Please note that parking in the area is extremely limited and caution must be exercised whenever stopping here.
Sarsen Stones - The Geology
Sarsen – a derivation of Saracen – stones, are blocks of hard sandstone – a sedimentary rock, which consists of quartz grains bonded together by a naturally formed siliceous cement. They were formed some 60-50 million years ago, when layers of silt and sand (mainly quartz), laid down as river or marine sediment, were saturated with silica-rich groundwater. Later on, possibly due to either acidification, or evaporation, of the groundwater, the silica precipitated out and filled the gaps between the grains of sand. This eventually acted like a mineral glue (siliceous cement), sticking enormous numbers of sand grains together to form a layer of solid rock just below the water table. Subsequent ice and water action, broke this rock layer up into the boulders (Sarsen Stones) that we see today. The stones vary in texture and colour, depending upon grain size, surface erosion, and mineral impurities, e.g. small amounts of iron oxide can make them look darker, especially when wet. They are found across much of the United Kingdom, and vary widely in size and shape, the most famous Sarsen stones being those found at Stonehenge (together with Bluestones).
These stones are not, however, native to Eastern England, and there have been at least two theories postulated as to how they got to this part of the country. The first is carried along and deposited by glacial action during the last ice age, and the second is that they were deposited by the River Thames – which used to flow well to the North of its present course, entering the sea probably in the vicinity of what is now Clacton, but possibly even further North than this.
Some uses of Sarsen stones have been as boundary markers, protective stones for the corners of buildings, and even briefly as building materials – although they were found to attract too much damp for this purpose.
Carboniferous Limestone - The Geology
Carboniferous Limestone is a sedimentary rock made of calcium carbonate. It is generally light-grey in colour, and is hard. It was formed in warm, shallow tropical seas teeming with life. The rock is made up of the shells and hard parts of millions of sea creatures, some up to 30cm in length, encased in carbonate mud. Fossil corals, brachiopods and crinoids are very much in evidence as components of Carboniferous Limestone; indeed the rock is full of fossils.
Carboniferous Limestone has horizontal layers (beds) with bedding planes, and vertical joints. These joints are weaknesses in the rock, which are exploited by agents of both denudation and weathering. They also lead to the most important characteristic of Carboniferous Limestone – its permeability. Water seeps through the joints in the limestone. This creates a landscape geologists call karst, which lacks surface drainage but which has all manner of characteristic surface and subsurface features. The Carboniferous Limestone has been folded and faulted by massive Earth movements which can be seen by the fact that the rocks are now above sea-level and no longer horizontal. The rocks generally dip (slope) gently eastwards and, in some places, clear folds in the rock can be seen especially at the Great Orme and Bryn Alyn (Denbighshire).
Pleshey Boulders - Your Homework:
In order to claim this Earth Cache, you will need to send me the answers to the following questions:
1. Which stone is which? Please describe each stone in terms of their colour and texture.
2. One stone contains many fossil corals. How do you suppose these got here?
3. Measure the approximate size of each rock.
4. How long ago was the sarsen stone formed, and in what geological era?
5. What is the basic chemical composition of the sarsen?
If you would like to take a photograph, please feel free to do so!