The land on which we live – The Geology of Dorset
In the first of a regular series, John Chaffey looks at the fundamentals of what makes Dorset so geologically special
Published in December ’11
Over the last two hundred years the study of the rocks of Dorset has made a fundamental contribution to the science of geology. The best exposures of the rocks of Dorset are inevitably along its coastline, but the contribution of inland Dorset to our current understanding of geology and landscape development cannot be entirely ignored. Geology in the interior forms the basis of some of the finest scenery in southern England, and has yielded a wide variety of building stones that have contributed much to the vernacular architecture of village and town alike.
The rocks of Dorset belong to the Mesozoic and Cenozoic eras of geological time. Within the Mesozoic, Dorset’s rocks belong to the Jurassic and Cretaceous Periods. The Jurassic Period lasted from 204 million to 145 million years ago and the Cretaceous lasted from 145 million years ago to 65 million years ago. The Cenozoic is divided between the Tertiary Period (now also often divided into the earlier Palaeogene and the later Neogene) from 65 million years ago to 1.8 million years ago and the Quaternary Period, from 1.8 million years ago to the present time.
In early Jurassic times southern England was below sea level, on the margin of a supercontinent known as Laurasia, and occupied a position about 30? north. Laurasia had already separated from South America and Africa which had, with Laurasia, hitherto formed part of a much larger super continent known as Pangea.
South America and Africa were already beginning to rift apart, as the South Atlantic Ocean began to form, and this rifting continued northwards in Jurassic times. Dorset was part of the so-called continental shelf of Laurasia, where the depth of water varied from time to time. In early Jurassic times deep water saw the deposition of limestones and mudstones in Dorset. In Middle Jurassic times shallower water allowed the accumulation of limestones such as the Inferior Oolite. Deeper water in late Jurassic times led to the accumulations of great thicknesses of clay, such as the Oxford Clay and the Kimmeridge Clay, with a further shallowing leading to the accumulation of the Portland Stone.
Later the Jurassic rocks were uplifted and the early Cretaceous rocks were laid down in shallow lagoons and later by rivers on land. Sea levels began to rise in later Cretaceous times as the North Atlantic opened, and Britain drifted farther to the north: during this time great thicknesses of Chalk were deposited. 60 million years ago further uplift occurred, and erosion of the Chalk began, much of it being removed completely from western Britain. After 10 million years the sea level began to rise again, and pebbly beds, sands and clays were deposited in southern England, to be followed by more deposition by rivers and in lagoons and estuaries. By 20 million years ago Africa, now separated from South America, was forced northwards against Eurasia, leading to the formation of the Alps and more folding of the rocks of southern England, particularly in southern Dorset, in Purbeck and the Weymouth areas.
About two million years ago temperatures began to fall in the northern hemisphere and ice sheets affected considerable parts of Britain during the Ice Age (Pleistocene times). No permanent areas of ice occurred in Dorset, although large snowfields were common over the higher parts of the Dorset Downs in winter. Dorset endured a periglacial climate, similar to conditions in Alaska, northern Canada and much of northern Scandinavia or Russia today. Dorset would have been under a permafrost regime, where the ground is frozen permanently to some depth. During the transitional seasons alternate freezing and thawing in the surface layers would break up rock material. In the brief periglacial summer the surface layers would thaw out, forming the so-called active layer. When this layer became sufficiently saturated it moved downslope, a process known as solifluction.
Rocks of Quaternary Age are usually referred to as superficial deposits, in order to distinguish them from the more consolidated rocks of the Tertiary Period and the earlier Mesozoic Era. Superficial deposits include periglacial debris such as frost-shattered and soliflucted material, and the gravels laid down by rivers and streams during the Pleistocene. Younger superficial deposits belong to the Holocene or Recent times, and include peat, river alluvium, dune sands and beach gravels.
Essentially, the rocks exposed along the Dorset Coast decrease in age from west to east. Along the coast of the World Heritage Site, which extends from Lyme Regis to Studland, Jurassic rocks first make their appearance at the Devon/Dorset boundary in the west, and continue, with a small number of breaks to Durlston Head at Swanage. Cretaceous rocks, which overlie the Jurassic sequence of rocks, first appear in Dorset in the upper part of the cliff at the Devon/Dorset boundary. Eastwards they form the summits of the cliffs as far east as Thorncombe Beacon to the east of Seatown. They reappear in the Chalk cliffs of White Nothe to the east of Weymouth, and continue to form important parts of the coast as far east as Worbarrow Bay, cut in the Wealden Beds.
Cretaceous gives way to Jurassic rocks again as far as Durlston Head. They reappear at Durlston Bay as the Purbeck Beds. The Wealden Beds appear again in Swanage Bay, and then the Chalk continues again from Punfield Cove at the northern end of Swanage Bay as far as the southern end of South Beach, Studland. Where Cretaceous Beds overlie the Jurassic rocks they are separated by a plane known as an unconformity, which represents a long period of geological time during which the Jurassic rocks were sometimes folded, uplifted and eroded before the Cretaceous rocks were deposited on top of them.
The Cenozoic rocks of Dorset’s coast first appear at the southern end of South Beach, Studland where Tertiary rocks overlie the Chalk, separated from it by another unconformity. Apart from the sand dunes of Studland Bay, Tertiary rocks form the remainder of the cliffed coast as far east as the Hampshire boundary at Chewton Bunny. Quaternary sediments are also referred to as superficial deposits, and along the coast they take the form of gravel beds that cap the cliffs from Poole Head at Sandbanks as far east as the cliffs at Highcliffe just over the Hampshire border.
Inland the pattern of outcrop of Dorset’s rocks is broadly similar, with the oldest rocks in the north-west and the youngest in the south-east. Jurassic rocks occur at the surface over a wide area extending from the West Dorset coast north-eastwards along the Somerset and Devon borders to Blackmore Vale and eventually to the Wiltshire border. The Chalk forms a huge swathe of Upper Cretaceous rocks that extend westwards from Cranborne Chase through the Dorset Downs and then swing round to form much narrower outcrops in South Dorset and the Isle of Purbeck. Inland, Dorset’s youngest rocks, the Tertiary sequence, occur in the Frome syncline or downfold, and extend east from near Dorchester to Bournemouth and the Hampshire boundary. Quaternary gravels, laid down by swollen rivers during the Pleistocene or Ice Age, first appear in the west near the headwaters of the Frome and the Piddle, but become increasingly extensive eastwards to form the huge spreads on which much of Bournemouth and Poole are built.
The rocks of southern Dorset have been much affected by the earth movements that built the Alps in Europe some 20 million years ago. The ‘outer ripples of the Alpine storm’ have buckled and broken the strata in the coastal region of south Dorset to form such important structures as the Weymouth anticline, or upfold, and farther east the Purbeck monocline, an asymmetrical structure. Along its northern edge, the Weymouth anticline is affected by minor folds and a major fault system. In both structures the dips are much steeper on the northern limb, which is sometimes overturned as at Lulworth Cove. Farther north these upfolds are complemented by the Frome syncline, or downfold, in which the younger Tertiary rocks are preserved. The Chalk of the Dorset Downs dips gently southwards, underneath the Tertiaries of the Frome valley to reappear in the hogback ridge of the Purbeck Hills to the south.