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Urkiola nature parks history



Mountains and valleys of the Urkiola Park. (Increase the size in a new window)

It is known by means of dating systems, that the sedimentary materials present in the Urkiola´s Nature Park are between 110 and 140 million years old, that is, the oldest rocks found here have 140 million years and the youngest 110 million years. All of them therefore were formed in a time span of around 30 million years. Does this mean that the hills and valleys of Urkiola´s Park have been here for such a long time? The answer is negative.

Limestone rock and trees in the Urkiola Park. (Increase the size in a new window)

Geology handles the various events undergone through time, in a determined space, that have culminated in a series of facts that can be observed at present, or in other words the reconstruction, step by step, of the different phases in the creation of the actual geological scenery.

And then? When does the actual Urkiola relief appear? Very recently, as can be seen by the following description

Studying the older rocks found in the Park´s area -(C-1), on the stratigraphical map we can see that they belong to a geological layer known as Neocomian, base or beginning of the Cretaceous era, belonging to the Secondary or Mesozoic era (the era of the Dinosaurs), its age of 140 millions years remains cemented in the fossils existing in the sediment. Likewise these fossils show evidence of marine origins, as marine organisms are frequently encountered such as sea urchins, gastropods and bivalves.

Above these sandy and clayish materials other sediments show up, much more powerful and bulky and of a calcareous texture: these are the Urgonian limestone also known as reef limestone, having a clear reference to its origins, coral reefs. 120 million years ago the Cantabric Sea did not exist as such or as we know it today. Which was represented, in that distant period, by a narrow and shallow marine enclave.

Thanks to the shallow depth of that primitive sea and due to the tropical climate existent at that period (corals do not survive in depths above 50 m, due to lack of light, and require the water temperature to remain between 18º and 25ºC) started to settle above the previously mentioned materials numerous living organism, especially colonies of coral, very similar to those that presently continue to develop in the Tropical coral Reefs, as well as others not of less importance, such as the Rudist colonies (organisms at present inexistent) and enormous oyster deposit banks.

Stratigraphic map
ALBIENSE MID-SUPERIOR (110 million years)
APTIENSE, ALBIENSE LOWER (120 million years) - (Reef faces - URGONIAN)
NEOCOMIAN (140 million years)
Stratigraphic layers

With the passing of time the organisms that grew towards the surface, squashed and compacted the ones already dead and below them, creating this way a new form, and on a continuous basis an enormous accumulation of materials of biological origin (dead corals and shells), of limy consistency, being this the reason these rocks are also known as organic-detritus lime. This accumulation of calcareous material constituted the reefs´ nucleus, and it is definitely the origin of the enormous lime masses that are predominant in the hills of Durango, the highest peaks of Urkiola´s Nature Park.

Peaks of the Urkiola Park
  1. Urtemondo (789 m)
  2. Mugarra (965 m)
  3. Untzillatx (935 m)
  4. Astxiki (791 m)
  5. Alluitz (1.039 m)
  6. Elgoin (1.240 m)
  7. Anboto (1.330 m)
  8. Izpizte (1. 062 m)
  9. Tellamendi (894 m)
  10. Leungane (1.008 m)
  11. Kanpantorreta (1. 0 16 m)
  12. Arrietabaso (1.018 m)
  13. Urkiolamendi (1. 0 11 m)
  14. Saibigain (945 m)
  15. Orixol (1. 128 m)

110 million years ago the primitive sea, until then narrow and quiet shallow, started to open up and at the same time gaining depth, therefore the golden era of the coral reefs came to an end. In its place sandy and thin clayish sediment deposits began settling in (C-3 in the stratigraphical map).

Urkiola Valley with the limestone mountain in the background. (Increase the size in a new window)

The solid lime rock, is the result of the compacting of numerous creations of constructive organisms, and is used by men in multiple forms, but all of them connected to construction activities, using quarries like the one situated in the surroundings of Mugara.

