The Rocks of Warwickshire as represented in the Geology Wall at Brandon Marsh
Introduction
The varied geology of Warwickshire tells a good story. WGCG are always looking for ways to involve people with possibly little interest in or knowledge of geology in getting to know the story underlying the landscapes of Warwickshire. A novel opportunity arose for a joint venture with Warwickshire Wildlife Trust after the Sensory Garden at their headquarters and reserve at Brandon Marsh, Coventry, was redeveloped and our display of rocks from Warwickshire was removed because it was in such a poor state. In 2013 a new Education Garden was planned alongside the Sensory Garden giving the chance to build a geology wall (shown below) on the boundary alongside the main path into the Reserve. The aim of the project is to represent a cross-section of Warwickshire’s geology roughly on a north-south line passing through Nuneaton, Coventry, Kenilworth and Stratford-Upon-Avon on the way. See also ‘Every Stone Tells a Story‘ leaflet, Interpretation Panel and Geological Timeline.
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Key to the materials in the wall
| 5 | Glacial sands and gravels | The thin layer of pebbles along the top of the wall represents the widespread scatter of Pleistocene, Holocene and Recent deposits across most of Warwickshire |
| 4 | Oolitic Limestone | Jurassic cream-coloured marine oolitic limestone |
| 4 | Hornton Stone | Jurassic orange/brown marine ferruginous limestone |
| 4 | Blue Lias | Jurassic grey/blue marine calcareous mudstone |
| 4 | White Lias | Triassic cream-coloured marine limestone |
| 3 | Mercia Mudstone | Triassic mudstones represented by bricks from brickworks in Leamington and Kenilworth |
| 3 | Bromsgrove Sandstone | Triassic buff/grey fluvial sandstones |
| 3 | Kenilworth Sandstone | Permian red fluvial sandstones |
| 2 | Carboniferous Sandstone | Upper Carboniferous mainly fluvial sandstones – ‘red beds’ |
| 2 | Coal Measures | Upper Carboniferous: Black paviours representing coal and Red Websters bricks representing mudstones |
| 1 | Diorite | Ordovician Intrusions |
| 1 | Hartshill Sandstone | Cambrian marine sediments later metamorphosed |
| 1 | Caldecote volcanics | Late Precambrian (Late Neoproterozoic) |
Interpretation of the Geology
The variety of rocks across the county can be clearly seen in the wall because of the contrasts in the colour and textures of the stones used. These differences can be used to divide the wall into five distinctive geologies, each representing time periods with contrasting locations, environmental conditions and rock types.
These geologies can be summarised as:
1. Warwickshire in the Southern Hemisphere (jump to section)
2. Warwickshire near the Equator (jump to section)
3. Warwickshire in the Desert (jump to section)
4. Warwickshire under the Sea (jump to section)
5. Warwickshire under the Ice* (jump to section)
* Note: 5 Glacial deposits are scattered throughout Warwickshire.
Display boards and supporting leaflet
The story of Warwickshire’s geology, represented in the geology wall at Brandon Marsh, covers 600 million years of earth history preserved in the rocks. That story is told on the two information boards near the wall, also available to view here (display 1 and display 2).
This leaflet tells a different story of how we can see the effects of the rocks on the world around us as it influences landscape and scenery, industry, building materials, and wildlife, as well as giving evidence of life in the past through the fossils that are found in the rocks.
1 Warwickshire in the Southern Hemisphere
650 to 450 million years ago
The oldest rocks in the County are found in a narrow belt running north-west to south-east through Nuneaton where they form a narrow ridge – the Nuneaton Ridge (photo 3) which is well seen at Hartshill Hayes Country Park.
They consist of four distinctive rock sequences:
- The Caldecote Volcanic Formation – Late Precambrian, (Late Neoproterozoic)
- The Hartshill Sandstone Formation – Cambrian
- The Stockingford Shale Group* – Cambrian – Ordovician
- Intrusive Rocks (Diorites) – Ordovician
*These rocks are unsuitable for building and are not represented in the wall.
The Caldecote Volcanics
The rocks in the wall are about 630 million years old.
