The White Cliffs of Dover
The White cliffs hass an amazing Geology. The region which faces the strait of Dover from England is part of the North Downs forming the white cliffs of Dover. The striking appearance of the cliff is due to the chalk accented by streaks of black flint. Around 70 million years ago, it is believed that Great Britain and much of Europe was submerged by the ocean. The bottom of the sea had been covered with white mud which formed from fragments of coccoliths, the skeletons of algae which floated in the surface waters and which sank to the bottom during the period of Cretaceous. In this era, the bottom living creatures got preserved as fossils in the muddy sediments. These sediments had been formed very slowly with only about a half of a milimeter adding up in an year which is equivalent to about 180 coccoliths piled on top of one another. Some areas had over 500 meters of sediment and the weight of these overlying sediments became consolidated into chalk.
With subsequent movements by the earth the Alps was formed and it raised the sea floor deposits above the sea level. Till the time of the last glacial period, the British Isles were part of continental Europe which were linked by the Weald-Artois Anticline, which is a ridge which acted as a natural dam to keep a large freshwater pro glacial lake, which is now submerged under the North Sea. The land remained connected until between 450,000 and 180,000 years ago when there was a glacial lake outburst flood destroying the ridge which connected Britain to Europe. During the last glacial period which was around 10,000 years ago, the rising sea levels finally cut across the last land connection.
The chalk face of the cliffs show horizontal bands of dark coloured flint which is composed of the remains of sea sponges and other siliceous planktonic micro organisms which has hardened into the microscopic quartz crystals. The quartz silica filled cavities which has been left by dead marine creatures are generally found as flint fossils, especially moulds of Micrasterechinoids. There are several different ocean floor species like crinoids and bivales which are present in the chalk deposits.
There are also certain areas which have layers of soft, grey chalk which is known as hardground complex. The hardgrounds reflect disruptions in the steady accumulation of sediment when sedimentation stops or the loose surface sediments are stripped off by currents or slumping, with exposed older hardened chalk sediments.