Limestone crenellations of the Urkiola Natural Park. (Increase the size in a new window)

In one of those back and forth trips of the geological history, the Iberian Plate and Euroasiatic Plate met in collision. As a result of these opposing forces emerged materials that once were marine sediments and today are, paradoxically, mountains.

Here the first phase of the Urkiola Nature Park´s geological history comes to an end, that is the one referring to the era when the formation of its materials took place, but it must be taken into account that we are submerged in the waters of an incipient sea.

The second phase of the geological history is unfortunately not recorded inside the limits of the Park, and for this the rock formation of the immediate surroundings would have to be studied. It is known that at the same time that Urkiola´s last rocks were being deposited in marine depths, powerful eruption of underwater volcanic lava was being dispersed throughout the immediate surroundings. So, facing the NW-SE direction, and towards the crest of Untzillatx-Anboto, and parallel to it, less than 2 km to the N (Axpe´s area), the basaltic deposits of the cushioned lava of clear submarine origin can be found.

The volcanic lava is a composition of rocky cast materials that come up to the surface of the earth, from the depths, through cracks. If it erupts on land, rivers of lava are formed that emanate from the volcanic cone, but when it takes place in the sea, and at deep levels, the pressure of the water hampers its explosion and cools it down rapidly. Therefore rivers of lava are not formed as on land, but the magma as it comes through the cracks forms clots, that when still hot and mouldable, turn and deposit themselves at the bottom of the sea forming balls or cushions, hence from where the name cushioned lava originates.

Outlines of Tectonic Plates.

Plate Tectonics Diagrams, showing the creation of the Pyrenees and the appearance of the Basque mountains.

U (Urkiola) situation before the collision between the Iberian and Eurasian plates.


The Iberian plate.

The first Iberian Plate, 130 million years ago (time of rock formation of the Urkiola´s Nature Park), was separated from the Euroasiatic Plate by a narrow, hot and not so deep sea, where coral reefs developed.

Movement of the Iberian plate. Start of the movement of the Iberian plate.

The Iberian Plate started a slow (taking millions of years) movement in an anti-clockwise direction moving closer to the Euroasiatic Plate, the Cantabric Sea began opening up become wider and deeper.

Collision between the Iberian plate and the Euroasian plate. Collision between the Euroasian plate and the Iberian plate.

The Iberian Plate collided with the Euroasiatic Plate, placing itself under it. The marine sediments then existent between these layers were compressed together, folded and lifted.

The Pyrenees Mountain range is formed

The lifting of the marine sediments was completed 40 million years ago, with the fusing together of the Tectonic Plates, as a result of the force exerted on the Iberian Plate, by the African Plate, which was pushing from south towards north, resulting in the definite fusion with the Euroasiatic Plate. And as result the Pyrenees Mountain Range and the Basque continental range was formed, with its rocks filled with marine fossils from previous eras.

Cushioned lava is presently being formed in the axles of oceanic ridges, where continents separate and therefore the sea become bigger and wider.

100 million years ago deep cracks were formed in the depths of the primitive Cantabric tropical Sea, becoming deeper and wider as it moved away from its primitive coastlines, represented by the Iberian Plate and the Euroasiatic Plate. The submarine lava is witness to this.

This ripping marine movement is attributed the terminology "opening of the Cantabric", and continued over a period of time that lasted over 50 million years.

The opening of the Cantabric could have continued, if it was not for a transcendental fact, that changed the course of happenings.

Lets not forget that we are still submerged in a sea that is expanding and growing. Nevertheless 45 million years ago, the Iberian Plate that initial had moved away from the Euroasiatic Plate, is now being pushed by the African Plate towards the Euroasiatic Plate, encrusting itself under it. We now enter the third phase of Urkiola Nature Park´s geological history: the Pyrenees Orogene.

