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Intro ...
Why be a geologist?
Slide 1 of 40
1. See the world
Slide 2 of 40
Nice
shirt!
2. Meet all sorts of people ...
Slide 3 of 40
Southern Venezuela near Colombian border
3. Make new friends ...
Slide 4 of 40
4. Never stop learning new things !
... such as ...
... the geology of Blockley ...
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Blockley's Bountiful Jurassic Geology:
Brick Clay, Building Stone, Groundwater Springs
... and World-Class Fossils!
Dr Roger Higgs
Blockley Discussion Group
13th March 2023
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Slide 7 of 40
Evolution of life
Earth is c.130 times
older than Jurassic
4.6 billion years
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Blockley
Jurassic
Period
NB Southeast England
mega-folds:
•London Basin
•Wessex Basin
= distant compressional
effects of Alpine Orogeny
Slide 8 of 40
N Atlantic just
starting to open !
S Atlantic did
not exist yet
Only one
continent ...
'Pangaea'
Middle Jurassic
palaeogeography
c.170 million years ago
Slide 9 of 40
Site of today's British Isles
Middle Jurassic
palaeogeography
The piece of continent bearing today's
British Isles was at latitude 30-40°N,
2,000km further S (today at 50-60°N)
Shallow sea areas (grey): deposition
of shelly sand & 'oolitic' sand (like
modern Bahamas & south Florida)
Land
Shallow
sea Slide 10 of 40
Blockley
Explanatory booklet.
Buy from BGS;
or read for free online
1:50,000 geological map.
Buy from BGS website
Slide 11 of 40
Rocks layers ('stratigraphy'), Blockley region
Upper Jurassic limestone
Lower Jurassic mudstone
(aka shale)
Lias Group (of formations)
Inferior Oolite Group
Great Oolite Group
Brick clay,
plesiosaurs
'Cotswold stone'
Roofing
tilestone
+ Abundant fossils
in most formations
Wool cleansing
(oil, dirt, odour)
Major groundwater aquifer, due to high porosity,
permeability & thickness (10s metres).
Springs occur at base of Inferior Oolite limestone
(at contact with impermeable mudstone)
Minor
aquifer
RESOURCES IN RED
Slide 12 of 40
Rock hardness
(resistance to
erosion) controls
topography
Vale of Moreton = Lias Group,
mudstone (soft)
Blockley Down =
Inferior Oolite Group,
limestone (hard)
LOOKING
SOUTH
NOTE: No natural rock exposures.
Best local exposure is Bourton Hill Quarry ...
Slide 13 of 40
Heart of England Way,
descending to Batsford Arboretum
Bourton Hill Quarry
Inferior Oolite Group, limestone
Note: Beds are nearly horizontal
backpack
45cm tall
Slide 14 of 40
1 km
Moreton
Blockley Brook
Valley floor is Lias
mudstone (brown)
Flanked by Inferior
Oolite limestone
(yellow)
Blockley
brickworks
Aston
Magna
Batsford
Bourton Hill
Quarry
Slide 15 of 40
Warren
Paxford
Dovedale
Many springs
along this contact,
base of 'Inferior
Oolite' limestone
(yellow)
Blockley
Downs
Line of cross section
(next slide)
Blockley
Downs
Elevation of base of
Inferior Oolite
(essentially horizontal):
155m at Russell Spring
215m at 'Park Farm
Spring'
Difference = 60m
= vertical displacement
on 'Park Farm Fault'
(black dashed line)
Slide 16 of 40
Major fault ...
Park Farm
concealed
'Park Farm Fault', c. 60m throw.
'Normal' fault, i.e. extensional
1 km
Slide 17 of 40
WEST EAST
Cotswold Inferior Oolite, oolitic limestone,
microscope thin section. Blue = porosity
(empty space).
https://www.oolithica.com/cotswold-
carbonates/
Cotswold limestone, 2 main types
1. Oolitic limestone
Grains are sand-size spheres called 'ooliths'or 'ooids' ...
= concentric sheaths of calcium carbonate (CaCO3)
precipitated from sea water, around a nucleus (e.g. quartz
grain, shell fragment)
Microscope 'thin section' below:
•blue = porosity
•white fringe = CaCO3 cement, binding the ooliths
c.1mm
Garden wall, Blockley
Slide 18 of 40
Cottage in Blockley,
oolitic limestone
Distinctively white
Skeletal limestone ... brown
Slide 19 of 40
c.1mm
Cotswold limestone, 2 main types
2. Broken-shell limestone ('skeletal limestone')
Microscope 'thin section' below:
•blue = porosity
•white fringe around skeletal grains = CaCO3 cement
Garden wall, Blockley
Slide 20 of 40
Blockley Church
Slide 21 of 40
'Cross bedding'
made by ripples & dunes
migrating across a sea floor
or a desert
Bourton Hill Quarry
Slide 22 of 40
Underwater dunes of broken-shell sand or oolitic sand
near Dry Tortugas islands, Florida
Closest modern analogue of
Middle Jurassic depositional
environment of England =
Bahamas & southern Florida,
latitude 23-27°N
Aircraft wingtip
Shipwreck 'Arbutus',
70 feet long
Cuba
Florida Atlantic
Slide 23 of 40
Bahamas
Resources
1. Brick clay
2. Building stone
3. Springs (& spring-fed Blockley Brook)
4. Fossils
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Slide 25 of 40
NB Skeletal limestone with whole shells
which are empty ... very high porosity!
