Serra Grossa inglés

STOP 1. La Sangueta.

Our first point is located at the tram stop La Sangueta. This enclave belongs to the Sierra de Santa Ana or El Molinet, the smallest block of the Serra Grossa (a small mountain range). In this stop we are going to distinguish two points of observation, A and B. We start at point A, on the tram platform in the direction of Alicante, with the aim of observing the front wall. This wall was artificially carved to house an oil refinery that operated from 1875 to 1966. As an interesting fact to point out that the facilities were expanded after the Spanish Civil War, thus creating inside the mountain range, large deposits of crude oil, that nowadays, although empty, still exist.

Now we can see the bare wall in front of us, highlighting in a unique way both the calcarenite strata and the fracture network that affect them.

From the Burdigalian to the Serravallian (approximately 18-10 million years) the area must have been an elongated and very active sedimentation basin. The bibliography relates the formation of this basin to the activity of the Crevillente Fault, perhaps the largest tectonic accident that exists in our province. This is a strike-slip fault with an important movement in the horizontal, that when breaking and stretching, created a depression or basin, technically called, pull-apart basin. The basin was filled with marine sediments in several successive episodes.

Later, during the Upper Miocene (Tortonian), it began a mainly compressive activity that has been lifting, folding and fracturing the materials until now.

The fractures that we observe form a dense network of normal, low throw faults that sometimes cross each other (these are the main faults and their conjugates). Normal faults occur because compression is not continuous in time and when it ceases, the fractured massif relaxes, producing the displacements we observe. You can follow the trace and the order of formation of the faults in the wall and one could also calculate the displacement that all together imply and how much the massif it has been "stretched".

Therefore, it constitutes an outcrop of high didactic value.

In addition to the faults it is also interesting to observe the strata. In this wall we can see an alternation of yellow calcarenites, more or less thick and more or less outstanding of the wall, rich in red algae, fragments of bryozoans, varied remains of shells (molluscs, echinoderms, fish teeth) and glauconite.

If we follow the strata with our eyes, we see that some are wedged and disappear. In the lower part of the wall, this phenomenon forms an inclined surface that seems quite continuous.

To finish the observation of this structure we will move to point B. We leave the platform through a spacious tunnel and about 80 meters beyond, we reach a large esplanade called "La Cantera" (“The Quarry�). From here we can contemplate the whole wall with greater perspective. The inclined surface can be followed through all the cliff, as well as the network of fractures. This surface is called "onlap" and is a type of stratigraphic discontinuity that separates two different episodes of sedimentation in

such a way that the strata of the upper unit are wedged on a prior relief as the sea level rises.

Finally, we would like to point out that from this and other quarries in the Sierra, the socalled "San Julian’s stone" was obtained, from which emblematic buildings such as the Town Hall or the San Nicolås Co-Cathedral are made. However, the exploitation has created all these scarps that increase the natural instability of the massif and constitute an important geological risk.

STOP 2. Fault plane.

To complete our observations about the different types of faults in the Serra Grossa massif we head towards stop 2. From la Cantera (The Quarry) we cross to the opposite pavement to the Avenida de Villajoyosa and take the direction to La Albufereta’s beach (north), walking about a kilometre until we reach a turn to the right that crosses over the Tram tracks. If we walk a little further along this road that goes down towards the coastal breakwaters, we can see on the left and on the other side of the tracks, a striated wall that is the objective of this stop.

This wall is one of the fault planes that cut and limit the materials of the Middle Miocene of Serra Grossa, creating the entire coastal cliff of about 160 m high.

The exceptional exposition of the plane allow us to analyse it.

It is approximately vertical and somewhat undulating. Friction has polished the surface to the point that the stratification can hardly be distinguished. This is what is called a fault mirror. The most striking feature of the plane is the striae inclined about 45Âş in the plane itself to the right. These bands indicate to us, as if they were scratches, the direction of the last movement of the raised block, towards the west; while the sunken block has remained under the sea. Moreover, the small cracks of tension parallel to each other, that we can see to the right of the plane and transverse to this one, are due to the tearing of the materials when moving.

The plane is large and can be followed by the road to Alicante (south), although in that direction, just above the mouth of the new tram tunnel, turns south-west. This fault is different and of greater magnitude than the network of faults observed at stop 1. The first thing is the direction, clearly parallel to the current coast, the second is the throw of more than 160 m. This makes us think that the mountain range is limited by faults like ours, of north-east/south-west direction that stem from the activity of the Crevillente tear fault.

A fault mirror in the middle of a city can rarely be seen at first sight. However, in this case and curiously, thanks to the anthropic action we can admire it. It is known that Serra Grossa is still tectonically active, so that any tremor or small earthquake in the area would dramatically affect the stability of the cliffs.

STOP 3. Rampant dunes.

We continue our route on the road that leads down to the Albahía building. We get behind it and go up again to the Avenida de Villajoyosa. Along the way we can observe the measures that are necessary to protect all these buildings from possible landslides.

Halfway up and next to a tram tunnel, we can see a mass of reddish sandstones that are already totally different from the yellow calcarenites of the main massif. We go up to the Avenida de Villajoyosa and cross it. Following this pavement towards La Albufereta, we will find a lot used as a car park, between the "Monte y Mar" urbanization and the "Mariola" building. Here we find again those same sandstones that are the objective of the stop. The coordinates are: N 38 ° 21'44.0 "/ W 00 ° 26'57.9".

The bibliography describes these deposits as eolianites formed by rampant dunes attached to the Serra Grossa during the Tyrrhenian Quaternary (0.1 mya). But let's go by parts.

Dunes are structures formed by deposits of sand-sized particles transported by the wind, when it meets an obstacle. These deposits are internally arranged in sheets inclined to leeward, which indicates the direction of the wind in the time in which they accumulated. The change in the direction of the laminations in the fossil dune means, therefore, that during the deposit there were continuous variations in the direction of the wind, originating the general aspect that we observe in this slope.

After the sedimentation of the sandy particles, the compacting and cementation processes transform them into sandstones. These sandstones of wind origin are called eolianites. In our case, this process of lithification or formation of the sandstone did not occur under the sea but because of the meteoric water infiltrated in the subsoil loaded with calcium carbonate. These carbonated meteoric waters cause the cementation of the particles. It is no longer a dynamic structure but a static one: an eolianite.

On the other hand, the term "rampant" refers to the climbing or ascent of the dunes by a wall, in this case the slope of the Serra Grossa. In other words, the dunes had to be moved until they found the cliff wall and went up to their current position. As the dunes "climbed" up the side of the cliff, the detachments of its wall dropped blocks of calcarenite that we see included and floating inside. On the eolianites we can observe side materials that were covering the wind deposit after the cessation of its activity.

If we look around, we can ask ourselves where do the sands that formed these dunes come from? This sand must come from the bottom of the sea since it contains fragments of shells. During the last glaciation the sea level dropped more than 80 m, exposing the sandy bottoms of the continental shelf of the bay of Alicante. In glaciations the climate becomes drier and windier. This climate change would mobilize the sand, forming dunes throughout the coastal area and causing part of the deposits to rise on the slope of San Julián. This process continued until the rise in the temperature of the planet that melted the ice, and the sandy bottoms were covered.

These curious deposits are not the only ones in the province, on the slope of the cliff of El Albir, we find them much more extensive and better preserved. In fact, they are considered a heritage treasure known internationally. The rampant dunes of Serra Grossa are, nevertheless, in clear danger of disappearing since there are only a few remnants left among the human constructions.