| The photos on these two
pages are of the site at Itchington, a short distance east of Thornbury,
and of the information board which WEGA has caused to be made and
positioned. The board, made by Landmark Design at a cost of £2000 is
intended to promote interest in geology and perhaps to encourage people
to join WEGA.
The site has been well cleared by Messers. Ropeworks of Bristol at
minimal cost to WEGA. (One of Ropeworks "leading lights" used to be our
treasurer!)
From time to time volunteers will be required to remove grass and weeds
from the face of the exposure - don't throw away those gardening gloves!
What the Board Says
Rocks A - Carboniferous Limestones
The lime-rich sediments forming the lower part of
this face were laid down on the sea-floor about 350 million years ago
when this region was close to the equator. They were then buried to
considerable depth and became hardened into solid rock. They have been
extensiveley quarried over many centuries for roadstone and other
purposes.
About 280 million years ago these rocks, and those of the whole Bristol,
Mendip and South Wales region, began to be tilted, fractured and
uplifted by major earth movements (continental collision). As a
consequence the tilted beds you can see now flank a small downfold or
basin (part of the Bristol coalfield Basin). The other edge of the basin
can be seen near Chipping Sodbury.
Rocks B - Dolomitic Conglomerate
The movements raised the limestones and associated beds above sea-level
so that they became subject to intensive subaerial weathering and
erosion for a very long period of time. The equator now lay further
south and the climate was semi-arid, almost desert-like. Several
thousands of metres of rock were removed from the higher hilly areas,
such as the site of the Mendips, and deposited in nearby gullies and
valleys, often as scree and breccio-conglomerate. The near-horizontal
deposits at the top of the face in front of you, and resting on the
up-tilted Carboniferous Limestones, are examples of these erosional
products and are known in this region as the Dolomitic Conglomerate
though at this locality large rock fragments are generally absent. The
main constituents are derived directly from the underlying limestones.
In addition, diffused dolomite mineral grains can be present having been
introduced later from invading mineralising fluids coming from outside
sources.
The Dolomitic Conglomerate is of Triassic age and is between 240 and 220
million years old. The boundary betweenh the two discordant sets of
sediments is known as an angular unconformity and represents a time-gap
of at least 50 million years during which time erosion predominated over
deposition. This identifies it as one of the really major unconformities
of the British Isles.
The Ongoing Saga
Avid readers of the Newsletter will have seen that
WEGA has set up a notice board at Tytherington, indicating the presence
of a spectacular unconformity between the Carboniferous Limestone and
the Dolomitic Conglomerate. Time, Mother Nature and WEGA move on, and
accruals of vegetative matter have been removed by the stakhanovite
efforts of John Toller, Glo Castle, Sandra Stead, Babs Bennett and
someone else.
Keep a date in your diaries for September 2003 so that you too can
become a hero(ine) of geological labour. Give the traffic on the M5
something to look at! We might try to make it a bit more fun-filled! A
barbecue and a couple of beers has been suggested.
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