teaching - Earth Science Teachers' Association

TEACHING EARTH SCIENCES ● Volume 31 ● Number 4, 2006
ESTA Conference Field Report 2
Tedbury Camp / Vallis Vale, Sunday September 17th 2006
RICK RAMSDALE
We were able to take advantage of the
small group to split the transport
between both ends of the vale and
extend the route to include the full distance
between Hapsford Bridge and Mells – and still
get back in time for a leisurely lunch outside the
Frome pigeon fanciers Flying HQ before the rain
started. As is usual with good field experiences
we raised and debated more questions than we
answered. Many thanks to Martin Whiteley, our
guide and chauffeur, and others for providing
the additional input. Briefly, the main field
activities were:
● Inspecting the erosion surface between the folded
Carboniferous Limestone and the Inferior Oolite an
extensive, more-or-less planar junction dipping gently
southwards. It clearly had an extensive erosional
history before the onset of Bajocian deposition, and
was examined at two localities.
● Inspecting the Jurassic colonisation sequence of the
erosion surface which can be deduced using crosscutting
relationships, (after the work of Copp). The
fossils are Jurassic boring worms and molluscs.
● The Triassic / Carboniferous contact was examined
and the nature of its relationship to the Carboniferous
discussed. These ‘fossil’ scree and valley fill
deposits are commonly developed on the steeply
dipping flanks of the Mendips. The enigmatic “fissure
deposits” at Spleenwort Shelter were observed:
are they pre-Triassic, post-Triassic, post-Jurassic or
more recent? The jury is still out.
● We observed the karst features of the Carboniferous
Limestone, which are poorly developed here compared
with other parts of the country. They are similar
to most other parts of Mendip, having developed
in a periglacial setting rather than the glacial conditions
that affected Carboniferous Limestone successions
further north.
● The influence of the large (and getting deeper) quarries
on the water table was reviewed. Here we had
the advantage of some very recent research and
hydrology maps. Thanks to a cunning groundwater
engineering scheme by Hanson, which drains the
quarry and also recharges the groundwater, the draw
down, which is considerable immediately adjacent
to the quarry, is almost nil elsewhere.
Geological cross-sections showing the explored underground
sections of the area between the quarry and the
surface drainage demonstrate the groundwater flow
zig-zags, alternating between the bedding and joint
planes, until it reaches the rivers. Also there are a series
of limestone sinks which drain and feed the River
Mells, depending on the season.
And finally, two questions for the next pub trivia
quiz:
What is the most unlikely site for an iron works on
the planet?
Answer: the Fussells iron works just east of Mells, and
Why are the crayfish in the Mells River so large?
Answer: they’re American.
Notes: Keep a look out next spring for Andy Farrant’s
(BGS) publication called Foundations of Mendip. It will
contain two specially modified geology maps at
1: 25,000 which allow the topography to show through,
along with a guide book that describes a number of
spectacular geological locations throughout the
Mendips. This publication will be linked to a website
containing much more detailed information on certain
localities, such as Martin’s recently completed structural
map of the Tedbury Camp erosional surface.
The ESO-S Primary material for Vallis Vale and Tedbury
Camp will be posted up on the UKRIGS website
in the next month or so. (www.ukrigs.org.uk click
; click ).
Rick Ramsdale
Email: rick.ramsdale@btinternet.com
Figure 2
Discussing the
hydrology of the
Mells River.
Figure 1
Discussing the
Inferior Oolite
23 www.esta-uk.org