clayey flysch covered by Pliocene and/or Quaternary calcareous sediments, sourced from thriving cold water coral communities. Exposed bedrock surfaces, coarse sediments and local sand dunes attest to the strong bottom current conditions at the present time. Around 5.9 million years ago, the connection between the Mediterranean Sea and the Atlantic Ocean along the Betic and Rifan Corridor was progressively restricted until its total closure, effectively causing the salinity of the Mediterranean to rise periodically within the gypsum and salt deposition range, during what is known as the Messinian salinity crisis. In this water chemistry environment, dissolved mineral concentrations, temperature and stilled water currents combined and occurred regularly to precipitate many mineral salts in layers on the seabed. The resultant accumulation of various huge salt and mineral deposits about the Mediterranean basin are directly linked to this era. It is believed that this process took a short time, by geological standards, lasting between 500,000 and 600,000 years. It is estimated that, were the Strait closed even at today's higher sea level, most water in the Mediterranean basin would evaporate within only a thousand years, as it is believed to have done then, and such an event would lay down mineral deposits like the salt deposits now found under the sea floor all over the Mediterranean. After a lengthy period of restricted intermittent or no water exchange between the Atlantic Ocean and Mediterranean basin, approximately 5.33 million years ago, the Atlantic-Mediterranean connection was completely reestablished through the Strait of Gibraltar by the Zanclean flood, and has remained open ever since. The erosion produced by the incoming waters seems to be the main cause for the present depth of the Strait ( at the narrows, at the Camarinal Sill). The Strait is expected to close again as the African Plate moves northward relative to the Eurasian Plate, but on geological rather than human timescales. Biodiversity The Strait has been identified as an Important Bird Area by BirdLife International because of the hundreds of thousands of seabirds which use it every year to migrate between the Mediterranean and the Atlantic, including significant numbers of Scopoli's and Balearic shearwaters, Audouin's and lesser black-backed gulls, razorbills, and Atlantic puffins. A resident orca pod of some 36 individuals lives around the Strait, one of the few that are left in Western European waters. The pod may be facing extinction in the coming decades due to long term