Geodiversity Audit of Active Aggregate Quarries
Quarries in Devon
Project Overview Report
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2.7.
Permian and Triassic Rocks of East Devon
The geographical division of Devon into the predominantly grey soils of relatively old rocks in
the west and the distinctly red soils of younger rocks in the east owes its character to these two
geological systems. Commonly called the New Red Sandstone, the ‘red beds’ of Permian and
Triassic age demonstrate the characteristics of having formed in a continental desert or semiarid environment which covered most of Britain and northern Europe between 290 to 210
million years ago. This new environment was the direct consequence of the Variscan
continental collision and mountain building episode which brought to an end the
predominantly marine conditions of the Devonian and Carboniferous. The supercontinent of
Pangaea came into being.
Building sand has been worked for many years at Bishops Court Quarry near Exeter and large
deposits of alluvial sands and gravels have been worked at several places from the Triassic
Budleigh Salterton Pebble Beds Formation, notably the working quarries at Blackhill, Hillhead
and Whiteball.
2.7.1
Bishops Court Quarry
Sand dune cross bedding
Photo BI 11a
From SX 9639 9156
Facing WNW
The Dawlish Sandstone Formation of Permian age, about 260 million years old, worked at
Bishops Court Quarry displays large scale cross bedding as shown in the photo. These are
classic desert dune sands comprising red and yellowish brown, well to moderately-well sorted,
medium grain-size sand with occasional layers of fine silty sand and red-brown mudstone. The
sand grains are mainly quartz with minor heavy minerals. Coarser grains are often spherical
(millet seed sand) with frosted surfaces. The prevailing wind direction at the time has been
shown by measurements of cross bedding directions to have been from the south east. The true
dip of the beds is difficult to measure directly because of the cross bedding, but occasional thin
interdune mudstone beds, probably water-lain, indicate a shallow, but variable, dip to the east.
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Geodiversity Audit of Active Aggregate Quarries
Quarries in Devon
Project Overview Report
___________________________________________________________________________________________
Bishops Court Quarry
Photo BI 07d
Detail of sand deposit
From SX 9647 9147
Facing NE
The photo shows a close view of typical medium grain-size sand with a 1mm thick black
manganese-rich band. Many of the sand grains are well rounded and often spherical (‘millet
seed’ sand grains) believed to be a characteristic of coarser varieties of wind blown material.
Most of the Dawlish Sandstone Formation is very weakly cemented and relatively
unconsolidated and can be dug by hand, yet historically the pit faces remain moderately stable
at angles more than 80°. It is possible that the deposit gains bulk strength from the numerous
thin (5mm to 20mm) iron and manganese cemented horizons that are shot through the deposit.
Bishops Court Quarry
Photo BI 08a
Iron /manganese cemented bands
From SX 9649 9147
Facing NE
Rain and wind erosion has sculpted the exposed face leaving the irregular iron and manganese
cemented bands standing proud of the rock face.
Localised iron and manganese cementation of the sand occurs sporadically throughout the
deposit at Bishops Court Quarry. The nature of cementation generally falls into two types.
•
Thin planar horizons ranging in thickness from 20mm to 80mm.
•
Very thin and impersistent, sinuous, non-planar and migrating horizons 5mm to 20mm
in thickness that run both parallel to and across all bedding features and structures.
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January 2004 2237/30 PO
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Geodiversity Audit of Active Aggregate Quarries
Quarries in Devon
Project Overview Report
___________________________________________________________________________________________
2.7.2
Blackhill Quarry
Typical conglomerate face of the Budleigh Salterton Pebble Beds Formation with interbedded
silty-sand horizon
Photo BL 12a
From SY 0637 8528
Facing NW
The Budleigh Salterton Pebble Beds Formation (BSPBF), of early Triassic age about 250
million years old, is quarried at Blackhill Quarry, Woodbury, East Devon. The 30m thick
deposit forms the lowest division of the Sherwood Sandstone Group in the area.
The BSPBF comprises red and purplish brown, sandy to very sandy, poorly sorted, fine to
coarse gravel with occasional fine silty-sand horizons as seen in the photo. The clasts comprise
a dominant proportion of well-rounded quartzite with much lesser proportions of vein quartz,
sandstone, gritstone and igneous rock gravel. Occasional boulders are up to 0.4 metres.
The pinkish coloured quartzite clasts forming the bulk of the deposit contain Ordovician shelly
fossils which can be matched to sources in north-west France. Coupled with evidence from
sedimentary structures such as cross bedding and channel directions, this indicates transport
and deposition of the deposit in a powerful northward-flowing braided river carrying a heavy
load of coarse sediment.
