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Page 1 of 5
BARROW HILL L.N.R., DUDLEY: EARTH SCIENCE BRIEFING FOR THE SITE
© UKRIGS ESO-S Project
Introduction
At any one site it is helpful to think of interpreting the evidence preserved in the rocks as a recurring pattern
of events, often referred to as The Rock Cycle. These events are:
a) Transport and deposition of weathered and eroded fragments, to form new sedimentary rocks;
b) Deformation (including folding, faulting, intrusion by igneous rocks or metamorphism); and
c) Uplift, weathering and erosion, leading to deposition of new sedimentary rocks at the beginning of the
next cycle.
The evidence for the events in The Rock Cycle can be “read”, or interpreted from the rocks in any exposure.
However, some parts of the story are always missing, because geological evidence has many “gaps” in it
caused by a combination of never having been deposited and preserved in the first place, or loss by later
erosion, and by the fact that much is still buried and unknown. This means it is important to remember that
the interpretation at any one site is but a fragment of a single Earth Science story that has an “invisible
“prologue” and “epilogue” each millions of years long, but for which we cannot see the evidence at any one
site, because it is not available to us.
The youngest sedimentary rock to be deposited on the Barrow Hill site is the Etruria Marl (a clay), which was
formed about 300 million years ago, and shortly after was intruded by a basic igneous magma which
crystallised to form dolerite. This visit investigates the fragment of the Earth Science story from the rocks
remaining on and around Barrow Hill, and the evidence for it, as well as the uses humans have made of
stone.
Transport and Deposition.
Towards the end of the Carboniferous period, about 300 million years ago, and after the end of the main coal
forming events, the area now known as Barrow Hill was the site of deposition of beds called the Etruria Marl.
These beds are mainly fine grained clays, with some cross bedded sandstones and very thin coal bands.
The clays vary in colour from yellow, purple and red, to grey, and contain fossil evidence of plants and
rootlets. The vegetation type was mainly delicate plants classified as conifers. When the water table on the
floodplain was high, the muds were grey, whilst, when the water table was lower, the iron in the clays
oxidised to reds and yellows in colour. Some of this colour variation can still be seen today in the sides of
ponds and footpaths across the area.
The coarse grained sandstones in the Etruria Marl were laid down mainly in river channels cut into the clays,
each sandstone becoming finer towards the top. This indicates that the river strength was initially high, but
then slackened off. The beds are cross bedded, and ripple marked, indicating a flow towards a north easterly
direction. These sandstone beds, locally called “Espleys”, are evidence for the river channels cutting across
the floodplains.
These clay and sand beds indicate a floodplain environment, with river currents transporting sands along the
channel, and depositing mud onto the floodplains during times of high water. On the floodplains plants were
growing on top of the soil horizons that formed, and then frequently were buried, eventually to form thin
coals, as more sediment was deposited at the next flood. This landscape was then buried by debris from a
volcanic episode.
On top of the Etruria Marl close to Barrow Hill lie volcanic tuffs (fine ash) and then volcanic breccias (made of
larger blocks, called volcanic bombs), indicating the sudden onset of nearby violent eruptions. This material
blanketed the river landscape and buried it under more than 30 metres of volcanic sediments. (See Figure 1)
Exposures of these sedimentary and volcanic rocks are not accessible today. The soft Etruria Marls are
covered with vegetation, and, where the overlying beds were once exposed in a clay pit to the west of Barrow
Hill, they are no longer visible.
Deformation.
At the time of the volcanic eruptions at the surface there was an intrusion underground of a magma which
later cooled and solidified to form a basic igneous rock, called dolerite. It is this dolerite, now exposed by
erosion at the surface which was quarried on Barrow Hill. The composition of the bombs in the volcanic
sediments thrown out on top of the Etruria Marl, indicate they are chemically related to the volcanic rock at
Barrow Hill. This dolerite intrusion seems to represent a solidified underground part of the magma flow which
fed the volcanic eruptions at the surface.
Embedded in the solidified magma are lumps of sedimentary wall rock up to 5 metres across, called
xenoliths. Xenolith means “strange rock” and here they are lumps of sedimentary rock broken away from the
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BARROW HILL L.N.R., DUDLEY: EARTH SCIENCE BRIEFING FOR THE SITE
© UKRIGS ESO-S Project
roof or walls of the intrusion, as the liquid magma was emplaced. They sank into the magma becoming
baked and trapped as the igneous rock solidified. These xenoliths, many of which are Etruria Marl, show
contact metamorphism, and appear today baked like terracotta. Otherwise contact metamorphism by these
igneous bodies is very slight, and may be related to the wet condition of these newly formed sedimentary
rocks, suggesting they were close to the surface at the time of the intrusion. The presence of water in the
sediments seems to have reduced the heating and metamorphic effect of the magma which was probably at
a temperature close to 1000 degrees Celsius when it was intruded. It is thought that the dolerite formed
about 1000 metres below the Carboniferous land surface.
Figure 1. Sketch of the present day outcrops and the eroded Barrow Hill volcano.
