Geology of Merseyside

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This article describes the geology of the ceremonial county of Merseyside which includes the City of Liverpool and the modern metropolitan boroughs of Knowsley, Sefton and St Helens which prior to 1974 were part of the historic county of Lancashire and the Metropolitan Borough of Wirral which was until then part of the historic county of Cheshire.

The geology of Merseyside in northwest England largely consists of a faulted sequence of Carboniferous Coal Measures rocks overlain in the west by younger Triassic and Permian age sandstones and mudstones. Glaciation during the present Quaternary Period has left widespread glacial till as well as erosional landforms. Other post-glacial superficial deposits such as river and estuarine alluvium, peat and blown sand are abundant. [1]

Contents

Carboniferous

Carboniferous rocks underlie all of Merseyside but are only exposed to the east of the north-south Boundary Fault. The sequence encountered locally comprises (in descending order, youngest at top): [2]

The oldest rocks to appear at or near the surface within the county are from the Namurian Epoch. These are alternate units of sandstone ('flags' and 'grits') and mudstone with occasional coal seams (known locally as 'mines') forming a part of the Millstone Grit Group. They are brought to the surface on the eastern side of the Upholland Fault and dip eastwards beneath Billinge Hill. The full Namurian sequence hereabouts is:

Overlying the Millstone Grit sequence is the thick Westphalian sequence of sandstones, mudstones and coal seams collectively referred to as the Pennine Coal Measures Group and which forms the Lancashire Coalfield, the western part of which extends into Merseyside. The youngest Carboniferous strata in the area are the non-productive (of coal) reddened beds of the Warwickshire Group.

Permian

The rocks of the Manchester Marls and Collyhurst Sandstone formations, which are ascribed to the Middle Permian Appleby and ?Upper Permian Cumbrian Coast groups respectively, have been detected at depth through exploratory boring but are not exposed at the surface. [3]

Triassic

A pile up to several hundred metres thick of Triassic sandstones, mudstones and siltstones underlies Wirral, Liverpool and the coastal plains to the north. The following sequence is encountered within Merseyside:

The Mercia Mudstone Group together with the Helsby Sandstone Formation are ascribed to the Middle Triassic (Anisian) whilst the lower part of the sequence shown above is ascribed to the Early Triassic.

The relatively hard-wearing Helsby Sandstone forms many of the more prominent hills in the area such as Irby Hill, Storeton Hill and Bidston Hill on Wirral and also the tidal island of Hilbre Island in the Dee Estuary. Quarries opened up in the Chester Pebble Beds and Helsby Sandstone formations have provided building stones as used in Liverpool Anglican Cathedral and Lime Street station for example. The quarry at Storeton Hill has been active since Roman times; some of this rock was used more recently as cladding for the Empire State Building in New York. The quarry has also yielded fossils such as the footprints of the Triassic reptilian Chirotherium. [4]

The Thurstaston Sandstone Member of the Wilmslow Sandstone Formation outcrops at Caldy Hill, Thurstaston Hill and nearby Heswall Dales.

Structure

The sequence of both Carboniferous and Triassic rocks is criss-crossed by geological faults generally oriented north–south of NNW-SSE. The most significant is the north–south oriented Boundary Fault which runs west of Widnes, St Helens and Skelmersdale and marks the western edge of the Carboniferous Coal Measures outcrop. A local high in the Palaeozoic basement running WSW-ENE through Merseyside separates the East Irish Sea Basin from the Cheshire Basin to the southeast.

The Triassic rocks of Wirral are affected by the Caldy, Grange, Thurstaston, Frankby, Greasby, Woodchurch, Seacombe, Barnston and Neston faults.

From Eccleston southwards, the roughly north–south aligned Eccleston West and Eccleston East faults define a graben in which Triassic rocks are exposed at the surface whereas to east and west are Coal Measures. Elsewhere in the St Helens area, the coalfield is broken by the generally NNW-SSE aligned Roaring Meg, Derbyshire Hill and Twenty Acre faults.

Quaternary

Glacial legacy

During the last ice age the area was over-ridden by the Irish Sea ice sheet moving south and southeastwards resulting in the development of iceways such as the channel now occupied by the Mersey between Liverpool and Wirral. Glacial meltwater cut tunnel valleys through the bedrock, the majority of which were subsequently filled by later deposits. Glacial till is smeared across much of the area. After the ice sheets had melted away from the Irish Sea but before vegetation had taken hold and before the sea reflooded that basin, blowing sand drifted across Merseyside to form what is now known as the Shirdley Hill Sand, a sheetlike deposit up to 2.5m thick in places. The presence of this sand was responsible for the establishment of the glassmaking industry in St Helens though the raw materials are now sourced in Cheshire. [5]

Post-glacial sediments

Subsequently, coastal and estuarine sands, silts and muds have been deposited widely throughout the Mersey and Dee estuaries whilst alluvium fills the floor of other river and stream valleys. [6] Considerable areas of recent blown sand cover the coastal zone around Southport and southwards to Formby and peat has developed across lower ground as at Holland Moss and Sefton Meadows inland of Southport and Formby.

