found along this depression.
From here onwards, the stage enters the slopes of Jurassic and Triassic materials (200-150 million years ago) in the last section of climbs and descents that will end in Gijón. Incidentally, Peñarrubia Beach in really a Jurassic abrasion platform (200 - 145 million years ago) where rocks form the beach surface.
Stage Term: Abrasion Platform
Abrasion is the term that describes the erosion produced by water or wind as they hit other materials with the sediments they carry. In coastal cliff areas, the waves cause abrasion, pushing the coastline inwards little by little, forming a flat surface just beneath the level of the waves.
Jersey wearers after the stage 4
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