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Somerset is a rural county in the southwest of England, covering It is bounded on the north west by the Bristol Channel, on the north by Bristol and Gloucestershire, on the north east by Wiltshire, on the south east by Dorset, and on the south west and west by Devon. It has broad central plains with several ranges of low hills. The landscape divides into four main geological sections from the Silurian through the Devonian and Carboniferous to the Permian which influence the landscape, together with water related features. The low lying areas of the North Somerset Levels and Somerset Levels have been subject to thousands of years of flooding and man's attempts to control the flow of water. In the north of the county the Limestone of the Mendip Hills dominates the landscape, while in the south the Blackdown and Quantock Hills rise out of the levels. The highest areas are on Exmoor. The wide variety of landscapes has led to several areas being designated as Sites of Special Scientific Interest for geological reasons, and support a range of flora and fauna as can be seen from the List of Sites of Special Scientific Interest in Somerset.

The oldest rocks are of Silurian age (443–419 million years ago), the most southerly known outcrop of rocks of this age in Britain. They make up a sequence of lavas, tuffs (volcanic ash), shales and mudstones in a narrow outcrop to the northeast of Shepton Mallet, in the eastern Mendip Hills. Rocks from the Devonian (419–359 million years ago) are found across much of Exmoor, the Quantocks (including Hangman Sandstone and Cockercombe tuff), and in the cores of the folded masses of the Mendip Hills. Carboniferous Period (359–299 million years ago) rocks are represented by the Carboniferous Limestone that forms the Mendip Hills, rising abruptly out of the flat landscape of the Somerset Levels and Moors. The limestones are very fossiliferous, and contain evidence of the abundant marine life that existed at the time of their creation, including fossil crinoids (sea lilies), corals and brachiopods. At the end of the Permian (299–252 million years ago) and Triassic periods, the Variscan orogeny resulted in the formation of several mountainous areas including Dartmoor in the south, Exmoor and the Quantocks, and the Mendips. In the Taunton area Permian (299–252 million years ago) red sandstones and breccia outcrop, although rocks of Triassic age (252–201 million years ago) underlie much of Somerset and form the solid geology of the Somerset Moors and Levels. There are no glacial deposits. The Triassic rocks consist of red marls, sandstones, breccias and conglomerates which spread over the older rocks. The Dolomitic Conglomerate is an old shingle beach of Keuper Marl age. The Rhaetic Beds are full of fossils due to invasion of the Jurassic Sea. The Lias consists of clays and limestones, the latter being quarried and are famous for their fossils. Blue Lias was burnt locally to provide a source of lime for making lime mortar. It is still used as a decorative building stone. Blue Lias is believed to have been quarried on the Polden Hills as early as the 15th century and was quarried in Puriton from the early 19th century until 1973, when the local cement works closed. Above the Lias is the Lower Oolite Series which are chiefly clays and oolitic limestone. The famous Bath Stone is obtained from the Great Oolite bed. Oxford Clay is the chief member of the Middle Oolite Series; behind only the Bay of Fundy and Ungava Bay in Canada. Normal high tide may be enhanced by between and during storm surges. This feature has meant that large areas of the county have been liable to flooding by the sea. Thus the present coastline is partly due to a belt of marine clay at the coast and partly due to seawalls built to reclaim areas previously flooded at high tide. The coastline contains exposures of Devonian sediments and tectonics west of Minehead adjoining the classic exposures of Mesozoic sediments and structural features which extend eastward to the Parrett estuary forming cliffs along the coastline near Clevedon and near Minehead, with low sandhills near Burnham on Sea. There are sandy beaches mainly at Burnham on Sea, Brean and Weston super Mare. There are also storm ridges, salt marsh, and sand dunes.