When these rocks were formed the land that was to become Warwickshire (and England and Wales) lay about 60° south of the Equator. It was on an island microcontinent, called Avalonia, offshore of the supercontinent Gondwana. Avalonia lay on a subduction zone and was very volcanic. A modern equivalent would be Java and Sumatra in Indonesia, offshore of continental Asia. The resulting volcanic rocks are now found deep in quarries to the north-west of Nuneaton, the most important of which is the SSSI Boons Quarry. (photo 4) Rocks there suggest that the volcanoes were very explosive and are of two types: 1) a Crystal Tuff formed from an ash fall on land where the crystals are consolidated and welded together (photo 5); 2) a Bedded Tuff formed when the ash fell into the sea and settled in layers. (photo 6a and photo 6b).
The Hartshill Sandstone
The rocks in the wall are about 545 – 520 million years old.
In the 100 million years since the Caldecote Volcanics formed continental drift had opened up an ocean (the Iapetus Ocean) between Avalonia/Gondwana and two other continents – Baltica and Laurentia. The Hartshill Sandstone (and the Stockingford Shales) are clastic sediments formed by material eroding from the continents. The Hartshill Sandstones are buff to purple, blocky, well-bedded sandstones. (photo 7) Rocks this old usually have a turbulent history and in this case the original sand grains have been re-cemented with quartz as a result of burial and subsequent weak metamorphism. The rock is in consequence very hard and is difficult to work. It has largely been used crushed for road stone and railway ballast. It is commonly used for walling near its outcrop and occasionally in buildings – Hartshill Castle, Hartshill Church and Vicarage (photo 8) and, rather oddly, St John’s Church, Kenilworth. There is evidence of early life-forms in the sandstones (and also in the Stockingford Shales). Soft-bodied creatures moved about, sheltered and fed in the unconsolidated sands and muds on the ocean floor leaving behind trace fossils. (photo 9 and photo 10) Hard bodied creatures such as trilobites were evolving in the seas leaving behind hard-body fossils. (photo 11)
Ordovician Intrusive Rocks (Diorite)
The rocks in the wall are about 445 million years old.
In the intervening period Avalonia had been drifting across the Iapetus Ocean nearing Laurentia. Avalonia was subducting under Laurentia. The Late Precambrian and Cambrian rocks were incorporated into a growing mountain chain (the Caledonides) resulting in them being folded, faulted and uplifted. (photo 12) There was also a lot of volcanic activity associated with these earth movements which is now preserved in the lavas and tuffs of Snowdinia and the Lake District. Warwickshire lay on the fringes of this activity and molten rock was intruded as sills into the sandstones and shales forming the dark grey to black crystalline rock called diorite. (photo 13) The diorite has and is being extensively quarried for road stone in the Nuneaton area. (photo 14)
About 100 million years passed before the next rocks in the wall were laid down. During that time large events took place. The Avalonia microcontinent collided with Laurentia so the Iapetus Ocean ceased to exist. As a result, Scotland, which originated on Laurentia, became attached to England and Wales forming the basis for the landmass of Britain that now exists. These collisions resulted in the formation of a major, Himalayan-sized, mountain range – the Caledonian Mountains, the eroded remnants of which are now the mountains and hills of Scotland, the Lake District and Snowdonia.
2 Warwickshire near the Equator
350 – 300 million years ago
These rocks belong to the Carboniferous Period and are sedimentary, formed from deposits laid down by rivers flowing from the eroding Caledonian Mountains. They are now found in the Warwickshire Plateau, the undulating hilly area between Nuneaton and Coventry. (photo 16)
They consist of two distinctive rock sequences:
- The Coal Measures
- The Carboniferous Sandstones (Warwickshire Group) – the ‘Red Beds’
Coal Measures
These rocks are about 326-311 million years old. Note however that coal is an unsuitable rock for wall building and black paviours have been used to represent this period in the wall. The paviours are made in Warwickshire from Carboniferous clays.
