Download From the moment that the Earth`s surface divided into plates, large

Pangaea
Pangaea
1. Pre Variscan orogeny.
Early Palaeozoic rocks
2. The compressive forces of the Orogeny reactivated
movement along ancient, deep-seated fault lines, raising
the land between them and forming a prism shaped block.
Inkberrow Fault
East Malvern Fault
3. The raised block quickly eroded away in the wasting
desert climate of the early Permian.
= Location of Great Britain
From the moment that the Earth's surface
A globe showing the supercontinent of Pangaea 240
million years ago. Image copyright to Ron Blakey,
divided into plates, large portions of the crust
Colorado Plateau Geosystems, Inc
have been moving against or towards each
other at incredibly slow rates, on average
between one and ten centimeters per year. Mountains, volcanoes, earthquakes and
ocean trenches are all the result of plate movements as is the migration, over millions
of years, of the continents over the Earth's surface. This is the process of plate
tectonics. Throughout geological time the configuration of Earth’s oceans and
continents has taken many forms due to this movement.
The sandstone of the Nubbins, known as the Bromsgrove Sandstone Formation, was laid
down in the Budleighensis river system. Material transported by the river built up layer
after layer of sand, added to by wind-blown material from the immediate surroundings.
The layers of sandstone in the Nubbins exhibit many features which indicate the
environment under which they formed. The red colour of the rock is indicative of oxidation
within the prevailing desert conditions. Cross-bedding reveals that sand bodies migrated
within the river channels and the occasional layer of pebbles indicates periodic high
energy events. The thickness of some of the layers of rock (in excess of 150cm) suggest
prolonged periods of steady deposition.
Geological Timescale
Permo-Triassic strata
Schematic sketch diagrams of the formation of the Worcester
Basin (after Barclay, Ambrose, Chadwick and Pharaoh 1997)
In the Worcestershire area a vast rift valley, the
Worcester Basin, was the dominating landscape
feature. To explain its origin we need to travel back a
little further in time to an episode of immense mountain
building, known as the Variscan Orogeny (see left).
Mesozoic Cenozoic
Dates given in millions of years
Palaeozoic
4. During the Permo-Triassic, extensional rather than
compressional forces prevailed. The central block started to
subside creating an elongate valley tens of kilometres wide.
Into this subsiding basin Permian and Triassic rocks were
deposited. At the deepest point the Permo-Triassic deposits
of the Worcester Basin are an impressive 3km thick.
By Triassic times the continents had joined together
forming a supercontinent called Pangaea. Britain was
landlocked, located some 31 degrees north of the
equator in the middle of Pangaea, and as such lay within
the latitudes of the northern hot deserts of today.
Quaternary
Neogene
Palaeogene
Cretaceous
Jurassic
Triassic
Permian
Carboniferous
Devonian
Silurian
Ordovician
Cambrian
Precambrian
During the Triassic a braided river system meandered its way across the
Worcester Basin, the Budleighensis River. The river’s source was in the
Variscan Uplands (Brittany), it flowed through southern England,
Worcestershire, Cheshire and out to the vicinity of the Irish Sea. As the
river flowed it deposited a variety of sediments, each reflecting the
conditions under which they were carried.
A present day braided river. Christchurch, New Zealand
The energy of a river generally dictates the weight of the load that it can
carry. Flash floods allow transportation of boulders; fast flowing rivers
may carry pebbles and sands whilst slower currents are only able to
move finer silts and muds.
Interpretation by Herefordshire and Worcestershire Earth Heritage Trust
www.EarthHeritageTrust.org.uk
01905 855184 / [email protected]
Registered Charity No 1144354
2.6
23
65
145
199
251
299
359
416
444
488
542
4600
A layer of cross-bedding in the rock face near the southwestern end of this lower quarry level
Another infrequent yet obvious feature seen in
the rock faces are white streaks. When dilute acid
is dripped onto them they effervesce, showing
the presence of calcium carbonate. Water is
drawn up through the Bromsgrove Sandstone by
capillary action. It is thought that this water
passed through the limestone hills to the north,
dissolving calcium carbonate from them before
reaching the porous sandstone. Here, it
precipitated out of solution forming components
of the white streaks.
Sandstone extracted from these quarries would have been used mainly as a building
stone. The parish church of St Peter's, Martley, is primarily constructed from Bromsgrove
Sandstone, as are the large blocks that make up some of the village walls. Though soft
and friable in nature the sandstone was used to construct many buildings across the
county; its proximity, rather than quality, being the attractive factor in its choice.
1. A cross-bedded layer forms
when layers of sediments are
deposited on an inclined surface
such as a channel in a river.
2. Typically, a period of erosion
follows when the water wears
away the upper part of the bed.
3a. New layers are then
deposited above the eroded
surface, often in the same pattern
as those below.
3b. The slope of the new cross
bedding may be in a different
direction if the current direction
has changed.
Sketch diagram showing the
formation of cross-bedding
St Peter’s Church, Martley constructed from Bromsgrove
Sandstone
The quarry faces, both on this and the upper level of the Nubbins quarries, exhibit a number of features
which are testament to the site's industrial past. Such features are restricted to the sections of the quarry
composed of massive layers of sandstone (layers which are greater than 1m thick); thin, flaggy, layers
serve no purpose as building stones.
Though details of the extraction history of the site have yet to be unearthed it is thought that the stone was
extracted by hand using the 'plug and feather' method (feather marks have been observed in a private
garden at the eastern end of the Nubbins). The method is a means of breaking large quarry stones into
smaller blocks. Firstly, a row of holes is drilled in the stone along a line where the break is desired. Secondly,
a pair of feathers (semicircular cross-section rods) is inserted in to each hole before plugs (iron rods) are
placed between each feather pair. The plugs are then hammered in succession until the stone fractures.
Plug and feather extraction