Silchester Gravel
At an elevation of about 100m above sea level the Silchester Gravels deposits are roughly 50m above the present day river drainage and sediment transport networks of the River Kennet to the north and the River Loddon to the east.
Previously referred to as Plateau Gravels the Silchester Gravel deposits represent fluvial deposits that are thought to have originated before the Anglian Stage glaciation of the middle Pleistocene, so more than about 478,000 years ago [ref 1].
Typically the Silchester Gravel is 2 to 4 metres thick, but can in places be between 5 and 6 metres. The base of the Silchester Gravel is an undulating erosional surface cut into the underlying Tertiary beds. They comprises a mixture of silt, sand and flint gravel with little apparent structure. Locally there are layers or beds of laminated clay and channels infilled with sand.
Clastic Sedimentary Rocks
Clastic sedimentary rocks are composed of silicate minerals and rock fragments that were transported by moving fluids: water, wind or sediment gravity flows. They were then deposited as the energy of the fluids reduced. Clastic rocks are composed largely of quartz, feldspar, rock (lithic) fragments, clay minerals and mica, other minerals may also be present as accessories.
Clastic sediments, and also clastic sedimentary rocks, are classified according to the dominant particle size and can be further subdivided by the transport and deposition process.
Most geologists use the Udden-Wentworth grain size scale to describe the clast or particle size and divide unconsolidated sediment into four fractions:
Gravel >2 mm diameter
Sand 2 to 1/16 mm diameter
Silt 1/16 to 1/256 mm diameter
Clay <1/256 mm
The shape of the particle can be described by how well worn it has been by the transport process prior to deposition. Angular fragments indicate the clast has not been transported far and smooth round particles tell us that the fragments have been transported a longer distance.
Long periods of sediment transport is also a very effective way of sorting particles, leading to a sediment of uniform size with a high degree of roundness. These can be called very well sorted or mature sediments. The greater the range of particle sizes present in a sediment then the less well sorted it is. A very poorly sorted sediment with angular clasts can be described as immature as it has not been transported much and will be found close to its source.
The Field Tasks
The location of this Earthcache is an old shallow gravel extraction pit with easy access from one of the many paths across the Common.
The path can be muddy in the wet months.
Please contact us through our profile with the answers to the following questions. Don’t wait to log the Earthcache, we’ll contact you in the unlikely event that there is a problem.
Examine the exposures of Silchester Gravel immediately south of the given coordinates. There are other exposures you might like to look at within 10 to 20m.
1) Describe the gravel clasts, look at a representative sample – what size, shape and colour are they, do you know what they are made of?
2) Estimate the proportions of gravel, sand and silt.
3) Do you think this sedimentary deposit has travelled a relatively short, medium or long distance? Explain why you think this.
Reference
British Geological Survey HMSO 1996. British Regional Geology London and the Thames Valley