Liquid magmas rise through the rocks of the Earth’s lithosphere because, being hot and liquid, they are less
dense than the surrounding rocks. Often these intrusions will follow lines of weakness in the rocks. In
several places this igneous intrusion is guided by fault lines, some of which seem to have occurred as the
intrusions happened. The faults are normal faults, which are caused by tension, and result from brittle
fracture during a stretching of the crust. This period of extension coincided with the period of volcanicity,
which might have lasted for as short a period as only one million years.
After this period of intrusion and faulting, the area was subjected to a period of uplift and erosion. There
then followed nearly 300 million years of Earth history, unrecorded in the rocks here, involving more cycles
of deposition, uplift and erosion and deposition of rocks which are now eroded completely away from Barrow
Hill. However, not far away to the west, are outcrops of the red sandstones, deposited during the Triassic
period, and which provide evidence of the start of a later rock cycle that has been completely removed from
Barrow Hill by an ancient period of erosion.
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BARROW HILL L.N.R., DUDLEY: EARTH SCIENCE BRIEFING FOR THE SITE
© UKRIGS ESO-S Project
Uplift, Weathering and Erosion
The high level volcanic structures of the Barrow Hill volcano, such as the cone itself and any lava flows are
no longer visible. They have been destroyed by erosion. The igneous structures now visible at Barrow Hill
represent a level, perhaps as much as a kilometre below the eruption surface. (See Figure 1)
The rocks in the area have been uplifted more than once and eroded into the present day landscape. The
more resistant dolerite now forms the higher parts of Barrow Hill, reaching 178 metres above sea level
whilst the less resistant Etruria Marls and tuffs have been weathered to form the surrounding lower ground.
On the horizon, on a clear day, a series of other hills, made of more resistant rocks can be seen. From the
west in a clockwise direction they are: The Clee Hills and The Long Mynd; from the east, working round to
the south: Rowley Hills and Clent Hills: towards the west: the Abberley Hills. (See the leaflet “Barrow Hill,
The Dudley Volcano”, from Dudley museum: Tel 01384 825575)
Quarrying of the dolerite probably began in the early 1800s, when material to surface the turnpike roads
running through the area was needed. One ran from Dudley to Himley, whilst the other ran from Dudley to
Kingswinton, now respectively the B4176 and the A4101. The dolerite was physically resistant, and was
very close to these roads, and so was easy to transport, as well as being strong enough for the task.
Although quarrying for roadstone continued for many years through the nineteenth century the dolerite body
was not big enough to allow a major quarrying centre to develop.
The site today shows a long complicated history of quarrying and comprises some quarry faces, and many
slopes of quarry waste more or less covered with soil and saplings. The area has the status of Local Nature
Reserve (LNR) and is being considered for Site of Special Scientific Interest Status (SSSI).
Human Use of Stone
The towns in the area around Dudley grew, mainly as a result of the use of coal and other mineral deposits
in the area providing jobs. Churches were also built, with St Marks being constructed during the 1840s. Its
walls are made of “Gornal Grit” a sandstone of Silurian age formed before our Barrow Hill story began, and
outcropping today 2km to the north.
The church has a roof of Welsh slate from Blaenau Ffestiniog and decorative tiles made from local Etruria
Marl. (Firing clay in a kiln can be thought of as man-made metamorphism, turning less resistant rocks in to a
more resistant man-made artefact). There are also decorative tiles in front of the main church door. Inside
the church the main pillars are probably late Carboniferous sandstones. All of these materials were chosen
because they are suitable for their purpose: sandstone for the walls and pillars because of its physical
strength; slates for the roof because of their impermeability and being able to split easily into thin sheets;
tiles from Etruria Marl which are easily moulded into shape and fired to a hard finish in a kiln.
The churchyards became full of memorials which likewise reflect the availability, suitability, attractiveness,
and cost of different headstones for graves. Early memorials tend to be of local stone (e.g. red Triassic
sandstones which have become deeply weathered over time) whilst later ones reflect international trade in
suitable, attractive igneous and metamorphic stones, made economic by improved transport systems.
Some interesting examples are ceramic, or brick, reflecting a more local industrial material, fired in kilns and
suitable for a long lasting memorial. These are a popular feature in Black Country churchyards.
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BARROW HILL L.N.R., DUDLEY: EARTH SCIENCE BRIEFING FOR THE SITE
© UKRIGS ESO-S Project
Figure 2. A map showing the outcrops in the area.
EARTH SCIENCE PRINCIPLES
In this area it is possible to demonstrate the following Earth Science principles.
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1)
The Principle of Uniformitarianism: The biological, physical and chemical processes we see
today, operated in much the same way in the past. “The present is the key to the past.”
2)
The Principle of Original Horizontality: bedding planes represent the original horizontal at the
time of deposition of sedimentary rocks.
3)
The Principle of Superposition: in a bedded sequence of strata, the oldest layers were deposited
first, and are found below the younger layers, which were deposited later.
4)
The Principle of Cross-Cutting Relationships: Structures, like faults and joints, which cut through
rocks must be later, and therefore, younger than the structures they cross cut. They must also be
older than the ones that cut across them.