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Dolphin Chalk Quarry, WEST THURROCK, Thurrock District, TQ571780, General geological site
Site name: Dolphin Chalk Quarry
Grid reference: TQ 571 780
Brief description of site:
Between Stonehouse Lane and Canterbury Way (the Dartford Tunnel approach road) is the Dolphin Chalk Quarry, now used as an industrial estate. It has fine exposures of Upper Chalk with remarkably regular bands of flint, possibly representing climate cycles in the Cretaceous period.
Summary of the geological interest:
Formerly known as the Metropolitan Works Quarry, the old Dolphin Chalk Quarry now contains a cement manufacturing plant and an industrial estate. The 20 metre high vertical chalk face that forms the western side of the quarry is the most remarkable chalk cliff in Essex as it shows regularly spaced bands of flint nodules which may represent cycles of climate change during the Cretaceous period some 80 million years ago.
Due to the recent construction of industrial and commercial buildings alongside this chalk face the flint banding is no longer clearly visible and the cliff is now inaccessible for most of its length. However, a piece of land in the south-west corner (from the Dolphin Way tunnel to the southern end of the quarry) remains undeveloped allowing a clear view of this cliff face from Dolphin Way.
Scientific interest and site importance
Chalk is a special type of limestone formed on the floor of a tropical sea about 80 million years ago during the Cretaceous period. The Chalk Sea is thought to have inundated most of northern Europe, the purity of the chalk being evidence that, at that time, coastlines were far away and sea level was very high. Flint is an extremely hard, black form of quartz that originates from the skeletons of sponges that were dissolved by sea water and precipitated as mostly horizontal layers or bands.
The flint bands in Dolphin Quarry are about one metre apart and continue for the full height of the cliff. Based on the extremely slow rate of chalk deposition on the Cretaceous sea floor each cycle may represent a time period of between 20,000 and 40,000 years. This is within the periodicity of the Milankovitch climate cycles, caused by changes in the Earth’s revolution and orbit, which alters the amount of the Sun’s heat reaching the Earth’s surface. These frequencies have been identified as the cause of the glacial-interglacial cycles during the Ice Age but they have operated throughout geological time. These rhythms may therefore have affected the sediment on the Chalk Sea floor, which made conditions more suitable for flint formation, but exactly how this happened is not precisely understood.
These flint bands are not clearly visible in any other Chalk exposures in Essex and, due to aspect and differential weathering, are not even apparent in the adjoining quarry.
The stratigraphy of the Chalk here, and elsewhere in the area, has been described in detail in Mortimore et al 2011.
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Reference: Smith & Batten 2002 (p.4), Wray & Gale 2006, Mortimore et al. 2011
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