Geology Site Account

Site name: Thorndon Country Park North, Brentwood (site includes part of Little Warley Common)

Grid reference: TQ 604 915 (entrance to country park)

Brief description of site: Country park with varied landforms and outcrops of London Clay, Claygate Beds, Bagshot Sand and glacial gravel. There are two exposures of glacial gravel.

----------------------------------------

Summary of the geological interest:

Thorndon Country Park North and Little Warley Common have a varied geology, the oldest surface rocks dating back to the Eocene period. These rocks consist of London Clay, Claygate Beds and Bagshot Sand, three types of sediment laid down successively in a sub-tropical sea some 50 million years ago. There are no permanent exposures of these rocks although Bagshot Sand is frequently visible in the roots of fallen trees at the northern end of the Country Park.

To the south of the Park and on Little Warley Common are large patches of glacial gravel dating from the Ice Age. This gravel was deposited some 450,000 years ago by colossal torrents of melt water issuing from an ice sheet, the edge of which was then situated only a short distance north of here. Ice then covered almost all of Britain to a thickness sometimes in excess of one kilometre. The gravel therefore provides evidence of an exceptionally cold period of the Ice Age, a time when Essex was an extremely barren and inhospitable landscape.

Two permanent sections through glacial gravel have been created by re-excavating the edge of old gravel pits at Scrub Hill on Little Warley Common (TQ 6045 9110) and in Thorndon Country Park (TQ 6127 9100). The exposure in Thorndon Country Park is very fine and was cleaned and re-excavated in 2021. It is described in detail in a recent article in Essex Naturalist (Lucy 2013). The exposure on Scrub Hill is only small and now much degraded.

------------------------------------------

Scientific interest and site importance

The prime value of this site is education but there is some potential for research on the permanent exposures of glacial gravel. The gravel consists of coarse sand and extremely well-rounded pebbles of flint that are mostly derived from the pebble gravel (Stanmore Gravel) on the high ground of Warley, which is thought to have been laid down by a river during a much earlier period of the Ice Age. The pebbles, however, were clearly originally shaped by rolling on a beach and were almost certainly originally derived from Eocene pebble beds. Some flint nodules are also present, no doubt washed from the boulder clay and originating from the Chalk of Cambridgeshire. A detailed analysis of the gravel constituents has yet to be carried out.

Thorndon Park North also has some interesting landforms. To the south the spread of glacial gravel is dissected by several streams, which have cut through the Claygate Beds exposing the underlying London Clay. It is possible to follow steeply sloping paths which pass over different rock types and appreciate the horizontal nature of the strata.

This site is important because in few other places in the district can glacial deposits be seen. The gravel provides a rare opportunity to study the evidence of the Anglian ice sheet, which, at its greatest extent, reached as far south as Brentwood and Hornchurch.

----------------------------------------------

Other information

For its biodiversity importance much of Thorndon Country Park North now forms part of Thorndon Park Site of Special Scientific Interest (SSSI). Thorndon Country Park is owned by Essex County Council. At the north end of the park is Thorndon Countryside Centre.

Little Warley Common, which is now mostly woodland, is owned by Brentwood Borough Council. Access to the common, and the gravel pit on Scrub Hill, is available via a gate from the country park. Close by, on the grassy common beside Childerditch Lane, there are views across the Thames valley to Kent. The Anglian ice sheet diverted the Thames to its present course which means that the modern Thames valley was formed after the deposition of the glacial gravel.

Adjacent to Thorndon Visitor Centre is a Jurassic fossilised tree stump from the Isle of Portland in Dorset that was donated to the country park by the Essex Field Club when the Passmore Edwards Museum in Stratford closed. It has no geological connection with the park but it is of educational value and of interest to visitors. It is provided with a descriptive plaque.

Thorndon Country Park section being finished off by hand in 2011