Itinerary around Rebolledo de la Torre

The forces of the Earth

We must travel in time in order to explain the origins of the rocks of the Reserve and its different geological structures.

The journey begins in the Secondary or Mesozoic Era, 250 million years ago (Ma). The Earth was very different from now by that time. All the continental masses were united forming a sole supercontinent called Pangea, surrounded by an ocean plate on which Panthalasa, the great ocean, was located.

Two hundred million years ago this big continent started fracturing and dividing into several continental masses, which little by little, started moving away from each other. The space generated by the separation of those continents was occupied by oceans. That is how the openings of the Tethys Sea and the Atlantic Ocean started to take form during the Jurassic. Firstly, until around 120 Ma, South America, Africa, Antarctica, India and Australia started detaching, forming a big mass called Gondwana, and North America, Europa and Asia formed Laurasia.

Subsequently, India and Australia migrated northwards, moving away from Antarctica. The migration of India is spectacular because 55 Ma it travelled around 310 miles, collided with Asia and started forming the Himalaya Mountain Range.

The Iberian Peninsula (Iberia) has had a very important paleogeographic evolution in the last 250 Myr. When Pangea started fracturing, Iberia was united to Laurasia.

One of the continental fracture zones separating Gondwana from Laurasia was located exactly to the south of Iberia and would give place to the opening of the central part of the Atlantic Ocean.

During the Upper Jurassic (145 Myr) Iberia started moving away from Euroasia through a continental fault that caused the opening of Vizcaya Gulf. Thirty five million years after (Lower Cretaceous) Iberia became the Iberian Microplate, separating itself definitely from the large plates of Africa and Eurasia.  

The Vizcaya Gulf reached its maximum opening 85 Ma (Santonian(Upper Cretaceous))  and the sea invaded several parts of Iberia, which had previously emerged, and marine sediments were deposited on continental material (Upper Cretaceous limestone  on Cenomanian Continental sandstone on facies Utrillas/ Middle Cretaceous facies).This material was folded and fractured during the several periods of the Alpine Orogeny deformation. 

The opening of the North Atlantic was a very important event at the end of the Cretaceous period, since it predetermined the relative motion of the Eurasian and African plates and several small plates located between them, among which Iberia was to be found. In fact, the North Atlantic opened and expanded at higher velocity than the Central Atlantic. Thus, Eurasia moved away from America quicker than from Africa, causing a change on the relative motion of both plates and therefore, Africa and Eurasia did not moved away from each other, but rather started converging, which is still occurring. 

The change in the relative motion between the two large Eurasian and African plates meant they got closer and collided, starting the compressive stress which affected wide areas of southern Europe, northern Africa and Iberia. It was the beginning of the Alpine Orogeny.

The Tethys closed progressively and turned itself into small secondary seas. The northern margin of the Iberian plate collided with Europe, causing the Pyrenees formation and the union of both plates. From the Oligocene (30 Myr) onward, Iberia was again part of the Eurasian plate.

This collision caused a huge deformation both in the material deposited during the Mesozoic and in the areas which were emerging during that period, therefore large blocks lifted through the faults, giving place to the reliefs of Toledo Hills, the Central System and the Cantabrian Mountain Range. 

This means that the structures on the Geological Reserve rocks (Thrust faults, folds and others) are directly related to these periods of the Alpine Orogeny deformation.

1. Sedimentary Rocks

Rocks originated by the accumulation of material (fragments of other rocks, substances dissolved in the water, hard parts of organisms, and others) in a sedimentary basin (sea, river and others). They often appear on layers or strata and usually contain fossils.

Sedimentary rocks with different characteristics and from different ages are to be found throughout the itinerary: Sandstone and limestone, marl, lutite with carbonaceous sediments and biocalcerenite. The oldest are 200 Myr and belong to the Jurassic. The youngest rocks are about 85 Myr and belong to the Upper Cretaceous.

What the rocks tell us

As we have seen, the study of the rocks provides us much information about how our planet was millions of years ago.

A large part of this spectacular route takes place throughout a limestone layer or stratum. There are several tracks describing the place where the calcareous sludge was deposited, the organisms that used to live here and how the sedimentation of this limestone stratum was produced within about two million years.

2. Lignite

Amongst the different Cretaceous layers or strata which can be found within the itinerary, there is a peculiar one presenting carbonaceous sediments.

Although the coal quality was very bad and despite its high content of ash, small family-run farms used it by the middle of the last century. Tips and, in some cases, small pitheads can be seen along the layer. 

3. Thrust fault

Besides folds, the presence of great faults is another type of deformation on the rocks caused by internal forces. These faults are caused by the relative motion of the fractured parts. 

One of these important geological accidents affecting large rocky masses aged between 200 Myr (Jurassic rocks) and 90 Myr (Cretaceous rocks) can be observed within the route.

4. Peña Mesa Crag

Mesozoic moorlands or Loras are actually large folds on the rocks caused by forces taking place inside the Earth.

Peña Mesa Crag is one of the Reserve most interesting and outstanding places where to observe this type of folding, which started appearing around 60 Ma, reaching its apogee 30 Ma. 

RECOMMENDATIONS: Although this interesting route is very short, there are some points where you should be extremely cautious because of steep slope and loose rock. Walk through the signalled trail and do not get close to the cliffs. Avoid this route on rainy, foggy, snowy or icy days. Wear suitable footwear. Smaller groups are easier to control and produce less impact on the area. Above all, respect nature, you are its guest!