The deposit dips gently eastwards about 3° to 4° and rests directly on the Littleham Mudstone
with a pale green leached zone up to 1.5m thick at the contact. The mid Triassic Otter
Sandstone which overlies the deposit to the east has been eroded from the quarry area.
Most of the BSPBF is very weakly cemented and relatively unconsolidated, yet historically
remains stable for many years in pit faces with face angles in excess of 70°.
A north-south groundwater divide corresponding to the topographic ridge at Blackhill sheds
water to watercourses both to the west and east. In the Otter Valley to the east, the BSPBF
together with the Otter Sandstone forms an important aquifer and groundwater resource.
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January 2004 2237/30 PO
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Geodiversity Audit of Active Aggregate Quarries
Quarries in Devon
Project Overview Report
___________________________________________________________________________________________
2.7.3
Hillhead Quarry
Detail of Hillhead-type conglomerate
Photo HI 11c
At ST 0611 1375
Facing SW
At Hillhead Quarry near Uffculme, some 30km north of Blackhill, the photo example of
typical conglomerate shows a difference in colour from the Blackhill photo but this is largely
an artefact of different weather conditions on the day and the colour is not so different.
The main difference at Hillhead is that the conglomerate beds in the BSPBF are less coarse,
less well sorted, and contain a greater variety of clasts, including rock-types not seen at
Blackhill and likely to be from local sources. These include Upper Carboniferous sandstone
and a small proportion of weak, white and friable de-calcified limestone pebbles, the lime
content having been completely dissolved away. A 10cm example, larger than usual, is shown
above.
The decalcified limestone pebbles are important because they must have been hard when
transported and deposited and the decalcification has taken place in situ after deposition. The
pebbles contain silicified remains and moulds of shelly fossils such as crinoids and corals but
the source has not yet been identified.
Nevertheless, the conglomerate also contains a substantial proportion of quartzite clasts of the
type identified as having a source in northwest France. This, together with flow direction
indicators in the sedimentary structures, confirms transport and deposition in a powerful
braided river, probably seasonal, flowing from the south, but in this case with significant local
tributary contribution.
Borehole evidence from intersection of the top and base of the deposit demonstrates an
eastwards dip of 3o to 4o, closely similar to that at Blackhill. The top and base of the BSPBF
are both seen at Hillhead. The deposit here is up to 34m thick.
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Geodiversity Audit of Active Aggregate Quarries
Quarries in Devon
Project Overview Report
___________________________________________________________________________________________
Hillhead Quarry
Photo HI 10
Otter sandstone contact at top of BSPBF showing yellow sandstone bed
with red sandstone above and complex micro-faulting
At ST 0623 1368
Facing SE
The yellow sand horizon is similar to that seen at the base of the Otter Sandstone in the sea
cliff at Budleigh Salterton, here resting on purple sandy clay with conglomerate beneath. Micro
faulting displacing the bed of yellow sand is similar to faulting on a larger scale affecting the
BSPBF in the east side of the Hillhead Quarry area, known as Houndaller. The small faults are
lined with fine granular manganese.
Hillhead Quarry
Aylesbeare Mudstone contact at base of BSPB showing pale-green
leached layer at base of gravel
Photo HI 23b
At ST 0685 1392
Facing North
Contact of BSPBF with underlying Aylesbeare Mudstone Group with the usual pale-green
leached zone 0.4 m thick at the base of the pebble beds. Minor groundwater seepages occur all
along the contact margin. The lighter coloured top layer is disturbed ground.
This lower contact is not seen in the natural coastal exposures because of landslipping.
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Geodiversity Audit of Active Aggregate Quarries
Quarries in Devon
Project Overview Report
___________________________________________________________________________________________
2.7.4
Whiteball Quarry
Working area showing leached base of deposit in quarry floor and top of deposit near the tree
at top left
Photo WB 08b
From ST 0790 1674
Facing SSE
The base of the deposit is also exposed at the current working area at Whiteball Quarry, Town
Farm Pit about 3km northeast of Hillhead,. The pale greenish layer at the base is seen in the
floor of the pit marking the working floor. A thickness of the BSPBF up to more than 20m is
recorded in boreholes, the highest beds having been eroded.
Whiteball Quarry
Detail of iron cemented BSPBF
Photo WB 20k
At ST 0846 1826
Facing SE
Patchy occurrence of iron-oxide cemented gravel in the northern quarries at Whiteball and
Hillhead can cause operational difficulties in excavation because of its hardness, and quality
control in the product because of the agglomeration of particles. Further north the deposit
becomes heavily cemented and working would be impractical.
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January 2004 2237/30 PO
DAVID ROCHE GeoConsulting
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