Most maritime traffic between the Atlantic Ocean and the North Sea and Baltic Sea passes through the Strait of Dover, rather than taking the longer and more dangerous route around the north of Scotland. The strait is the busiest international seaway in the world, used by over 400 commercial vessels daily. This has made traffic safety a critical issue, with HM Coastguard and the Maritime Gendarmerie maintaining a 24-hour watch over the strait and enforcing a strict regime of shipping lanes. In addition to the intensive north-east to south-west traffic, the strait is crossed from north-west to south-east by ferries linking Dover to Calais and Dunkirk. Until 1994 these provided the only route across it except for air transport. The Channel Tunnel now provides an alternative route, crossing beneath the strait at an average depth of below the seabed. The town of Dover gives its name to one of the sea areas of the British Shipping Forecast.
Geological formation
The strait is believed to have been created by the erosion of a land bridge that linked the Weald in Great Britain to the Boulonnais in the Pas de Calais. The predominant geology on both the British and French sides and on the seafloor is chalk. Although somewhat resistant to erosion, erosion of both coasts has created the famous white cliffs of Dover in the UK and the Cap Blanc Nez in France. The Channel Tunnel was bored through solid chalk. The Rhine flowed northwards into the North Sea as the sea level fell during the start of the first of the Pleistocene Ice Ages. The ice created a dam from Scandinavia to Scotland, and the Rhine, combined with the Thames and drainage from much of north Europe, created a vast lake behind the dam, which eventually spilled over the Weald into the English Channel. This overflow channel became the Strait of Dover about 425,000 years ago. A narrow deep channel along the middle of the strait was the bed of the Rhine in the last Ice Age. A geological deposit in East Anglia marks the old preglacial northward course of the Rhine. A 2007 study concluded that the English Channel was formed by erosion caused by two major floods. The first was about 425,000 years ago, when an ice-dammed lake in the southern North Sea overflowed and broke the Weald-Artois chalk range in a catastrophic erosion and flood event. Afterwards, the Thames and Scheldt flowed through the gap into the English Channel, but the Meuse and Rhine still flowed northwards. In a second flood about 225,000 years ago the Meuse and Rhine were ice-dammed into a lake that broke catastrophically through a high weak barrier. Both floods cut massive flood channels in the dry bed of the English Channel, somewhat like the Channeled Scablands or the Wabash River in the USA. A further update in 2017 attributed a series of previously described underwater holes in the Channel floor, "100m deep" and in places "several kilometres in diameter", to lake water plunging over a rock ridge causing isolated depressions or plunge pools. The melting ice and rising sea levels submerged Doggerland, the area linking Britain to France 6,500–6,200 BCE. The Lobourg strait, a major feature of the strait's seafloor, runs its wide slash on a NNE–SSW axis. Nearer to the French coast than to the English coast, it runs along the Varne sandbank where it plunges to at its deepest, and along the latter's south-east neighbour the Ridge bank with a maximum depth of.
Marine wildlife
The submarine depth of the strait varies between at the Lobourg strait and at the highest banks. It presents a succession of rocky areas relatively deserted by ships wanting to spare their nets, and of sandy flats and sub-aqueous dunes. The strong currents of the Channel are slowed around the rocky areas of the strait, with formation of and calmer zones where many species can find shelter. In these calmer zones, the water is clearer than in the rest of the strait; thus algae can grow despite the average depth and help increase diversity in the local species – some of which are endemic to the strait. Moreover, this is a transition zone for the species of the Atlantic Ocean and those of the southern part of the North Sea. This mix of various environments promotes a wide variety of wildlife. The :fr:Ridens|Ridens de Boulogne, a deep rocky high ground partially covered with sand located to the west of Boulogne, boasts the highest production of maerl in the strait. A area of the strait is classified as a Natura 2000 protection zone listed under the name :fr:Ridens|Ridens et dunes hydrauliques du Pas de Calais. This area includes the sub-aqueous dunes of Varne, Colbart, Vergoyer and Bassurelle, the Ridens de Boulogne, and the Lobourg channel which provides calmer and clearer waters due to its depth reaching.
In the late 17th century during the "Little Ice Age", there were reports of severe winter ice in the English Channel and Strait of Dover, including a case in 1684 of only a league of open water remaining between Dover and Calais.