The main valleys between the hills are filled with alluvial deposits from the hills or sea. The county has many small rivers, most of which flow into the Bristol Channel. Many of the latter rivers now have clysts (the local name for a sluice) on them to control the sea, but formerly they were tidal for some way inland. The main exception to this is the River Parrett, which still has a tidal bore. However the Chew and the Frome flow into the Avon which forms most of the northern county boundary with Gloucestershire. The Cale flows into Dorset through the Blackmore Vale, while the Exe flows into Devon. The (Dorset) Axe, the Culm and the Otter rise in Somerset but flow into Devon. The courses of the rivers Parrett, Somerset Axe, Brue and Cary run across the Somerset Levels and have generally been changed to improve the flow. The River Axe rises from Wookey Hole Caves, due to water draining into the ground at swallet holes on top of the Mendips. The river passes through Panborough Moor, Wedmore Moor, Ox Moor, Stoke Moor and Mark Moor and reaches the sea at Uphill (near Weston super Mare) on Bridgwater Bay. The River Brue rises at Brewham, close to the county border with Wiltshire. It flows through Bruton and is joined by the rivers Pitt and Alham. The river then flows past East and West Lydford to Baltonsborough and then turns north to Street across Butts Moor, South Moor and Kennard Moor. Originally it then joined the Axe but now it flows west across Westhay Moor, Tealham and Tadham Moors, Chilton Moor, Mark Moor and Huntspill Moor. The Brue reaches the sea near Burnham on Sea. The River Cary originates in Castle Cary in the east of Somerset. It flows south west through Cary Moor to Cary Fitzpaine. where it changes course to go north east to Taunton. It continues east through West Moor, Curry and Hay Moors and Stan Moor to Burrowbridge where it meets the Parrett. The River Exe rises at Exehead on Exmoor and flows south east to Exton where it is joined by the River Quarme. It then flows south to Exebridge where it meets the Barle and passes into Devon.

The North Somerset Levels are to the east of Weston super Mare, while the Axe valley is separated by the Isle of Wedmore from the Brue valley. There are coastal marine clay deposits, and further inland there are many peaty areas. These have been settled from the Mesolithic onward, with wooden causeways linking them to higher ground. There are also rocky outcrops, such as Brent Knoll and Glastonbury Tor which have also housed ancient settlements. The water levels in the moors and levels are controlled by a series of small narrow canals called rhynes (known as 'rhines' in Avonmouth and Gloucestershire both are pronounced 'reens'). Some parts are allowed to flood in winter. The area is mainly used for grazing but some peat extraction is carried out.

This is the area between the River Avon to the north and the Axe valley. The north of Somerset is dominated by the tableland of the Mendip Hills, an area of outstanding natural beauty, stretching from Frome in the east to Crook Peak in the west, with outliers of Bleadon Hill and Brean Down as well as Steep Holm in the Bristol Channel. The highest point is Black Down at There is an escarpment facing south to the Levels and Moors while the dip slope to the north is broken up. The Old Red Sandstone is a series of red sandstones, marls and conglomerates. It rises as an anticline in the Mendips and appears in the Avon Gorge and at Portishead. Carboniferous Limestone, of marine origin, covers the sandstone and appears in the Avon Gorge and at Weston super Mare where it contains volcanic rocks. There are outlying hills at Worlebury, Middle Hope, the Failand Ridge, Broadfields Down, Portishead Down and Wrington Hill. The main geological component of the Mendips is Carboniferous Limestone. It represents the remnants of a much higher range of hills that existed hundreds of millions years ago. Coal measures appear in the Radstock district, and surrounding Somerset Coalfield (largely concealed by Triassic and newer rocks). There are two series of coal bearing sandstones and shales separated by Pennant Sandstone. Locally the beds are folded and faulted. There were mines in the Radstock and Nailsea areas but these have closed. This was one of the first areas in the world to undergo systematic geological study and mapping by John Strachey and William Smith in the 18th century. They observed the rock layers, or strata, which led Smith to the creation of a testable hypothesis, which he termed the Principle of Faunal Succession. The Mendips were mined for lead, silver, coal, ochre, fuller's earth and zinc but this has finished. They were also quarried for stone, notably at Bath and Doulting.