As a result of the slow but inexorable collision the Iberian and Euroasiatic Plates, the portion of marine sediments existing between both, accumulated throughout the various eras and prior to the consummation of the impact, are folded and compressed, and at the same time lifted to unbelievable heights. Forming this way the highest peaks of the Pyrenees Mountain Range as well as all other mountain ranges in the periphery of this Continent, which were until then marine sediments belonging to other eras. This tells the story of the origins of all the Basque mountains, and the explanation of the birth of its rocks, as in the Pyrenees, the fossils and marine life of other eras can also be found here.

We can be amazingly surprised by finding old marine organisms, away from the coast, so far inland and at such great heights above sea level, or even complete tropical reefs at an altitude of more than a 1 000 m, forming part of the outside layer of the Durango Mountain Range. But even so, this is not the exact image of the mountains formed over 40 million years ago, after the total lifting up of the marine depths.

We are now entering the fourth and last phase of the mountain ranges´ formation of Urkiola´s Nature Park: the erosive phase

Erosion means wear and destruction. If in the sea depths the sedimentation, accumulation, deposit or construction is the ruling factor, the same does not occur with the Continent´s mountain ranges that are constantly subjected to all type of exposures, resulting in a continuous wearing away and lowering of these. With the result that all eroded materials will finally finish up at sea, after undertaking a longer or shorter distance during a slow process, using transporting agents, generally the river network drainage systems or winds. This is the sediment cycle: erosion wears away the Continent´s mountain ranges whose rejected materials or potential sediments (clay, sands, crusts and stones) are dragged down to the sea where they are deposited forming layers at the bottom. If an orogene movement lifts these marine sediments creating new Continent mountain ranges, the sedimentary cycle begins once again.

Right after the continent´s mountain range formation, 40 million years ago, started the process of erosion and wearing away, where the rocks were softer or more exposed, which resulted in the actual shaping of the mountain ranges, not only of Urkiola´s Nature Park, but also all other mountain masses that surround it. What we have today is the result of 40 million years of constant erosion, and 40 million years of mountain surface sculpturing.

The geological history can go further, and we could be talking about a fifth phase, due to the fact that the modelling of the mountain range continues its guided course led by the hand of erosion, a natural and slow process, but suffering frequent alterations implemented by another erosive factor, more lethal, faster and brutal, that is the human species and its technology.


Inside Urkiola´s Nature Park a singularly shaped scenery can be found, that is common to all the enclaves with lime prominence in its stratum phytology: the Karstic mountain range.

The limestone, as previously mentioned, is a sedimentary rock formed by calcium carbonate, of a high concentration level. On the other hand, this chemical composition is very soluble when mixed with carbonic water, this being the reason why limestone is so easily worn out by water, modelled and eroded by means of dissolution.

Graphite (1); Drains (2); Calcareous deposits (3); Abysses (4); Subterranean gallery (5); Hills (6); Caves (7); Rivers and subterranean lakes (8) and Underwater streams or springs (9). (Increase the size in a new window)

Karstic scenery. The surface limestone when dissolving presents rocky structures deeply cracked and divided in blocks called graphite (1). In other cases the dissolution creates vertical channels or abysses (4) that connect directly with the subterranean gallery (5). If these abysses are, places where water filter in, coming from some small surface streams, then we are talking about drains (2). The denudation of the land leaves ample vertical cliffs or calcareous deposits (3), while in flatter surfaces, are more frequent the circular depressions or hills (6) formed by the collapsing of the subterranean galleries (5), protruding to the exterior in the form of openings or caves (7). The drainage water dissolves the rock always in search of lower levels (8), where it forms rivers and subterranean lakes, flooding totally the mentioned levels, depending on the quantity of infiltrated water. When it finds an impermeable level the water looks for an exit to the outside creating an underwater streams or springs (9).

