Geology

Seaton has two main geological stories

1. Two Major Landslips: The Axmouth Undercliff and the Hooken Landslip

2. The Walk through Time: This is the only area on the coast where you can easily walk through three time zones and see Triassic, Jurassic and Cretaceous geology in a single day.

The location of Seaton, at the mouth of the Axe Valley, is ideally suited to the demonstration of several of the fundamental principles of the earth sciences, in particular the relationship of geology to landscape and human occupation. A wide variety of rock types that formed in environments ranging from hot deserts to periglacial tundra, and which represent tens of millions of years of Earth history, are readily accessible at beach level within easy walking distance of the proposed Centre.

Triassic rocks

The oldest rocks in the area are red mudstones and sandstones of Triassic age (c 200 to 230 million years old) that form low cliffs in and adjacent to the town between Seaton Hole and Culverhole Point. These are some of the best exposures of late Triassic rocks in Europe. They were deposited in hot deserts by flash floods that formed ephemeral streams and lakes that supported a rich flora and fauna, but the dry, oxidising conditions were not conducive to the preservation of fossils. Nevertheless, the Triassic rocks of the east Devon coast have yielded more organic remains than any rocks of this age in Britain. They include a wide variety of plants including conifers and horsetails, and the ancestors of many modern insects, fish, amphibians and reptiles. Some of the archaeosaurs and rhynchosaurs, the ancestors of the dinosaurs, are only known from this area and Eastern Europe. The presence of ripple marks, abundant root traces, tracks and burrows made by insects and brine shrimps, and the evapourite minerals gypsum and salt, confirm that the Triassic muds were deposited in lakes which for short periods of time teemed with life, but which were for long periods hot, saline and inhospitable.

When traced eastwards from Seaton, progressively younger Triassic strata come down to beach level, and the sedimentary structures, minerals and fossils within the mudstones show that the climate became progressively wetter with time. At Culverhole Point, grey mudstones and white limestones with fossil shells that indicate that they were deposited in shallow, saline lagoons adjacent to a large ocean. Mary Anning of Lyme Regis, sometimes referred to as the first modern palaeontologist, collected shells and the teeth and bones of extinct reptiles from these rocks.

Cretaceous rocks

The deposition of the Triassic rocks was followed by about 100 million years of uplift and erosion in south-west England. When the sea returned to the Seaton area it deposited the Cretaceous Upper Greensand and Chalk (c 90 to110 million years old). These rocks are exceptionally well displayed on the higher ground on either side of the town. To the east, the imposing Haven Cliff exposes the full thickness of the Upper Greensand and much of the Chalk and to the west, between Seaton Hole and King’s Hole the same succession can be examined at beach level. White Cliff at Seaton Hole is particularly imposing and is regarded by geologists as the most complete and best exposed section in Upper Greensand anywhere in Britain.

The cliff sections at Seaton Hole are also of particular interest because this is one of the few places in southern England where one can see a major fault. This is an earthquake fracture line that was intermittently active for at least 300 million years, and possibly for more than 500 million years. It extends downwards for several kilometres, and runs northwards to form the boundaries of the Axe Valley, the Quantock Hills and the Bristol Channel. It was last active about 15 million years ago when the area west of the fault subsided about 60 metres. At Seaton Hole it brings the red Triassic mudstones into contact with the markedly dissimilar Cretaceous rocks.

When fresh, the Upper Greensand consists of grey sandstones richly speckled with the bright green mineral glauconite. These become oxidised in the cliffs to produce the yellowish brown and faintly greenish grey sandstones seen in Haven Cliffs and White Cliff. The presence of channels and pebble beds together with the remains of a rich marine fauna in the sandstones shows that they were deposited in a current-swept, relatively shallow (20m to 60m deep), clear sub-tropical sea. Pieces of wood bored by marine worms indicate that land was not far distant. Plankton, ammonites, fish and marine reptiles lived in this nutrient-rich sea, and shellfish, sea urchins, and crustaceans lived on and below the sea floor. However, fossils are relatively rare except for an abundance of oysters, snails and other strong shells because of the abrasive nature of the current-swept sea-bed sands.

Between Seaton and Beer Head the white limestones of the Chalk are brought down to sea level in a syncline, a basin-like fold that was formed by compressional earth movements adjacent to the Seaton Hole Fault. This has resulted in strikingly attractive coastal scenery, culminating in Beer Head one of the highest vertical cliffs in southern England. The Chalk in this area was deposited on a sub-tropical marine shelf in water that was still relatively shallow (50m to 100m deep) but which was not disturbed by strong currents and which received very little sediment from land areas. As a result, most of the Chalk is a pure limestone composed of the remains of microscopic algae (coccoliths) and finely broken debris derived from the shelly parts of other animals. As with the Upper Greensand, the sea was a well-oxygenated, nutrient-rich environment which supported abundant plankton, ammonites, shellfish, sea urchins, sponges and crustaceans, fish and marine reptiles.

Flints make up a conspicuous part of the Chalk at Seaton and Beer. They occur in rows in which individual flints mostly have similar shapes, sizes and colours (from grey to black). These represent the positions of the sea bed at different times, the flints being the silica infillings of the dwelling and feeding burrows of crustaceans and other animals that lived just below the sea floor.

