Hadleigh Chalk Exposure
Chalk which was laid down (90-70 million years ago), in an extensive shallow tropical sea, is the oldest rock exposed at the surface in Essex. In Essex, the majority of the Chalk is concealed below Tertiary and Quaternary deposits, to leave only minor outcrops. The chalk can be seen in quarries in the south of Essex, which represent the remnants of the once thriving Portland cement industry.
Chalk is a soft, white, porous sedimentary rock, a form of limestone, comprising over 95% calcium carbonate and contains thin beds of marl and nodules of flint, either scattered or in bands. composed of the mineral calcite. It forms under relatively deep marine conditions from the remains of skeletal bits of plank tonic green algae, associated with varying proportions of larger microscopic fragments of bivalves, foraminifera and ostracods.
It is common to find flint and chert nodules embedded in chalk. Flints are a well known component of Chalk and they are present as nodular seams, tabular beds and linings to fractures. The silica was derived from the dissolution of the siliceous skeletons of sponges and other organisms and has been redistributed in the form of nodules during several stages of crystallisation.
Tertiary
A rise in sea level around 50 million years ago led to the widespread deposition of the London Clay; a uniform bluish-grey clay deposited in a semi-tropical sea. Harder concretions and layers within the Clay, known as Cementstones, have yielded a large number of marine fossils including starfish, crabs, lobsters, fish, turtles, bivalves and gastropods. The youngest part of the London Clay is sandier and is known as the Claygate Beds. These rocks represent the transition between the deeper water London Clay and the succeeding shallow marine to estuarine Bagshot Sand. Outcrops of Bagshot Sand form several ridges (“Bagshot Hills”) across the central and southern part of the county.
Quaternary
Over the last two million years the climate of Britain has varied tremendously with periods of temperate climate interrupted by repeated advances and retreats of glaciers and ice sheets. Collectively these periods have become known as the Ice Age (we are still in one of the temperate phases) and the actions of the ice sheets have been instrumental in forming the landscape we see today.
Deposits from this time are widespread in Essex and occur typically as river deposits on valley sides or as boulder clay (till). During the early part of the Quaternary, the forerunner of the River Thames flowed north-east across the northern half of Essex. The River Medway was an important southern tributary of this river system, joining it at Clacton-on-Sea. River sediments, laid down as a series of terraces, representing former floodplain levels, are known as the Kesgrave Sands and Gravels and were deposited across parts of Essex by this combined fluvial system. The gravels have great commercial value and are worked in numerous pits between Harlow, Chelmsford and Colchester, which was the route of the ancestral Thames at least 600,000 years ago.
In the 10,000 years since the melting of the Devensian ice sheet, sea-level has risen by 30 m and is currently rising at an estimated 2 mm per year. The relative levels of land and sea during this period have dominated the physical development of the Essex coast. The mudflats and salt marshes of Essex have been created and sustained during this time by mud carried by the sea from the north. Thousands of acres of salt marsh formed over the last 2,000 years, however, their extent has been greatly reduced in the past few centuries due to human land-claim. The remaining salt marsh forms part of the Greater Thames Estuary coastal lowlands that stretch north from the Swale Estuary in Kent to the Essex/Suffolk border.
Hadleigh Country Park lies between South Benfleet, to the West, and Leigh, to the East. The park consists of a mix of scrub, woodland and grassland hillside running down to grazing marsh and seawall, which is adjacent to a narrow strip of salt marsh. This wide range of habitat provides many places for plants and animals to live. In summer this is one of the best places in Essex to see butterflies, dragonflies and damselflies.
The rangers tell me that the best place to park is in the main car-park (N 51º 33• 130' E 00º 35• 724'), however, this is £2 for the day. The car-park currently closes at 7:00, but check the time. Alternative parking can be found at Benfleet Station or surrounding roads (N 51º 32• 668' E 00º 33• 917'), however, there are charges and restrictions.
To claim this earthcache you will need to perform four tasks:
1. Estimate the height / thickness of this section of the chalk band
2. Estimate the length of this section of the chalk band (from this point to where the chalk band disappears into the undergrowth on the bank close to railway stanchion- heading east)
3. In what geological period or epoch was this chalk laid down, this is part of the Mesozoic era?
4. Take a close-up picture of your GPSr against this chalk band. Also take a photo of your GPSr at the sign mentioned on the co-ordinates.
Message me the answers to three tasks to me at the same time as you log your find. Post your picture with your log.
For more information and an opportunity to see a copy of the excellent ‘Essex Rock’ see:- http://homepage.ntlworld.com/rog.coleman/Archive/ERock/Essex%20Rock.pdf