Approximately 25-15 Ma ago, within a geological process known as the Alpine Orogeny, the island of Mallorca was structured into a series of elevated zones (horsts) and depressions (grabens).
Precisely one of the horsts corresponds to the Serres Centrals, whose highest point is the summit of the Puig de Randa, at 543 metres.
The Puig de Randa is constituted mainly by materials of the Burdigalian-Langhian (Lower and Middle Miocene, 20.4 – 13.8 Ma ago), deposited during the raising of the island.
The Santuari de Gràcia is located just at the contact point between two superimposed stratigraphic sequences: the Banyalbufar Formation and the Randa Calcarenites Unit, making it a good place for studying them.
At the base of the cliffs, only visible behind the sanctuary, there appear grey-green marls that correspond to the turbidite sediments of the Banyalbufar Formation, deposited at great depth.
Turbidites are a type of deposit formed by successive avalanches of materials in the continental slope, usually caused by tectonic activity.
Structure of the limit of the continental shelf, where the turbidites originate.
The marls present chaotic undulating forms which contrast with the horizontal stratification of the calcarenites that compose the slope. They correspond to slides within the turbidites themselves, called slumps
Detail of the slumps observed at the base of the cliff behind the sanctuary.
In contrast, the cliffs of the sanctuary, more than 40 m high, are composed entirely of bioclastic calcarenites with clearly-marked stratification of centimetric order belonging to the Randa Calcarenites Unit and corresponding to platform deposits.
The upper levels of this unit present a marked cross-lamination which can be seen in the paving slabs of the terrace of the sanctuary. This type of structures is a product of the back-and-forth movement of the waves and the marine currents in shallow seabeds
Detail of the stratifications present in the paving slabs of the sanctuary, corresponding to the upper part of the Randa calcarenites.
The Santuari de Nostra Senyora de Gràcia is also an exceptional spot for observing different stabilisation methods of rocky slopes
The entire cliff is covered by a triple-torsion mesh which in some parts has been reinforced with a network of steel cables fixed by means of steel bars.
The triple-torsion meshes, also called stone-retaining meshes, prevent the small rocky blocks from falling freely by trapping them in their interior. They do not work if the blocks exceed a certain size because they can break, so they are complemented with networks of steel cables. The meshes are fixed with metal stakes at the head and base of the cliff, while the nets are anchored to it with steel bars, which can also be used to fix large rocky blocks.
At the base of the cliff, dynamic screens have been installed. These are flexible metal structures designed to intercept the trajectory of the falling rocks. Each screen is designed to withstand a certain impact force (a determined maximum block size), and in contrast with the walls, which are rigid elements, they deform and absorb the energy of the impact without breaking. In order to increase their capacity to stop masses in movement, they are fitted with elements that absorb the impact.
Finally, the non-rocky slopes have been protected by means of projected concrete, also called gunite or shotcrete, or by triple-torsion meshes weighted at their base.
Stabilisation mesures.