The surface scenery that can be observed in a spot where limestone predominates, is very typical, here the rock is in a stripped form, and dissolution signs can be seen everywhere. In a small scale, the rocky surface appears worn away with circle groves, ripples or crests similar in form to waves. In other cases circular dissolution phenomena can be observed, the rock imitates being a cheese with holes. In a wider scale, the scenery shows rocky grounds and soils, extremely cracked, with total absence of water on the surface, as all pluvial water filters through the rocky crakes until reaching the interior of the limestone masses, dissolving and widening further and further the group of diaclastic system (cracks).

The water infiltrated through the surface gains depth as it dissolves the rock, forming caves and a quiet complicated network of subterranean galleries, until an impermeable level is reached, it searches an outlet giving way to underwater streams or spring formations.

On the other hand, the communication to the exterior, of the complex subterranean galleries, can be more obvious and the occasional surface water basins can disappear into the interior in an abrupt way through drains. Equally, on the surface, more or less vertical orifices connecting directly to the subterranean galleries can be observed, these orifices are called abysses. When in the place of vertical orifices, we observe depressions, more or less circular, similar to a crater or funnel; we are looking at dolinas, which are none other than a surface expression for a cave in, into the depths of some subterranean sector.

Inside the Park, the massive limestone of Aramotz-Mugarra and Ezkubaratz are presented in a plateau form, showing a variety of different forms on the Karstic mountain range: dolinas, abysses, graphite, that give shape to a very abruptive and storm-like scenery, with a tendency to show deeper escarpments on the outside edges of the plateau, especially on its NE slope.

The karstification of the limestone is closely related to the hydrological contribution and, at the same time, the volume of karstificable rock is directly related to the availability of subterranean water reserves. Formation of a limestone pavement. (Increase the size in a new window)

Graphite formation. The limestone dissolution is always carried out towards a network of cracks, also called diaclastic network. The diaclastic network has predominantly dominant directions perpendicular to each other, limiting off blocks that are more or less rectangular (Photo l). As the dissolution acts on, and the cracks become wide and deeper, these blocks become isolated (Photo 2). At the terminal stage of the process, we can observe (Photo 3) a forest with tooth shaped, vertically grooved and cleaved stones; these are the resulted of a surface dissolution, on a large scale, of the primitive crack network, This way we have shaped graphite, which is one of the typical morphologic elements of the Karstic scenery. On a small scale, the rocky surface that forms the graphite, presents corrosion marks, equally flashy and characteristic. (Photo 4).

The Anboto´s group of limestone crests, flows preferably towards the NW, through the Atxondo valley (Urtzillo underwater streams, 100-200 litres/second).

The reef limestone with Karstic development, present an hydrology of its own, without surface water streams, and with an internal circulation, that in the Aramotz-Mugarra range springs towards the SW foot of the hill, in the Dima valley, with the underwater streams of Orue (75-100 l/s), and on the NE, in Mañari, of Iturrieta (100-200 l/s). The Ezkubaratz range, on the other side, flows towards the N in the direction of Mañari (Zallobenta, 100-200 l/s), and also in the SW, towards the Indusi River (underwater streams de Urmeta, Angilarri, Indusi, Bernaola).

On the northern slope the permanent streams of surface water, are situated on top of more impermeable clay loams, of the Inungane and Iturriotz-Txakurzulo cliffs, that make up the Mañaria River and the Mendiola Cliff, which through a stream, of the same name, flows into the Elorrio River. This river, on the other side has collected the waters coming from the Txareta and Atxondo Cliffs.

Without any noticeable water streams the SE slope, of the Arangio and Tellamendi Mountain, drains towards the Aramaio River in the direction of the Deba.

The Karstic spring waters as well as the streams, previously mentioned, finally join up in the Ibaizábal River, of the Cantabric Basin.

In the southern slope, can be seen two important streams: the Urkiola and the Oleta streams, that deposit their water into the Ebro, through the Urrúnaga Dam, belonging therefore this sector to the Mediterranean Basin.


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