3. Formation and composition of Menorcan beaches

On peninsular coasts the main sediment supply forming the beaches comes from inland along the rivers, so it is said that their origin is fluvial. In Menorca, however, there are no large rivers to supply sediment, therefore the beach sand mainly comes from the sea, caused by geological erosion of the island and also largely from skeleton remains of sea organisms that are rich in calcium. In this way, part of the sand in our beaches is made of marès (limestone) and other calcareous rocks with a common origin but from a different geological period. Nevertheless, a large amount of the sand has its origin directly from sea animals.

Geogical origin of sediment
Biological origin of sediment
Sediment formation phenomena

Geogical origin of sediment

The material accumulated on beaches after being dragged due to the action of surf and sea currents has its origins in the very geological nature of the area where the sediment comes from. In Menorca there are two contrasted and different regions, Tramuntana (north) and Migjorn (south), which are separated by a virtually straight line that divides the island from the harbour of Mahon to Cala Morell beach, to the north of Ciutadella.

Geological map of Menorca according to the landscape units. Source: Geoservei SL. Geologia de Menorca - Consell Insular de Menorca

The Tramuntana region has the oldest materials of the island where we can find a grand mosaic of sedimentary rocks, sandstone, clay and calcareous stones from the Palaeozoic period (a geological period of the Earth that ended 250 million years ago) and also dolomites, marls and calcareous rocks from the Mesozoic (a geological period that ended 65 million years ago). On the other hand, in the Migjorn region, we can only find carbonated rocks with rather more homogeneous colours. The most frequent rock is the Miocene marès (period that ended 23 million years ago), which is a rock made up of remains of calcareous fragments of sea organisms. However we can also find a different carbonated rock called "piedra viva" (solid rock), whose nature includes a larger proportion of biological matter and less earth than the marès rock. These differences in the rock composition mean that sand from beaches in the north and south of the island present a range of different composition and colours.

Cala Pilar, with its golden and ochre shades as a result of the clayish origins of its sediments
Binigaus beach, with its characteristic white sand made up of marès, which is the most common origin of the southern beaches of the island

Biological origin of sediment

Apart from the fragments with biological origin present in sedimentary rocks that belonged to organisms living million years ago, remains of other organisms can currently also be found in the sand -skeletons, shells and corals- (a. Nacre – Pinna nobilis, b. Warty venus– Venus verrucosa, c. Warty crab– Eriphia verrucosa, d. Sea urchin– Sphaerechinus granularis, e. Starfish– Echinaster sepositus, f. Common cockle– Cerastoderma edule, g. Coral – Corallium rubrum).

It must be stressed the great relevance of small organisms of calcareous structure that live on the Posidonia oceanica leaves. As a result of their changing leaves every year, all these tiny skeletons remain at the sea bottom forming a cemetery that is the sand factory of the beaches around island. The biological remains then mix with the eroded matter from the land.

Sediment formation phenomena

The biological remains that appear in the Posidonia meadows then mix with the eroded matter from the land. Depending on the environment the proportion of matter may vary from one type to another and together with the changes in composition there is a subsequent change in the colour of beaches. The phenomena that determine this difference in composition can be either physical or biological.

Physical phenomena: waves

Sea waves are caused by the wind blowing across the surface of seas and oceans, and are the primary coastal shaping agent through processes of erosion. There are two types of movement: the first is a circular movement originated by the medium oscillation caused by the wind; and the second is the propagation of the wave itself, which is longitudinal and moves the wave in a determined direction and speed.

Wave movements caused by the wind. Source: The author: CARDONA, F.

A wave’s height depends on its speed, persistence and stability in the wind direction, as well as on the distance or surface of sea area upon which it acts. In Menorca’s case the most significant winds are the northerly winds, which originate in a depression in the Gulf of Lion and use the northern Pyrenees and the southeast of the French central range as acceleration fields. If we combine this strong wind with an extensive fetch (sea surface on which the wind acts), it results in large forceful waves from the north. These waves strike Menorca with the greatest intensity.

Area where northerly waves are created. Source: The author: CARDONA, F.

Storms from the east or southeast also have large fetches, although their intensity is not usually as pronounced. Southern winds are not usually dominant and those coming from the east or southeast do not reach Menorca with as much intensity since the island is sheltered by Mallorca.

Figure 3.6. Area where northerly waves are created

Depending on a wave's dimensions, its energy arrives to a certain depth, directly proportional to its size. When a wave approaches to the coast, the depth of the sea reduces until the wave touches the bottom and loses speed; as a consequence of this friction, the wave continues to slow and eventually breaks.

When waves touch the bottom due to shallow waters, they finally break. Source: The author: CARDONA, F.

Coastal erosion is caused by the action of waves deteriorating cliffs and tearing off rocky material that accumulates at the base, creating a deposit that initially remains underwater. This material incessantly suffers from the surf and is gradually spread all along the coast by sea currents, eventually settling in sedimentation areas where beaches will finally be formed.

Biological phenomena

Apart from waves, which are the main coastal shaping agent, there are living organisms that also help to create sediment, although in less proportion. This sediment is created by the calcium-rich nature of these organisms, and when they die, their skeletons are carried away by waves and currents, or even thanks to the action of the organism itself throughout its life, which works the existent sediment by making it smaller. This is the case of limpets or sea urchins, which slowly break down the rocks upon which they live and make a finer sediment from it, or sea cucumbers, which filter the already small grains of sediment. This effect is almost unnoticeable around Mediterranean islands in comparison with the surf effects, although in other seas or oceans the sediment supply coming from living organisms can constitute a significant part of the beach sand. A good example of this is the Caribbean, where large parrot fish tear off pieces of sediments when they eat, which are then digested by their digestive system and subsequently return to the sea dissolved as excrements. These sediments are also spread along the coast carried by sea currents.