The Purbeck Monocline is a geological fold

Fold (geology)

The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in...

. The term 'fold' is used in geology

Geology

Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...

 when one or more originally flat sedimentary strata

Stratum

In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...

 surfaces are bent or curved as a result of plastic (i.e. permanent) deformation. A monocline

Monocline

A monocline is a step-like fold in rock strata consisting of a zone of steeper dip within an otherwise horizontal or gently-dipping sequence.-Formation:Monoclines may be formed in several different ways...

 is a step-like fold, in which one limb is roughly horizontal. The Purbeck Monocline was formed during the late Oligocene

Oligocene

The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...

 and early Miocene

Miocene

The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...

 epochs

Geologic time scale

The geologic time scale provides a system of chronologic measurement relating stratigraphy to time that is used by geologists, paleontologists and other earth scientists to describe the timing and relationships between events that have occurred during the history of the Earth...

, about 30 million years ago. It is the northernmost 'ripple' of the Alpine Orogeny.

The Purbeck Monocline gives rise to the prominent ridge of steeply dipping Cretaceous

Cretaceous

The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 chalk

Chalk

Chalk is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. Calcite is calcium carbonate or CaCO3. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite plates shed from micro-organisms called coccolithophores....

 which now forms the Purbeck Hills

Purbeck Hills

The Purbeck Hills and South Dorset Downs are a ridge of chalk downs in Dorset, England. The hills extend from the Dorset Downs west of Dorchester, where the River Frome begins to form a valley dividing them from the larger area of downland to the north. The ridge then runs east through the Isle...

. This chalk band runs from Swyre Head

Swyre Head, Lulworth

Swyre Head, Lulworth lies on the Jurassic Coast between Bat's Head to the west and Durdle Door to the east, close to Lulworth in Dorset, England...

 via Flower's Barrow

Flower's Barrow

Flower’s Barrow is an Iron Age hillfort, built over 2500 years ago, above Worbarrow Bay in Dorset on the south coast of England.- Location :Flower’s Barrow is located about 16 kilometres from Swanage and about six kilometres south of Wareham....

 to Old Harry Rocks

Old Harry Rocks

The Old Harry Rocks are two chalk sea stacks located at Handfast Point, on the Isle of Purbeck in Dorset, southern England.- Location :Old Harry Rocks lie directly east of Studland, about 4 kilometres northeast of Swanage, and about 10 kilometres south of the large towns of Poole and...

. From here the fold continues under the sea to The Needles

The Needles

The Needles is a row of three distinctive stacks of chalk that rise out of the sea off the western extremity of the Isle of Wight, England, close to Alum Bay. The Needles lighthouse stands at the end of the formation...

 and forms the central spine of the Isle of Wight

Isle of Wight

The Isle of Wight is a county and the largest island of England, located in the English Channel, on average about 2–4 miles off the south coast of the county of Hampshire, separated from the mainland by a strait called the Solent...

. Here it is also known as the 'Purbeck-Isle of Wight Disturbance'. The monocline continues under the English Channel as the Wight-Bray Monocline.

The Purbeck Hills run east-west through the small broad peninsula known as the Isle of Purbeck

Isle of Purbeck

The Isle of Purbeck, not a true island but a peninsula, is in the county of Dorset, England. It is bordered by the English Channel to the south and east, where steep cliffs fall to the sea; and by the marshy lands of the River Frome and Poole Harbour to the north. Its western boundary is less well...

. The resistant beds of chalk and limestone

Limestone

Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

 form two ridges and the softer Wealden

Wealden Group

The Wealden Group is a group in the lithostratigraphy of southern England. The Wealden group consists of paralic to continental facies sedimentary rocks of Valanginian to Barremian age and thus forms part of the English Lower Cretaceous. It is composed of alternating sands and clays...

 rocks between them have been eroded to form a valley.

Some visible features along the monocline include the disharmonic folds and faults, known as the Lulworth Crumple, at Stair Hole

Stair Hole

Stair Hole is a cove that is forming just to the west of Lulworth Cove in Dorset, southern England. In a few hundred thousand years it may be as large as nearby Lulworth Cove. The folded limestone strata known as the Lulworth crumple are particularly visible at Stair Hole. Stair hole featured in...

, Lulworth Cove

Lulworth Cove

Lulworth Cove is a cove near the village of West Lulworth, on the Jurassic Coast World Heritage Site in Dorset, southern England. The cove is one of the world's finest examples of such a landform, and is a tourist location with over 1 million visitors a year...

, Arish Mell and at Peveril Point

Peveril Point

Peveril Point is a promontory forming the easternmost part of the Isle of Purbeck in Dorset, England, and is part of the town of Swanage.It is located at OS Grid Ref: SZ 041 787.On top is a coastguard hut...

 further east. These features also include the polygonal thrust ridges developed in the harder rock bands at Kimmeridge Bay and related to the growth of the monocline is the fault at Ballard Down

Ballard Down

Ballard Down is an area of chalk downland in Dorset, southern England.The down forms a headland, Ballard Point, between Studland and Swanage bays in the English Channel, and once formed part of a continuous chalk ridge between what are now west Dorset and the Isle of Wight, part of the Southern...

.