The area now lay near the Equator close to the shores of the Rheic Ocean. Humid tropical swamp forest grew on the sands and muds of the shifting river courses, deltas and alluvial fans. Coal was formed from the decayed and compressed vegetation. (photo 17; photo 18) Coal forms only a small part of the Coal Measure deposits; most are shales, mudstones with some pale, buff sandstones. (photo 19) The mudstones were used for brick and tile making and are represented in the wall by bricks from the now defunct Websters brick yard in Coventry. Records show that the coal has been exploited since the sixteenth century. Deep mining ceased with the closure of Daw Mill Colliery in 2013. (photo 20).
Carboniferous Sandstones – the ‘Red Beds’
The rocks in the wall are about 310 – 290 million years old.
The area was slowly drifting northwards and becoming more arid. It was still receiving sediment from the eroding Caledonian Mountains and these were deposited as sand banks and bars in ephemeral meandering or braided rivers and by flash floods. In periods of low river flow mudstones and shales were deposited. The sandstones underlie the upstanding ridges of the Warwickshire Plateau (photo 21) and the mudstones form the valleys. The sandstones are commonly used for building, especially those with high status. (photo 22)
3 Warwickshire in the Desert
300 – 200 million years ago
The area had now drifted to 20–25° north of the Equator and was part of the new supercontinent Pangaea, Avalonia/Gondwana having joined with Laurentia and Baltica. These earth movements led to a new belt of mountains (Variscan/Hercynian/Armorican Orogeny), the remains of which are found in Devon, Cornwall and South Pembrokeshire. An upland area stretched from there into Brittany and Belgium and it was rivers from there which brought the sediments making up the rocks of this age in Warwickshire. The area now lay in a desert/semi-desert climatic zone.
Three distinctive rocks are represented in the wall:
- Kenilworth Sandstone – Permian (Warwickshire Group)
- Bromsgrove (Warwick) Sandstone – Triassic (Sherwood Sandstone Group)
- Mercia Mudstone – Triassic
Kenilworth Sandstone
The rocks in the wall are 290 – 280 million years old.
These rocks occur in the area around Kenilworth. (photo 24) They are predominantly a red, iron-stained sandstone interbedded with mudstones. The sands were deposited in sand banks or bars in intermittent major river channels and mudstones represent finer-grained material deposited in low or still water conditions. Kenilworth Sandstone was an important building stone best seen in Kenilworth Castle, St. Nicholas church and the remains of Kenilworth Abbey/Priory (photo 25) The Ashow Formation mudstones were the basis for the Kenilworth brick industry
Bromsgrove (Warwick) Sandstone
The rocks in the wall are 240 – 225 million years old.
This is readily distinguished by its buff and grey colour and is most readily seen on the hill on which Warwick is built. It is a fresh water alluvial deposit of sands and mudstones formed in major river channels and flood plains, subject to flash floods. It has fossil reptiles and amphibians of national and international significance. (photo 26). The sandstone was commonly used for building e.g. Warwick Castle, Westgate and Eastgate Warwick. (photo 27) It is also an important aquifer and is the source of spa water in Leamington Spa.
The higher land around Kenilworth and Warwick where these two sandstones occur forms the south-western corner of the Warwickshire Scarplands.
Mercia Mudstone
This rock is 225 – 220 million years old. Note however mudstone is an unsuitable for wall building and bricks have been used to represent rocks of this period and the Permian mudstones (see above) in the wall. There are named bricks in the wall from the former Cherry Orchard brickworks in Kenilworth and from Leamington Bricks.
The mudstone is thought to have its origin as blown sand being deposited in temporary desert lakes (playa lakes). In terms of area it is the most common rock in Warwickshire and underlies the Triassic Lowlands formed by the valleys of the rivers Blythe/Cole/Tame in the west, the Avon in the south and south-east (photo 28) and the Anker in the north-east. (photo 29)
4 Warwickshire under the Sea
200 – 145 million years ago
Britain had now drifted to about 40° north. The supercontinent Pangaea began to split apart and the Atlantic Ocean was beginning to open up. A shallow tropical/sub-tropical sea, less than 100m deep, encroached over Britain covering the older rocks, bringing totally different environmental conditions. (photo 31) With the exception of the White Lias they represent well-functioning marine ecosystems with rich food chains and are very fossiliferous (Brachiopods; Molluscs including oysters and scallops; ammonites and belemnites; fish; crustaceans; coral; echinoids and crinoids; marine reptiles – ichthyosaurs) (photo 32 and photo 33). Relatively small changes in sea conditions led to the variety of rocks displayed in the wall. Together these rocks underlie the Warwickshire Scarplands of the south-east of the county.
