Geology Site Account
Fobbing Borehole, FOBBING, Thurrock District, TQ71278435, Historical site only
Historical site only. The Fobbing borehole is one of very few in Essex that have penetrated the deep 'Palaeozoic basement'. Rocks encountered at the base of the borehole dated from the Devonian period and were almost 400 million years old.
In the nineteenth century the Southend Waterworks Company was formed to supply Southend with fresh water from two or three deep wells in or near the town. However, the growth of population was so great that the company had to sink well after well extending as far as Fobbing. The Fobbing Auxiliary Well, north-west of Fobbing church, was sunk in 1904 and terminated in the Upper Chalk at a depth of 551 feet. However, what made this well of great geological interest was the water company’s decision two decades later, in 1924, to deepen it by sinking a borehole at the bottom which more than doubled its depth taking it right down to the ancient Palaeozoic basement rocks.
The Palaeozoic basement consists of shales, sandstones and quartzites, and when they were first encountered at Fobbing they were considered to be of Cambrian age. However, based on the fossils present they are now thought to be from the middle Devonian period, about 390 million years old, and probably of similar age to the Devonian rocks encountered in the nearby Canvey Island borehole.
The borehole revealed that lying directly on top of these ancient rocks is the Lower Greensand, a much younger deposit from the Cretaceous period. A considerable period of geological time – over 250 million years – is therefore missing from the rock record at this point which means that this was an ancient land surface. During the Jurassic and early Cretaceous periods there must have been inundations of this land by the sea but the sediments deposited have been removed by subsequent erosion. At this time dinosaurs were the dominant land creatures, there were no flowering plants, and Essex would at various times have been covered by forest dominated by huge conifers such as the monkey puzzle. At Fobbing, this ancient land surface was recorded as being present at a depth of 1,128 feet below the well house floor.
Old water company records state that the borehole passed through the following rocks, the thickness of each is given in brackets: London Clay (158 feet), Lower London Tertiaries (138 feet), Upper and Middle Chalk with flints (501 feet), Lower Chalk (112 feet), Upper Greensand (33 feet), Gault clay (150 feet) and Lower Greensand (32 feet) before penetrating the Palaeozoic rocks at a depth of 1,128 feet and finally terminating at a depth of 1,154 feet. Of particular interest was the Lower Cretaceous Gault clay which yielded numerous fossils such as ammonites, belemnites and bivalves, the species present indicating that it is the same layer of Gault that is present in the classic cliff section at Folkestone in Kent. Also of interest is the Lower Greensand which is usually absent beneath Essex but must be present in a trough in this restricted area.
Deepening the well must have been a success for the water company. At the base of the Lower Greensand water rushed in and rose to a height estimated at the time to be 83 feet above sea level. As the well house floor was only 65 feet above sea level this well must have been an ‘artesian’ well which means that water fills the entire well or borehole and overflows at the surface. Due to the shape of the London Basin several boreholes in Essex have been artesian as water in the permeable Chalk beneath Essex is confined under pressure beneath the London Clay, but in this case the water released appears to have been confined beneath the Gault clay.
A copy a drawing of the borehole with descriptions of the strata (produced by the water company in 1966 at a scale of 24 feet to one inch) is held by GeoEssex.
The borehole has a cast iron cover and is situated adjacent to a small brick building presumably owned by the water company and still in use as a pumping station. Unfortunately there appears to be no public right of way to the site and so the potential for geological interpreta-tion is limited. However, the land associated with the pumping station, the open grassland and small woodland has been suggested as a potential Local Wildlife Site.
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Reference: Lake et al. 1986 (p.4-5), Ellison 2004 (p.4-5), Dewey et al. 1925 (p.132-134), Cole 1925, Sumbler 1996 (p.70).
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