Harefield SSSI And Northmoor Woods RIGS
Led By Bryan Cozens
19th October 2003
Harefield
pit was one of many chalk quarries in the area which produced lime for the
cement industry. The pit was used as a landfill site and was filled by
approximately 30 years ago, after which only the top of the Upper Chalk and
overlying Upnor and Reading Formations and London Clay are exposed in the
remaining eastern wall. It was designated as an SSSI due to the exposure of a
section through these deposits and is the stratotype locality for part of the
London Clay. Since then the land has been grazed by cattle and the exposures
became overgrown. Bryan’s group, the Harrow and Hillingdon Geological Society,
have cleaned up some of the exposures which are now in pristine condition.
At the time
of deposition of the top of the Upper Chalk during the Santonian, about 86 Ma
ago, Britain was in tropical latitudes and sea level was relatively high. Marine
regression exposed the Chalk. There is less Lower Lambeth Group here than
further east and south, eg. at Newhaven, as less deposition occurred. More Chalk
was also removed here due to prolonged uplift and erosion further inland. There
is therefore an unconformity between the Chalk and the Palaeocene Upnor
and
Reading Formations (formerly the ‘Reading Beds’). This was the first of several
transgressions during the Tertiary.
The Upper
Chalk contains large flints and burrows. Bryan was keen to hear our thoughts on
these burrows as they have bee attributed to various species in the past.
Originally thought to have been produced by annelid worms and named
Terebella harefieldensis,
they were later thought to be
Thalassanoides
made by
crustaceans. There followed a lively discussion and a marine annalid worm was
thought to be the most likely burrower, there being no fossil evidence as it was
soft bodied. The burrows were in various orientations but mainly sub-vertical.
On the very
clean exposure the Upnor and Reading Formation is well exposed allowing the fine
lamination and stratification of the deposits to be seen. Current bedding and
channel deposits are clearly visible. The basal Upnor Formation is shallow
marine with glauconite. This is overlain by a pebble bed of glauconite-coated
flints. A fall in sea level is indicated by evidence of marshy mudflats which
are cut into by river channels containing plant-rich seams including rare leaf
impressions. Deposition in lagoonal setting in a warm wet climate is indicated.
Channel clays are mottled and contain the only known
Charophytes
in the
Reading Formation. These indicate deposition in non-marine/brackish lagoons.
Bryan told
us that calcretes had been noted by the Tertiary Research Group. These were
formed by water being drawn up through the chalk carrying dissolved carbonate
which precipitated in distinct horizons in the Tertiary beds. Surface crystals
of halite were observed on the exposure and the age of them were discussed. It
was concluded they were formed by salt coming out of the clean face. Other
observations included mud casts which were granular rather than clay, micaceous
roots/stems and other plant material, and stone warts representing an algal
complex.
Further
round the face the London Clay deposits have been covered up by a land slip. The
HHGS plan to clear it next year.
Coming away
from the quarry face, Bryan described the changing direction of the proto river
Thames as it responded to glaciations. One million years ago the river flowed NE
and several terraces were created. During glaciation ice blocked the river’s
path around the Watford area, forming a lake in the tundra south of the ice. The
backed-up water forced a way to the SE. During interstadials outwash retreated.
There were 3 episodes of this during the Anglian glaciation. This created the
Colne river which now flows in reverse direction into the Thames. The gravels
that accumulated during the glaciations were extracted during the 1920’s and
transported along the Grand Union canal.
Bryan also
talked about man’s impact on the area, particularly the industrial activities
which have accompanied the building of the Grand Junction Canal which is close
by. The canal was used to transport chalk for cement for building in London in
the 19th
century and gravels in the 20th
.
Lakes filling the gravel pits are now used for water sports. Mills on the river
Colne ground corn, milled paper and later copper.
We walked
along the canal a short way to see Weybeards Pit from a distance before going
there. This pit has been filled with houses rather than rubbish. The chalk face
has been retained and can be accessed. Here the chalk is blocky in appearance
with flint layers and faulting in conjugate sets. Those who had been to Newhaven
with Rory Mortimer could see the similarity to the chalk there, but it is not
known which part the Upper Chalk is present here.
After
lunch, we drove to Northmoor Woods, a RIGS site. A simple geology trail leads
past a swallow hole to an old former chalk pit which also shows the unconformity
at the junction between the Upper Chalk and the Upnor and Reading Formations.
Unfortunately in stark contrast to the exposure at Harefield, this site is in
poor condition, having been damaged by quad bikes. We could see massive flints
similar to Harefield but no sign of burrows. Above the chalk the gravels and
sands of the Upnor Formation can be seen. Back at the car park we thanked Bryan
for his very interesting trip.
Marnie White
Back to the top
Mam Tor Winter Weekend
31st October - 2nd November 2003
Led by Chris Arkwright
Saturday
1st November
On a
clear sunny day our party set out for our first stop at the back scarp of the
Mam Tor landslip (SK 131 835). We looked out over the exhumed topography of the
Lower Carboniferous as Chris gave us a brief history of the geology of the area.
Uplift and erosion in Tertiary times has revealed the southern edge of the Lower
Carboniferous platform with lagoonal and basin sediments interspersed with
various reef types. Evidence of marine volcanism in the form of ash and lava can
also be found.
