A simple EarthCache set in Richmond Park, London's largest Royal Park and a Site of Special Scientific Interest.
At GZ you will find King's Clump, a circular barrow (burial mound) located at the SW edge of a high plateau and one of several prehistoric sites in the Park. It was enclosed with an iron fence and planted with Scots Pines in 1910. Walk around the Clump and you will find a wooden bench facing a sandy, gravelly path heading downhill into deciduous woods (see photo). Take a good look at the surface of the path.
With an area of 955 hectares (2,360 acres) Richmond Park as we know it today first came into being almost 400 years ago when King Charles I designated it as a royal hunting ground. However, for this EarthCache we need to go back in time a thousand times further. Pick a pebble from the sand and examine it. The object in your hand has probably been in the same location for about 400,000 years.
When your pebble was deposited, Britain was beginning to emerge from the great Anglian Glaciation, the most extreme Ice Age of the last two million years, when about two-thirds of the British Isles were covered by an ice sheet up to 1,000m thick stretching from Ireland to Scandinavia. At its peak around 450,000 years ago, the ice reached as far south as the fringes of London and into Essex. Prior to its arrival, the River Thames flowed from Staines to the North Sea through Hertfordshire but was dramatically diverted onto its present course by the advancing ice sheet, which blocked its path near Hatfield. Later, as the ice began to retreat, huge volumes of melt water flowed south into the Thames, cutting through the Cretaceous chalk of the Chiltern Hills and the harder flint layers embedded within it. Pebbles and rock fragments were transported in the engorged river and deposited at its edges over a wide flood plain. As the swollen water gradually slowed and retreated, a series of sequentially lower gravel terraces were laid down over hundreds of thousands of years. In some parts, Thames gravel terraces contain sandstone pebbles picked up by the ice from the Midlands and in others even igneous rock fragments from North Yorkshire. Believe it or not, the high plateau in Richmond Park at GZ was once the bed of the River Thames, covered with pebbles like the one you are holding. Over the intervening 400,000 years, the river has slowly eroded the ground through which it runs at an average rate of around 1m every 10,000 years. At its nearest point today, the Thames is about 45-50m lower and about 1,650m to the WSW from where you are standing.
To log the EarthCache, you now need to answer three questions.
Q1. Describe the gravel pieces in the sandy path in terms of size, shape, colour and surface texture.
Q2. What is the main type of rock represented by the pebbles here?
Q3. What is the reason for the surface texture of most of the pebbles?
Please email or message me your answers using the link to my profile at the top of the cache page. A photo of you at GZ is welcome, but optional, and please do not include pictures that give clues for any of the questions. You may log the find as soon as you have sent me your answers.
Congratulations to FBH3 for FTF!