Lancashire (including Blackburn with Darwen, Blackpool)
The underlying geology of Lancashire is comparatively simple and is formed from four major rock types from three main geological periods.
The Lower Carboniferous is represented by the shallow marine Carboniferous Limestone which outcrops at Silverdale and the Ribble Valley, running through Clitheroe into Yorkshire. The deltaic, Upper Carboniferous Millstone Grit, forms the core of the upland area of the Forest of Bowland and the higher moors to the east of Chorley, while the succeeding Coal Measures underlie Blackburn, Accrington and Burnley in the south of the County. The coastal lowlands of the west of the County are generally formed of Permian and Triassic sandstones and mudstones.
Lancashire was completely covered by ice during the last glacial advance of the Quaternary, or Ice Ages, and as a consequence the solid geology is largely covered by layers of glacially derived sediments. These form a skin of superficial deposits, or till, which in places are so thick as to eradicate all visual clues as to the nature of the underlying solid geology. This drift has been eroded and shaped by fluvial, marine, aeolian and frost processes, which combine to create distinctive features and landscapes.
Carboniferous
Following a long period of continental conditions during the Devonian, there was a major marine incursion at the beginning of the Carboniferous (354-290 million years ago) which resulted in the establishment of a shallow shelf sea across much of Britain. The oldest rocks present in Lancashire comprise limestones and mudstones that were deposited in this sea and are now collectively known as the Carboniferous Limestone.
This distinctive unit of rock outcrops in two separate parts of the County. In the south it underlies the tract of land from north of Preston, through the Ribble Valley via Clitheroe to Barnoldswick and around the southern and eastern flanks of the Forest of Bowland. In the north the Carboniferous Limestone outcrops between Kirby Lonsdale and Silverdale on the coast, where the outcrop gives rise to some of the best examples of limestone pavement in the country. Generally, where exposed, the Carboniferous Limestone yields characteristic marine fossils such as brachiopods, trilobites and crinoids.
The limestones of the Carboniferous Limestone give way to marine shales, with thin limestones and sandstones and then to the more massive sandstones and shales of the Upper Carboniferous Millstone Grit Series. These rocks form the bulk of the upland area of the Forest of Bowland and also form the higher ground around Blackburn, Accrington and Burnley, including the moors to the south between Chorley and Rawtenstall and Pendle Hill to the north of Burnley.
These rocks were deposited as sediments in a coastal environment where large river deltas were building out into the shallow, tropical marine waters that covered much of Britain at this time. Continuing deposition over the millennia led to the further building out of the deltas and the formation of an extensive low-lying, swampy land area in which the succeeding Coal Measures were deposited.
In Upper Carboniferous times, the periodic flooding and building of the swamp deltas along the coastline resulted in the deposition of a series of layers of coals (representing the compressed remains of the luxuriant swamp vegetation) interspersed with layers of shale, clay, sandstone and mudstone. These sediments now form the Coal Measures, which overlie the Millstone Grit. Fossils within the various layers of the Coal Measures include plants, marine shells and animals that lived in brackish water conditions.
These fossils provide an indication of the environmental conditions at the time of deposition and show that there were repeated advances and retreats of the shallow sea over the deltas throughout the Upper Carboniferous. In the Blackburn, Accrington and Burnley area, which occupy a broad trough underlain by Coal Measures, the presence of the coal accounts for the early industrialisation of the area. Coal has been worked at depth and by opencast operations at the surface.
Permian and Triassic
The Permian (248-290 million years ago) and Triassic (248-205 million years old) Periods in Lancashire are represented by red mudstones and sandstones that underlie the flat coastal plain of the western half of the County from the southern end of Morecambe Bay to the southern side of the Ribble Estuary and inland along a line running between Garstang and Ormskirk. However, although these rocks constitute much of the floor of the Lancashire lowlands, the solid rock geology rarely emerges from beneath a thick covering of glacial and post glacial deposits. The Permian-Triassic rocks of Britain were deposited under arid, desert conditions.
The Lower Triassic sandstones of the Sherwood Sandstone Group consists largely of red, yellow, and brown sandstones that represent the deposits of large braided rivers that crossed the desert plain. These rocks occupy the western half of the Fylde Peninsula and occur beneath glacial sediments between Leyland and Ormskirk. The upper part of the Triassic sequence is formed by the Mercia Mudstone Group, which comprise mudstones, sandstones and thin horizons of gypsum and rock salt. These rocks, which underlie Blackpool and the southern side of the Ribble Estuary, represent desert plain and evaporated lake deposits.
Quaternary
Over the last two million years the climate of Britain has varied tremendously with periods of temperate climate interrupted by repeated advances and retreats of glaciers and ice sheets. Collectively these periods have become known as the Ice Age (we are still in one of the temperate phases) and the actions of the ice sheets have been instrumental in forming the landscape we see today.
Much of the bedrock geology of Lancashire is mantled by sediments deposited during the Ice Age by ice sheets moving over the area from a variety of directions. During the last main glacial advance, the Devensian, some 15,000 years ago, thick deposits of till (boulder clay) were deposited over large areas of the county from beneath ice sheets and glaciers.