To the south of Somerset there is an upland with a series of rolling valleys and scarps, from Penselwood in the east to the Blackdown Hills, another designated Area of Outstanding Natural Beauty, in the west. This is a geologically complex area of clays, limestone and marl. The honey coloured limestone at Ham Hill (also known as Hamdon Hill) is particularly important to geologists because of the assemblages of fossils which it contains, the sedimentary features which it displays and the way it relates to other rocks of equivalent age in the close vicinity. It has been quarried since Roman times at least. The Blackdowns are on the south west border of Somerset, extending into Devon. They are composed of Upper Greensand. The Quantock Hills are largely formed by rocks of the Devonian Period, which consist of sediments originally laid down under a shallow sea and slowly compressed into solid rock. In the higher north western areas older Early Devonian rocks, known as Hangman Grits, predominate, and can be seen in the exposed rock at West Quantoxhead quarry, which were worked for road building. Further south there are newer Middle and Late Devonian rocks, known as Ilfracombe beds and Morte Slates. These include sandstone and limestone, which have been quarried near Aisholt. At Great Holwell, south of Aisholt, there is a limestone cave, which is the only one in the Devonian limestone of North Devon and West Somerset. these are known as New Red Sandstone rocks which represent the deposits of large river systems that crossed a desert plain, At Kilve are the remains of a red brick retort, built in 1924, when it was discovered that the shale found in the cliffs was rich in oil. At Blue Anchor the coloured alabaster found in the cliffs gave rise to the name of the colour "Watchet Blue".

Exmoor is a dissected plateau of Devonian sedimentary rock, rising to at Dunkery Beacon. It extends into Devon but the majority of the area is in Somerset. Much of the area is a National Park. The landscape is one of rounded hills, with hogs back cliffs at the coast due to geological movements. Because of high rainfall there are boggy areas and the part by the Chains is a Geological Conservation Review site recognised as being nationally important for its south western lowland heath communities and for transitions from ancient semi natural woodland through upland heath to blanket mire. The Chains provides palynological record of a mid to late Flandrian vegetation history on Exmoor. The pollen sequence in the peat is calibrated by radiocarbon dating. The Glenthorne area demonstrates the Trentishoe Formation of the Hangman Sandstone Group. The Hangman Sandstone represents the Middle Devonian sequence of North Devon and Somerset. These unusual freshwater deposits in the Hangman Grits, were mainly formed in desert conditions. As this area of Britain was not subject to glaciation, the plateau remains as a remarkably old landform. Quartz and iron mineralisation can be detected in outcrops and subsoil. The highest point on Exmoor is Dunkery Beacon; at it is also the highest point in Somerset. However, the crest of this coastal ridge of hills is more than from the sea. If a cliff is defined as having a slope greater than 60 degrees, the highest cliff on mainland Britain is Great Hangman near Combe Martin at high, with a cliff face of Its sister cliff is the Little Hangman, which marks the edge of Exmoor. Exmoor's woodlands sometimes reach the shoreline, especially between Porlock and The Foreland, where they form the single longest stretch of coastal woodland in England and Wales. The Exmoor Coastal Heaths have been recognised as a Site of Special Scientific Interest due to the diversity of plant species present. The high ground forms the catchment area for numerous rivers and streams. There are about 300 mile (483 km) of named rivers on Exmoor. The River Exe, from which Exmoor takes its name, rises at Exe Head near the village of Simonsbath, close to the Bristol Channel coast, but flows more or less directly due south, so that most of its length lies in Devon. The river and the Barle Valley are both designated as biological sites of Special Scientific Interest. Another tributary, the River Haddeo, flows from the Wimbleball Lake. The action of streams has cut combes through the hills down to the sea, which are now wooded, although much of Exmoor is open heathland. There is an outlier of Exmoor at North Hill near Minehead. Iron working was formerly carried out, probably from the Roman period onward. Because Exmoor was a royal forest, i. e. a hunting reserve, it was unpopulated in Medieval times. The first house on the moor was only built at Simonsbath in 1654. It was not until the 19th century that farms were built around the moor. The Brendon Hills are an eastern outlier of Exmoor with the same undulating landscape but separated from the main area by the valley of the River Avill. Iron ore mining was carried out from Roman times up to the early 20th century.