The steep
back scarp of the Mam Tor landslip can be inspected at this location, with Mam
Tor beds overlying Edale Shales. The back scarp is very impressive here, a near
vertical cliff about 100m. high. The Mam Tor beds are down faulted against the
Shales at this point. Inspection of the exposure indicated that the Mam Tor beds
show repeated coarsening upward sequences with flute marks on the bases
characteristic of turbidites. The Edale Shales could be seen in a mudflow to the
left of the scarp. These are organic rich marine mudstones containing pyrite and
gypsum.
Our
second stop was at the small quarry at Windy Knoll (SK 126 830). Back reef
fossiliferous limestones overlain by a boulder bed are exposed at this site
which is famous for the discovery of Pleistocene bones in a cave in the 1870’s.
Many fossils could be seen in the limestones here. Solitary corals, bivalves,
crinoid debris, and algal mats were all identified. At the top of the quarry
hydrocarbons could be identified.
We walked
on through Winnats Pass (SK 136 827). The pass is an exhumed channel in the reef
complex of the Carboniferous platform edge. Mine adits here indicate that the
reef has been heavily mineralised. Beds are almost horizontal at the west end of
the pass, but dip sharply to the east as the edge of the reef is approached just
above Speedwell Cavern.
Our next
stop was a well-deserved lunch stop at Treak Cliff Cavern (SK 137 830). From
here we walked on to Odin Mine (SK 134 834). The mine is a 2m. wide strike slip
fault, which was worked for galena, barite and flourite from the 13th
century until 1869. Slickensides on the sides of the mine indicated movement on
the fault, and traces of mineralisation could still be seen on the walls. Chris
explained that the source of the mineralisation was from late Carboniferous
hydrothermal fluids picking up minerals from anoxic basin clays and depositing
them in up-dip fissures along the edge of the carbonate platform.
Passing
along the lower part of the old A625 Manchester to Sheffield road the true
magnitude of the slide began to be appreciated. This part of the road is still
used as access to Mam Farm, but is ridged and cracked by continual movement of
the earthflow. Chris explained the purpose of the many survey points seen along
the route. These were installed in 1996 and have been read in the spring of each
year using electronic measurements of distance and angle. The results of the
survey indicate that sections of the landslide move at different rates and
directions, with maximum movements of approx. 1m. occurring every four years
when the water table is high and the main slip planes are reactivated.
We
continued to climb up to the upper level of the A625, now closed, where we were
met with a scene of utter devastation! Rotated blocks of Mam Tor beds dipping
back towards the scarp have displaced the road surface by several meters.
Evidence for many years of failed remedial measures was seen in the form of
herringbone drains and successive layers of road construction, in places over a
meter thick. Lots of photographs taken here!
We ended
the day with a view of the Lafarge Cement Quarry, courtesy of one of our local
members who works at the quarry.
We had
had an excellent day thanks to Chris, including a detailed inspection of one of
the most spectacular landslips in the UK as well as a splendid overview of the
geology of the area.
Brian
Harvey
Sunday
2nd November
As a
complete novice to the study of geology and living close to the Peak District, I
hoped that a weekend field trip with the London branch of the Open University
Geological Society led by Chris Arkwright would help throw some light on the
mysteries of geology within the context of the Carboniferous period in
Derbyshire. The experience was indeed illuminating and surprisingly enjoyable in
a number of ways. The following account describes the group’s visits on the
Sunday to 2 sites in the White Peak. The sites are of interest because they are
good examples of relatively rare pockets of volcanic activity in what is
literally a sea of limestone covering the whole of the White Peak District. OK,
so what did I learn?
The first
site, Calton Hill quarry (SK112709), now designated a SSSI is of particular
interest because of the dolerite found there and which was quarried extensively
between the 1920’s and 1990’s. The dolerite at Calton Hill is specifically
described as ‘basanite’, an analcitic olivine dolerite which displays clear
hexagonal columnar jointing (both vertical and combajoint). In fact there are 3
types of igneous rock which can be identified in the quarry from what is
generally thought to be 2 main phases of volcanic activity. The first phase,
during the Visean period resulted in a mixture of tuffs, agglomerates and
basaltic lava flows of the Upper Miller’s Dale Lava series on an Asbian Low
Limestone surface. The later phase during the late Westphalian period involved
the intrusion of the basanite through old vents into the previous eruptions. The
basanite contains olivine-rich (ultramafic) nodules from the upper mantle which
are difficult to find due to weathering which has left small angular cavities up
to 5 cms in diameter. Silica –rich, hydrothermal late-stage mineralization is
clearly indicated in vesicles and fissures. The lavas contain quartz, calcite,
barite, and pyrite. The basanite contains analcite and chlorite, the latter a
greeny-black mineral leading to the description of the rock as ‘toadstone’.
There are
several good examples of spheroidal weathering of the dolerite, a reddish-brown
colouration indicating high levels of iron and greenish colouraton indicating
fluorite.
The
second site at Tideswell Dale quarry (sk155738) is also a dolerite site,
interpreted as a sill, known as the Tideswell Sill. The rock is about 12m thick,
a coarser and younger variety than the Calton Hill dolerite. In a gulley
adjacent to the picnic site can be seen an interesting layer of clay
‘marmorised’ by a later intrusion of lava.
The
weekend was very informative, the group very knowledge, everyone was only too
happy to help answer a long list of queries, and the experience was extremely
useful. The weekend was also great funvery pleasant and interesting company, in
a beautiful location with good food (and drink) thrown in. I’m looking forward
to my next field trip and hope it lives up to my first experience with LOUGS.
Duncan
Renwick
Back to the top