The composition of the till varies depending on where the main ice flow was from, so that till over the Fylde includes significant amounts of material derived from Triassic rocks on the bed of the Irish Sea. Very stony till with rocks from the Lake District was deposited in the northern part of the County and has intermingled with the Irish Sea till on the Fylde. On the flanks of the Pennines to the East, the local Pennine till contains sandstone and shale fragments derived from the Carboniferous rocks.
Following the melting of the last ice sheet large amounts of material were released and transported by the meltwaters to form important fluvio-glacial sand and gravel deposits in the vicinity of Carnforth and in the Ormskirk and Skelmersdale area. Several deep glacial channels have been recorded in the underlying bedrock, some extending well below present sea level. These channels are often associated with, but not always coincident with, present-day rivers and are filled with relatively poorly-consolidated sand, clays and silts. In the Rossendale and Calder valleys in the south-east of the county, clays were deposited in large ice-dammed glacial lakes.
Post-glacial deposits include raised-beaches and blown sands resulting from the retreat of the ice and the sea, and also alluvium in river valleys and estuaries. Peat deposits are of several basic types, extensive and sometimes deep basin peats, such as at Chat Moss, raised mosses on the poorly-drained till plains in the south and south-east of the County, and thin peat on the plateau areas of the Forests of Rossendale, Bowland and Pendle. On the Fylde Peninsula a low cliffline in the till plain a few miles inland marks the old shoreline of the former lake of Martin Mere, and is still traceable intermittently from the River Ribble to the River Dee. It is best seen at Hesketh Bank near Preston and at Hill House, east of Formby.
Geological highlights:
- The Carboniferous Limestone of the Lancashire area formed on the bed of a warm, shallow sea which covered this area some 340 million years ago. An important feature of the Carboniferous limestones of this area is the occurrence of mound-like structures, generally referred to as reef-knolls, which formed during deposition of lime-rich muds, partly as a result of the growth, on the sea-bed, of large colonies of marine animals, probably crinoids (sea-lilies) and bryozoans. The small hills running east-west between the villages of Worston and Downham near Clitheroe are some of the best known examples of these early Carboniferous reef-knolls.
- At Silverdale in the north-west of Lancashire the Carboniferous Limestone supports the most important single example of limestone pavement in Britain. This weathering feature of hard limestones arises through the gradual dissolution of the limestone by rainwater, which is slightly acidic. This process creates a number of diagnostic features commonly including massive, flat, tabular limestone blocks (clints) with intersecting vertical fissures (grikes). Pockets of soil formed in the grikes and surface depressions on the clints support the widest range of characteristic plant species recorded on any limestone pavement in Britain. The shady grikes shelter harts-tongue fern and other fern species, while plants such as tutsan and bloody cranesbill grow on the pavement. Due to the build up of organic material some of the surface depressions contain plants associated with more acid conditions such as tormentil, often in close association with typical lime-loving species.
- Pendle Hill, to the east of Clitheroe is a striking and prominent hill rising to 557m which is of geological and cultural significance. The hill is formed by Upper Carboniferous shales and sandstones with the Pendle Grit Formation, the oldest Millstone Grit sandstone seen in the central Pennines, forming the resistant cap to the Hill. This is underlain by the Upper Bowland Shale Formation, the resistant Pendleside Sandstone and the Lower Bowland Shale Formation respectively, their outcrops being marked by changes in slope. From a cultural perspective, Pendle Hill is a place of inspiration for Quakers, as it was here that their founder, George Fox (1652), allegedly had a visionary experience on top of the hill. The Hill is also strongly associated with the Pendle Witches of 1612.
- The post-glacial deposits on the Fylde Plain to the south of the Ribble Estuary largely comprise windblown sands, the Shirdley Hill Sand, with small patches of underlying till and marine clay. In addition, there are major areas of basin peat in the east around Simonswood Moss and coastal peats south-east of Hightown. The coastal peats together with the Shirdley Hill Sands have produced soils which are of high agricultural quality (Grades 1 and 2) over much of the area. The Shirdley Hill Sands are a very pure silica sand that has been extracted in the area for use in the glassmaking industry. Excavations have shown that glass making occurred in the Simonswood area during the 17th Century, probably by French Huguenot glassmakers. As late as the 1970's the fields around Bickerstaffe and Lathom were still being excavated for the Shirdley Hill sand by the Pilkington Brothers Glass Company.
- In certain areas of the County there are a number of drumlin fields. Drumlins are smooth, streamlined hills composed of till that have their long axis oriented in the direction of ice movement; the blunt nose points upstream and the gentler slope tails off downstream. One such area of drumlins occurs around the mouth of the River Wyre in north-west Lancashire, where a number of drumlins occur, all of which are orientated north-east to south-west. These were probably deposited during the melt of the last great ice sheet probably around 9000 years ago and consist of a variety of materials including sands, clays and gravels.
Local sites
The following localities represent, in part, the geology of this county. Each locality has a grid reference, a brief description of how to get there and a short summary of the geology you are likely to find. All the localities listed are openly accessible.
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