Around 450,000 years ago most of Britain was covered by a giant ice sheet up to a mile deep. The area around what is now Thorndon Park lay at the southern tip of this ice sheet and great torrents of meltwater flowed out from beneath the ice, spreading gravel and sand. After the ice retreated, most of the gravel was removed by new rivers, leaving only isolated patches on high ground. One of the largest expanses of glacial sand and gravel is at Thorndon Park.
At the given co-ordinates you will find a cliff of glacial sand and gravel created by Country Park staff and volunteers in 2011. The layers of sand and pebbles thus exposed show a cross section of river beds where torrential floods of icy water flowed from the nearby icecap 450,000 years ago. Four layers can be discerned.
The bank is topped with a thin layer of topsoil which supports shallow rooted plants; at the right-hand end you can see a black fossil soil layer which grades down into a few centimetres of pale brickearth or loess, a wind-blown deposit of well-rounded sand grains.
Below the topsoil layer lies the upper pebble bed. Many pebbles in this layer are broken and long stones are at all angles, even vertical; this was caused by the freezing and thawing of deep ‘permafrost’ during a period of very cold climatic conditions.
The next layer is a silty-clay bed which developed from a mudflow during the melting of permafrost during glacial times. The mud is from the Claygate Beds and some pebbles from glacial melt water were mixed into it. Its mottled grey/brown colouring shows that was turning into soil.
The bottom layer is a gravel and sand bed. This bed was deposited by huge river floods; the sands were left when the water flow slowed. Amongst the pebbles deposited are some rare fragments of rock from Yorkshire which prove the glacial origin of these gravels.
In order to log a find on this earthcache please send us an email via our the profile with the answers to the questions below.
Please Note : Logs without a corresponding email or message will be deleted. We're sorry to have to introduce this rule but we have had a few unsubstantiated logs since publication.
Questions to answer
1. Estimate the height of the cliff
2. Of the four layers, as visible in the section, which layer is the deepest and roughly how thick is it?
3 a) In the bottom layer pebbles from Yorkshire have been identified but they are in the minority. What name is given to a piece of rock that differs from the size and type of rock native to the area in which it rests? How does their presence prove the glacial origin of the gravel?
3 b) What is the rock type of the majority of pebbles in this layer?
Although not required to claim a find, it would be nice to see a photo of you or your GPS at GZ.
Many thanks to Ros Smith and Ian Mercer of the Essex Rock and Mineral Society for their invaluable assistance in developing this earthcache, and many thanks to Essex Country Parks for giving permission to place this EarthCache.