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Sarsen Stones:
By the roadside in many parts of north Essex are sandstone boulders, known as sarsen stones. The word sarsen originates in Wiltshire, where these stones also occur, and may be derived from the word ‘saracen’, which means stranger.
This EarthCache will take you to the two most prominent ones in Ingatestone, either side of Fryerning Lane.
Sedimentary rocks are formed by the deposition and cementation of material at the surface of and within bodies of water. The particles that form a sedimentary rock by accumulating are called sediment.
Sarsens are the remains of a tough overlay of chalk, now eroded away. These are very dense rocks; one cubic foot weighs 150 lb. They consist of quartz grains bonded together by a naturally formed siliceous cement. They were formed some 50 million years ago, when a thick layer of gravel and sand was laid down as river sediment and saturated with quartz-heavy groundwater.
After being deposited, this layer was raised above ground level, which saw sections of the sand being cemented into a hardened sandstone layer. This layer was extremely resistant to erosion but it eventually broke up into boulders, which remained on surface of the ground after the sand washed away.
The stones vary in texture and colour, depending on grain size, surface weathering, and mineral impurities.
Weathering
Physical weathering is caused by physical processes such as changes in temperature, and the effects of wind/rain. An example is freeze-thaw weathering; if water gets into a crack and freezes into ice, it expands the crack and makes it bigger.
Chemical weathering occurs when the minerals within the rocks are chemically altered. e.g. acid rain dissolves minerals within the rock, weakening the structure and often results in the appearance of staining on the surface.
Biological weathering is where rock is weakened and disintegrates from the impact of plants, animals and microbes. Examples of this would include moss or lichen growing through the rock.
QUESTIONS:
1) Describe and compare the two stones in terms of size and texture.
2) Focus on the stone upright against the wall. From the above lesson, estimate its weight in pounds (lb). (Size in cubic feet = height x width x depth).
3) There is evidence of one stone in particular being worn away here. Which one? What type of weathering caused this and what can you see that tells you this?