See also

Related Research Articles

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<span class="mw-page-title-main">Millstone Grit</span>

Millstone Grit is the name given to any of a number of coarse-grained sandstones of Carboniferous age which occur in the British Isles. The name derives from its use in earlier times as a source of millstones for use principally in watermills. Geologists refer to the whole suite of rocks that encompass the individual limestone beds and the intervening mudstones as the Millstone Grit Group. The term Millstone Grit Series was formerly used to refer to the rocks now included within the Millstone Grit Group together with the underlying Edale Shale Group.

<span class="mw-page-title-main">Geology of Alderley Edge</span> Hill in Cheshire, United Kingdom

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<span class="mw-page-title-main">Geology of Wales</span> Overview of the geology of Wales

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<span class="mw-page-title-main">Geology of Gloucestershire</span>

Gloucestershire is one of the most geologically and scenically diverse counties in England, with rocks from the Precambrian through to the Jurassic represented. These varying rock-types are responsible for the three major areas of the county, each with its own distinctive scenery and land-use - the Forest of Dean in the west, bordering Wales, the Cotswolds in the east, and in between, the Severn Vale.

<span class="mw-page-title-main">Geology of Yorkshire</span>

The Geology of Yorkshire in northern England shows a very close relationship between the major topographical areas and the geological period in which their rocks were formed. The rocks of the Pennine chain of hills in the west are of Carboniferous origin whilst those of the central vale are Permo-Triassic. The North York Moors in the north-east of the county are Jurassic in age while the Yorkshire Wolds to the south east are Cretaceous chalk uplands. The plain of Holderness and the Humberhead levels both owe their present form to the Quaternary ice ages. The strata become gradually younger from west to east.

<span class="mw-page-title-main">Marros Group</span> Geological term for rock in south Wales

The Marros Group is the name given to a suite of rocks of Namurian age laid down during the Carboniferous Period in South Wales. These rocks were formerly known as the Millstone Grit Series but are now distinguished from the similar but geographically separate rock sequences of the Pennines and Peak District of northern England and northeast Wales by this new name.

<span class="mw-page-title-main">Mercia Mudstone Group</span> Early Triassic lithostratigraphic group

The Mercia Mudstone Group is an early Triassic lithostratigraphic group which is widespread in Britain, especially in the English Midlands—the name is derived from the ancient kingdom of Mercia which corresponds to that area. It is frequently encountered in older literature as the Keuper Marl or Keuper Marl Series.

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<span class="mw-page-title-main">Sherwood Sandstone Group</span>

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Mynydd Garnclochdy is a gentle summit on the long moorland ridge which extends south from Blorenge towards Pontypool and which defines the boundary between the modern county of Monmouthshire to the east and the county borough of Torfaen to the west though historically it was entirely within the traditional county of Monmouthshire. Its summit, at a height of 448 m (1407 ft), is marked by a small cairn. The summit and the eastern slopes of the hill fall within the Brecon Beacons National Park. A southern top of the hill, Mynydd Garn-wen achieves a height of 436m, and carries a trig point further south again at an elevation of 425m. To the north the broad moorland ridge runs via a poorly defined 425m top and a col at 404m elevation just south of a minor east–west road, into Mynydd y Garn-fawr, east of Blaenavon.

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The geology of Pembrokeshire in Wales inevitably includes the geology of the Pembrokeshire Coast National Park which extends around the larger part of the county's coastline and where the majority of rock outcrops are to be seen. Pembrokeshire's bedrock geology is largely formed from a sequence of sedimentary and igneous rocks originating during the late Precambrian and the Palaeozoic era, namely the Ediacaran, Cambrian, Ordovician, Silurian, Devonian and Carboniferous periods, i.e. between 635 and 299 Ma. The older rocks in the north of the county display patterns of faulting and folding associated with the Caledonian Orogeny. On the other hand, the late Palaeozoic rocks to the south owe their fold patterns and deformation to the later Variscan Orogeny.

References

  1. ↑ British Geological Survey 1:50,000 scale geological map series sheets (England and Wales) 74, 83, 84, 96 & 97 and accompanying memoirs
  2. ↑ Aitkenhead et al 2002 British regional geology: the Pennines and adjacent areas (4th edn). British Geological Survey, Nottingham
  3. ↑ Howard et al 2007 geology of the Liverpool District - a brief explanation of the geological map Sheet explanation of the British Geological Survey 1:50,000 sheet 96 Liverpool (England and Wales), pp13-14
  4. ↑ Howard et al 2007 geology of the Liverpool District - a brief explanation of the geological map Sheet explanation of the British Geological Survey 1:50,000 sheet 96 Liverpool (England and Wales)
  5. ↑ Howard et al 2007 geology of the Liverpool District - a brief explanation of the geological map Sheet explanation of the British Geological Survey 1:50,000 sheet 96 Liverpool (England and Wales), pp21-22
  6. ↑ British Geological Survey 1:625,000 scale geological map Quaternary Map of the United Kingdom South 1st Edn. 1977
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