The Strait of Dover or Pas de Calais is the narrowist part of the English Cannel. Cap Banc Nez is, just visible, immediately over the anchor, which is set in front of the Dover Patrol Monument. It is here that there is a similar obelisk on the French coast. The Strait is one of the busiest international seaways in the world, regularly used by over 400 commercial vessels daily. This has made safety a critical issue, with HM Coastguard maintaining a 24-hour watch over the strait and enforcing a strict regime of shipping lanes. The Strait is believed to have been created through erosion. At one time there was land where the Strait is now, being a southeast extension of the Weald, joining what is Great Britain to continental Europe. The east end of this old longer Weald can be seen as the Boulonnais chalk area in the Pas de Calais . The predominant geology in the region, on both the British and French sides and on the sea floor between to quite a depth into the earth's crust, is chalk . Although somewhat resistant to erosion, such erosion of the chalk can be seen on both coasts as impressive sea cliffs, the famous White Cliffs of Dover, and Cap Gris Nez on the French side of the Strait.