Pebble Walk - Diapir EarthCache

This is one of a series of earth caches placed in Thorndon Country Park as part of Essex Mega 2015 to showcase the interesting geological features of this part of Essex. The series follows the route of the Pebble Walk, a discovery trail created by the Essex Rock and Mineral Society, and we have been fortunate to have the assistance of ERMS members in developing the series.

Thorndon Park stands on many geological layers. In the area of the Visitor Centre pebble gravel from ancient rivers overlies Bagshot Sand, Claygate Beds, London Clay, the Lower London Tertiaries, the Upper Chalk and the Gault Clay. Below these layers lies a deeply-buried platform of hard rocks: a large, eroded Jurassic 'Dinosaur island'. Erosion has removed the upper layers of pebble gravel and Bagshot Sand in the area of the park, so that the Visitor Centre sits on the Claygate Beds.

Occasionally, a lower, older layer of rock pushes upwards through overlying layers. Such an intrusion is known as a diapir. In this geological situation a diapir may have been triggered by differential gravitational pressure between water-laden, mobile London Clay beneath more permeable layers, here the Claygate Beds. See Simple Diapir Example below. At Thorndon Park there is a small example of this phenomenon which this earthcache will examine.

At the given co-ordinates you will find a stream. On the east bank you will see London Clay (it is extremely sticky so do not get stuck in it!) and on the west bank the Essex Rock and Mineral Society have exposed a patch of the Claygate Beds (it is framed by a patch of moss).  

To claim this earthcache, email us via our profile 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.

1.    Describe the texture and colour of the London Clay and the Claygate Beds at this point. How do they differ?

2.    Roughly how tall is the exposed section of the Claygate Beds?

3.    Which is the older layer? London Clay or the Claygate Beds? Explain the reason for your choice.

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.