Tertiary and Quaternary

The deposition of the Chalk was followed by a long period of uplift and erosion related to the earth movements in southern Europe that gave rise to the Alps and Pyrenees. Much of the Chalk, which had probably been deposited as a layer up to 2 kilometres thick in Devon, was removed by rivers that left only a patchy veneer of clay and sand. These Tertiary deposits (about 45 million years old) now cap the high ground around Seaton. Further erosion and uplift, including renewed earthquakes and movement along the Seaton Hole Fault, produced the dissected plateau that occupies most of east Devon.

Within this plateau, the Axe Valley at Seaton owes its position to two large faults that had a major effect on the siting and shape of the valley. These are associated with much fracturing in the rocks through which they pass, and it is this line of weakness that the present-day river and its ancestors have exploited. The western boundary fault and the secondary fracturing associated with it are exposed at Seaton Hole where the cliffs offer a rare opportunity to examine the nature of the disturbances. The eastern boundary fault is hidden beneath alluvial deposits and the Seaton to Axmouth road.

The Axe Valley is one of the best examples of a misfit valley in southern England, one in which the width of the alluvial floodplain is far greater than one would expect from the size of the river. In addition, the sediments in the river bed and beneath the floodplain are gravels and boulders that are far too large to be moved by the present river, even when it is in flood. The earliest geologists thought that this discrepancy was so striking that it represented unquestionable proof of the Biblical Flood. The modern explanation is that the present-day valley was carved out by meltwaters during cold periods in the Ice Age (about 10,000 to 350,000 years ago) when much of Britain was covered by ice and Devon, although ice-free, experienced an Arctic climate with high snowfalls. Each Spring, short-lived torrents carried with them large amounts of highly erosive flint and chert gravel derived from the local Cretaceous rocks. The present-day river carries only mud and silt as it meanders over its relatively inerodible gravel bed. The meander channels, oxbow lakes and abandoned channels that can be seen on the floodplain between Seaton and Axminster are among the best preserved in England.

Human occupation

The combination of proximity to a sea and river that teemed with fish, and to extensive woodlands with their diverse wildlife, berries and fruits, would have made the Axe Valley attractive to the earliest hunter-gatherers. The oldest known settlements in the area, sited on the river bank near Axminster, date from about 300,000 years ago. These were communities of the now extinct Neanderthals who made tools from the cherts and flints in the river bed. Examples of their tools have been found all over south-west England, the Axe Valley sites being the most prolific source of Palaeolithic implements in southern Britain. At a much later date, flints obtained from the Chalk at Beer Head became the most important source of finely crafted Neolithic implements in the south west. The flints continued to be used for flint-lock pistols from medieval times until the Battle of Waterloo when the victory over Napoleon caused a recession in the armaments industry.

The ‘modern’ settlement of the Axe Valley dates from the Roman port at Axmouth and its successor, the medieval port of Seaton. The Roman port was sited on the east bank adjacent to a large bend that carried the river to an outfall, later the site of the port of Seaton, on the west side of the valley. The river cliff that was formed when the river hugged this western course is still visible as a steep bank within the town. A succession of violent south westerly storms in the mid 16th century moved the shingle ridge eastwards to its present position. This blocked the entrance to the harbour and caused it to silt up and become overgrown by salt marsh. This was the most recent in a series of beach movements which began during the Last Ice Age about 20,000 years ago when sea level was 100 metres lower than at present due to the water locked up in the ice sheets, and the beach lay several miles south of its present position.

In addition to shaping the present-day Axe Valley, the Ice Ages initiated large landslips along the south coast, and some of these have become reactivated in recent years in the relatively wet climate of today. The Undercliff Landslip, on the east side of Seaton, is the largest landslide complex in Britain. A massive movement at Bindon Cliffs in 1839 caused several million tons of rock to slide seawards in a few hours. It attracted much publicity and became the first landslip in the World to be accurately documented and scientifically studied. The 1790 Hooken Landslip adjacent to Beer was equally spectacular but occurred a few years before geology had become established as a subject of interest, and received little attention. It is now the most accessible major landslip in southern Britain with coastal paths through and above it, and beach access below. Taken together, the viewpoints from these paths enable the 3-dimensional nature of the collapse to be demonstrated in great detail.

The rich variety of rock types and landforms at Seaton, and the succession of geological and geomorphological events that can be deduced from them, attracted many of the early 18th century scientists. These were not just the founders of modern geology but were instrumental in changing the study of natural science from one based on a literal reading of the Bible to one based on observation and hypothesis.

Prominent among these were Henry de la Beche, one-time resident of Lyme Regis and founder of the British Geological Survey, the World’s oldest national geological survey; William Buckland, a native of Axminster who described and named Megalosaurus (= giant lizard), the first of what was later recognised as a new group of extinct animals, the dinosaurs; and William Coneybeare, Vicar of Axminster who described the first Plesiosaurus (= near lizard). All three gained much of their geological experience on the coast around Seaton.

Buckland achieved popular acclaim in 1823 when he published the Reliquiae Diluvianae in which he attributed the gravels of the River Axe to the Great Flood as recorded in the Book of Genesis. To his great credit and despite strong opposition from many of his contemporaries, he later (1838) supported the glacial theory of their origin.