Four distinctive rocks are represented in the wall:
- White Lias – Triassic (Penarth Group)
- Blue Lias – Jurassic (Lias Group)
- Hornton Stone (also called Marlstone) – Jurassic
- Oolitic (Cotswold) Limestone – Jurassic (Inferior Oolite Group)
White Lias
The rock in the wall is 209 – 205 million years old.
This consists of pale cream rather thin well bedded limestones, mudstones and siltstones. They were formed in warm, shallow seas with a high concentration of carbonates which precipitated to form the rock. It is a common building stone in villages along the outcrop – e.g. Harbury, Ufton (photo 34)
Blue Lias
The rock in the wall is 205 – 195 years old.
This is a blue/grey rock which breaks into rather rectangular blocks. It formed in shallow off-shore water rich in lime but with a heavy load of mud making it contrast sharply with the White Lias. (photo 35) It is susceptible to weathering and so less used as a building stone but is sometimes used in walls (photo 36) for footings for timber-framed buildings (as in Stratford upon Avon) and as interior flagstones. The Rugby cement industry is based on this rock and the outcrop is pock-marked with quarries for example at Stockton, Long Itchington and in Rugby itself. (photo 37)
Hornton Stone
The rock in the wall is 195 – 189 million years old.
This is an orange/brown sandy-textured iron-rich limestone. It is the iron content which gives it its colour. It was deposited in water which was shallow enough to roll the sediment grains so that they are often spherical. Such rounded grains are called ‘ooids’ – (egg shaped as in fish roes) and the rocks are described as ‘oolitic’. The iron content supported, for a short time, an iron mining industry in the Burton Dassett Hills (photo 38). Despite a tendency for it to fracture rather irregularly it is a common building stone for houses and churches particularly on the top of the Edge Hill escarpment (photo 39 and photo 40). Henry Moore used it for sculptures. When freshly quarried it has a greenish tinge but weathers towards orange/brown. This variation and the fact that it is very fossiliferous has led it to be used as decorative cladding. (photo 41)
Oolitic Limestone
The rock in the wall is about 175 million years old.
This is a shallow water oolitic limestone with very little iron content. It is creamy/yellow in colour and occurs in rather thin beds of rectangular stone. This and its colour have made it a popular building and walling material. The side of the wall facing the Education Garden is built entirely of oolitic limestone. (photo 42)
These are the most recent bed rocks in Warwickshire and there is no evidence of deposition since the Middle Jurassic. For the last 30 million years or so the land has been above sea level and has been eroding to give the skeleton on the landscape we see now.
5 Warwickshire under the Ice
Within the last half million years
By now Warwickshire had drifted to its present location 50° north. The climate varied between cold periods, sometimes with glaciers and warmer periods (Interglacials) in between, one of which we are in now. At least once during the last half million years ice covered the whole area, leaving behind patches of glacial till – an unsorted mix of clay and rock boulders and fragments. Warm periods had ice and snow melt with powerful rivers depositing great thicknesses of sand and gravels. These surface deposits from the last half million years are scattered throughout Warwickshire. They are represented by a mortar cap on top of wall with pebbles and cobbles. (photo 44)
The sands and gravels are found especially in river valleys – eg Avon Valley in the South-West, Blythe/Cole/Tame valley in the west and north; and the Anker valley in the east. These are exploited in quarries for the aggregate industry. (photo 43)
In the gravels are the earliest evidence of human habitation in the Midlands revealed by the finds of Palaeolithic tools for example in the old quarries in the Baginton/Bubbenhall area. (photo 45)
Last Updated on 25 May 2021