Resources ...
1. Bricks
Photos: http://www.northcotbrick.co.uk
Brickpit, Wellacre Quarry, formerly 'Blockley Station Quarry'
Opened 1925; Northcot Brick Ltd since 1952
Charmouth Mudstone Formation, c. 190 million years old
Very fossiliferous, including plesiosaurs
(cf. Mary Anning, Dorset coast, same formation)
'Site of Special Scientific Interest' (SSSI)
'Regionally Important Geological Site' (RIGS)
Slide 26 of 40
Resources ...
2. Building stone
Slide 27 of 40
Heart of England Way,
at crest of Blockley Downs
Blockley
Church
Rain
Resources ...
3. Springs
Porosity = pores spaces + fractures ... determines fluid-holding capacity (water, gas, oil)
Permeability = interconnectedness of pores ... determines fluid transmissibility (flow rate)
Rain water percolates down through porous, permeable limestone
Springs occur ... at contact with impermeable Lias mudstone below
... at ends of high-permeability 'pathways' (e.g. high porosity and strongly fractured),
not 'underground rivers' (Cotswolds caves are rare)
Main
aquifer
Slide 28 of 40
PERMEABLE
IMPERMEABLE
Blockley Downs
'Park Farm Spring'
Spring-fed
Bishop's Pond
Park
Farm
house
Hammer
Slide 29 of 40
Russell Spring
Middle Jurassic limestone (behind wall)
Lower Jurassic mudstone (under road)
Spring line near base of wall
Behind the wall, presumably one or
more springs are gathered into a single
outlet pipe
Slide 30 of 40
Blockley prospered in 18th & 19th centuries
thanks to mills powered by
spring-fed Blockley Brook
Prime source = 'Hailstones Spring'
Lateral tributaries are fed by other springs Slide 31 of 40
mill
race
mill
All Blockley fossils were shallow-marine animals ...
Resources ...
4. Fossils
Valuable ...
(A) intrinsically and
(B) geologically, e.g. studies of evolution; 'correlation' to other places
Slide 32 of 40
Ammonite (fragment).
Swimmer
Top Bottom
Echinoid (sea urchin)
Clypeus ploti
Burrowed in sandy seabed
Brachiopod.
Lived on seabed,
attached to hard
surfaces
Slide 33 of 40
Pholadomya sp.
Burrower
?Pseudopecten sp.,
fragment
Trigonia sp.
fragment
Various bivalve molluscs (aka pelecypods, lamellibranchs)
Slide 34 of 40
Plesiosaurs
-Extinct Mesozoic marine reptiles (underwater counterparts of land dinosaurs)
- Two discovered in 1990, Blockley brickpit
- A third in 2000, much more complete
- See my article in 'The Voice', March 2023
Slide 35 of 40
Discovered in 2000
c. 85% complete
(white = missing)
c. 9ft long
Bought by Leicester
Museum, £40,000
Skull
Scale,
15cm/6in
Blockley plesiosaur no. 3
Slide 36 of 40
Blockley plesiosaur no. 2
Discovered 1990, then disappeared.
Offered in a 2010 Sotheby's auction, Paris, on behalf of an anonymous private German museum.
Estimate EUR 330,000-370,000 !
Sotheby's catalogue photo
Internet does not reveal:
... if it sold
... where it is now
Cast replicas of this 'semi-
replica' are sold by a
South Dakota company, at
US$35,000 each
c.9ft long
50% 'false' ... missing
parts (including skull)
are represented by
casts from other fossils
Rebuilt in 3D
Slide 37 of 40
Temperature,
global lower
atmosphere average
(satellite)
2020-23 temp.
suppressed by
La Niña
Will cause
slight cooling
until 2030? But
China & India
both rapidly
building more
coal-fired
power stations.
'Stair-step' warming mimics coal consumption, not CO2.
i.e. coal burning is warming the world, due to soot emissions (inefficient capture).
Sun warms airborne soot, in turn warming the atmosphere.
2020
2020
1990
2020
c.10-year lag
Green =
mostly
China
CO2,
ppm
Coal,
global
consumption
by region Slide 38 of 40
Suggestions ...
1. Future BDG talk by Thames Water?
History and infrastructure of their operations in Blockley,
extracting spring water for household & industrial use
2. Start a 'Blockley Fossil Group'?
Meet once or twice a year to admire & identify each other's new finds?
Starting this summer ...
'Blockley Cotswold Geological Walks', Saturdays, 2.5 hours, c. 3 miles, £20
For a flavour, google my 'Bude Geological Walks' (since 2009)
Contact ... rogerhiggs@hotmail.com Slide 39 of 40
Thank you for coming!
Nice
shirt!
Assam,
Digboi
Oilfield
Slide 40 of 40