Dog Rocks can be found on Plumstead Common. They are so called because of their shape seen in silhouette. This relatively type of rare rock formation is a RIGS, or Regionally Important Geological Site, which are selected according to the following criteria:
The value of a site
- for educational purposes in life-long learning
- for study by both amateur and professional Earth scientists
- in terms of historical advances in Earth science knowledge
- in the aesthetics of the landscape, in relation to promoting public appreciation of Earth sciences
Perfect material for an Earthcache, then.
A bit of local geological context
After a rise in sea-level approximately 56 million years ago, shallow marine conditions were established in this area once again. The crashing waves pounded existing flints into rounded pebbles and, in what is now the Lewisham, Greenwich and Croydon area of south London, this debris was deposited as the sea receded.
These fragments form the Blackheath Beds, which are composed of black well-rounded pebbles of flint in fine sand, with occasional seams of shells. They largely accumulated in marine channels that were cut into the underlying Woolwich Beds (which are formed of sands and fossiliferous shelly clays). Blackheath Beds vary in depth from 5 – 27 metres; depending on the depth of the channels they were infilling.
Blackheath Beds become puddingstone
Occasionally, these pebbles become bonded together by a naturally occurring cement formed of silica. This conglomerate rock is called puddingstone, because of the way the embedded flint pebbles resemble plums in a Christmas pudding (represented by the cement). Unusually for a conglomerate, because the pebbles, the fine sand and the “cement” are all made of silica, the rock has a uniform and coherent density. Thus, when it is broken, it fractures straight across the matrix and pebbles, unlike most other conglomerates, which break unevenly.
Puddingstone is rarely found in situ in the strata in which it formed, but its hardness has preserved examples which have been deposited after being transported by rivers or glaciers. These are commonly found in river beds, and less frequently exposed at the surface. Dog Rocks is one such less common occurrence.
So to the learning bit…
Question 1
Observe the surface of one of the Dog Rocks. Describe its properties in terms of texture and colour.
Question 2
Look at the flint pebbles which form the largest piece of puddingstone. What are the lengths of the largest and smallest stones forming the conglomerate?
Question 3
From the downhill side of the largest piece of puddingstone, what do you estimate its height to be? Why might this be significant?
If you wish to post a photo of yourself at GZ, please try not to include detail which gives away answers to the above.
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SOURCES
http://www.naturalengland.org.uk/ourwork/conservation/geodiversity/englands/counties/area_ID14.aspx
www.greenchain.com/pdfs/Geologytrail.pdf
http://en.wikipedia.org/wiki/Hertfordshire_puddingstone
http://www.ehgc.org.uk/html/puddingstone.htm