Geology Site Account

Site name: Flitch Way Ballast Pit and Tufa Springs, on the Flitch Way footpath, south west of Great Dunmow.

Grid reference: TL 6195 2160

Brief description of site:

Old railway ballast pit adjacent to the Flitch Way footpath providing limited exposures of Kesgrave (Thames) Sands and Gravels overlain, or butting up against, glacial till. The pit is adjacent to the disused railway line (now the Flitch Way Country Park). Also, hard-water petrifying tufa springs occur here.

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Introduction

The route of the former railway around the south of Great Dunmow is now occupied by the Dunmow by-pass. From this road the route (the Flitch Way) continues over the river on an embankment and enters a cutting, the eastern end of which has been enlarged to create the ballast pit. The site includes not only the ballast pit but also the section of the cutting westwards to Folly Bridge, which carries a farm track over the former line. Access is available at all reasonable times.

Summary of the geological interest

The Bishops Stortford, Dunmow and Braintree branch railway opened in 1869 and closed in 1969. The route of the line is now the Flitch Way Country Park, which is managed by Essex County Council. For construction of the railway a pit was excavated adjacent to the line near Great Dunmow to provide ballast on which to lay the track. This pit still exists and provides exposures of Kesgrave Sands and Gravels, which were deposited by the River Thames during the Ice Age when it flowed through here over 500,000 years ago.

Lying on top of these sands and gravels, and outcropping in the cutting to the west of the pit, is the Anglian Till (also known as chalky boulder clay), which was laid down by the Anglian Ice Sheet about 450,000 years ago. This ice sheet covered most of Britain as far south as Essex and was responsible for diverting the Thames to its present course.

This site provides a rare opportunity to study these two Ice Age deposits, and the junction between them, in one place. Also of particular interest are hard-water petrifying springs, which are the source of crystal-clear streams that run in the cutting, depositing calcium carbonate tufa and encrusting all the objects in the steam bed including leaves and twigs.

Scientific interest and site importance

The geology of the hill in which the cutting is excavated consists of London Clay bedrock overlain by Kesgrave Sands and Gravels and capped by Anglian Till. The London Clay outcrops at the eastern end but the ballast pit was excavated in Kesgrave Sands and Gravels. The base of the cutting (the track bed) here rises in a westerly direction at a gradient of 1 in 86 (Paye 2010). This gradient, together with the rising of the ground, means that the till appears at the highest level in the cutting and increases in thickness as you walk west, with a corresponding reduction in the thickness of the outcrop of the sands and gravels until they finally disappear beneath the floor of the cutting. The geology of the ballast pit was first recorded by Whitaker et al. (1878). The pit was next recorded by Hey (1980) who provided a pebble count from the gravel on 16-32mm fractions. Although it was only a small sample of 190 pebbles he found 71% flint, 20% quartz, 8% quartzite and 1% pinhole chert (Lower Greensand). No volcanic or other exotic rocks were found in Heys sample but they will be present. This pit is also referred to in Appendix 1 of Rose et al. (1999) and in Whiteman (1992). The latter has divided the Kesgrave Sands and Gravels into at least eight Thames terrace aggradations and states that this site provides an exposure of the Stebbing Gravel.

A visit in 2011 revealed no visible exposures but coarse, ferruginous, sandy gravel of classic Kesgrave type was seen in the roots of fallen trees in the south side of the pit. Close to Folly Bridge the presence of till is indicated by chalk clasts in the soil. The site is important as it provides a potential exposure of Kesgrave Sands and Gravels and Anglian Till, and the contact between them, and is close to a town centre.

Also of scientific interest are the hard-water springs, which are the source of crystal-clear streams that run in the cutting. Dissolved carbon dioxide (CO2) in rainwater (H2O) soaks into the ground and has reacted with chalk (calcium carbonate - CaCO3) in the boulder clay to form soluble calcium bicarbonate (Ca(HCO3)2). When the groundwater reaches the surface and comes into contact with the air, carbon dioxide is lost from the water and a hard deposit of cream-coloured calcium carbonate (tufa) is formed. Along the bed of the streams the lime-rich water has encrusted all the objects in the steam bed including twigs and other organic material. The stream beds are fortunately protected from damage by the board walk along the floor of the cutting.

Folly Bridge, which carries a farm track over the former railway line (now the Flitch Way). The bridge is the eastern limit of the geological site. Photo: G.Lucy