Hardgrounds are thought to reflect disruptions in the steady accumulation of sediment when sedimentation ceased and/or the loose surface sediments were stripped away by currents or slumping, exposing the older hardened chalk sediment.
In some areas, layers of soft, grey chalk known as a hardground complex can be seen. Several different ocean floor species such as brachiopods, bivalves, crinoids, and sponges can be found in the chalk deposits, as can sharks' teeth. Quartz silica filled cavities left by dead marine creatures which are found as flint fossils, especially the internal moulds of Micraster echinoids. The cliffs' chalk face shows horizontal bands of dark-coloured flint which is composed of the remains of sea sponges and siliceous planktonic micro-organisms that hardened into the microscopic quartz crystals. At the end of the last glacial period, around 10,000 years ago, rising sea levels finally severed the last land connection. A land connection across the southern North Sea existed intermittently at later times when periods of glaciation resulted in lower sea levels. The land masses remained connected until between 450,000 and 180,000 years ago when at least two catastrophic glacial lake outburst floods breached the anticline and destroyed the ridge that connected Britain to Europe. Until the end of the last glacial period, the British Isles were part of continental Europe, linked by the unbroken Weald-Artois Anticline, a ridge that acted as a natural dam to hold back a large freshwater pro-glacial lake, now submerged under the North Sea.
Due to the Alpine orogeny, a major mountain building event during the Cenozoic, the sea-floor deposits were raised above sea level.
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