Essex Rock & Mineral Society, Field
Trip
Grays Chalk Quarry, Sunday 23rd May 2004
Leader: Gerald Lucy
Our
first stopping point was this vertical exposure of Thanet Sands (60Ma
approximately), which was capped (unconformably) by Thames river gravels
dating from the Ice Ages. The gravels cannot be clearly seen in this view
as they are fairly well hidden by the undergrowth.
Our leader, Gerald Lucy, explained the geological history of this exposure. A tale of the former course of the Thames and how these river gravels had survived whilst the river system carved through the landscape to its present day course.
Of particular interest was the base of this section. Although covered mostly by talus and overgrown by indigenous plants, it had been cleared (not by us) in one spot to show the junction with the chalk floor.
This
photograph is of the Cretaceous/Tertiary boundary.
The age of the chalk was said to be about 80 million years old.
The next layer up is the Bull Head Bed, a layer of glauconite (green) stained nodular flints in a clay matrix. These represent the base of the Thanet Formation and were deposited about 60Ma ago.
It is perhaps the nearest one can get in Essex to the KT event of 65Ma. All the same, this horizon is missing as 20 million years of geology had been eroded away before the bull head flints were deposited at this location.
For me it was a joy to see the Bull Head Bed, and only 5 miles from home, it sort of filled in the bit that was missing at Reculver, Kent (Herne Bay trip 2004). I had intended to go to Pegwell Bay to continue to chase the sequence down to this Major Unconformity, the boundary of the Cretaceous and Tertiary Periods, the change from the Mesozoic to the Caenozoic Era. No need now. But the next question to answer is: "Are there any older Tertiary exposures than the Bull Head Bed reasonably local i.e. England? " We have or will have had a trip to look at Chalk and Tertiary deposits in and around Newhaven on the 10th October 2004. Perhaps the experts can tell me then?
We
crossed over the Devonshire Road and entered the old Grays Chalk Quarry
to do a bit of weeding!
Now
what's under this mound of Ivy? Ron Flack is tearing it away with gusto.
Other mounds were getting similar treatment by us all.
After
a thorough cleanup, a "Sarsen Stone" is revealed.
The mammilated surface is well preserved in the specimens at this location. A testament to the fact that they are not that far travelled from there original source.
Another
Sarsen Stone, note the 50 pence piece sitting on one of the bumps.
Gerald explained the current interpretation; that they formed in the Woolwich Beds during hot arid conditions during the Miocene. The water table stabilized at one level in these sands. Given ground water enriched with silica, and the right environmental conditions, triggered the chemical reaction to cement the sands together.
This was a "cold rock forming process" not to be confused with metamorphic processes.
The mammilated surface is thought to have developed during cementation. (Expansion during crystallization). Which begs the question, what does the underneath look like? I will return on day. Another interpretation states the sands were originally deposited onto a surface that was scoured to the inverse shape of the surface we now see. I don't buy this, the shapes are just too bizarre for the mould theory in my opinion, (which mostly isn't worth nought.)
How did they get to their current location? "Gerald explained the were removed as part of the overburden of Thames sands and gravels (they wouldn't have come from the Thanet sands as these were deposited earlier than the Woolwich beds) as they quarried for the chalk. The stones were placed along the rim of the quarry. (could it be to provide sort of cairns to mark the edge?) The one above has not moved in over one hundred years, there is a photograph dating from 1903 in which it appears much the same.
The
quarry is managed by the Essex Wildlife Trust to preserve an outpost of
chalk loving fauna and flora in Essex.
Pictured here is an example of one of those species, a "man orchid".
Photograph courtesy of David Turner
Later
in the afternoon we went in search of more Sarsen Stones to Davy Down,
South Ockendon. There is a prime example in front of the old pumping station
(now a visitor's centre). This Sarsen came from the old Marley depot not
too far away, where it was once perched outside their main office.
Photograph courtesy of David Turner
Gerald
led us on further into the wilds were we found a whole ridge of partially
covered Sarsen stones which seemed to have been placed to one side, possibly
as part of the old sand quarry workings.
The above just about covers the day, except that sometime was spent looking for fossils in the chalk of Grays quarry.
All text and pictures © Roger Coleman, January 2005.