The Purbeck Monocline is a

geological fold In structural geology, a fold is a stack of originally planar surfaces, such as sedimentary strata, that are bent or curved during permanent deformation. Folds in rocks vary in size from microscopic crinkles to mountain-sized folds. They occur ...

in southern England. The term 'fold' is used in

geology Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Ear ...

when one or more originally flat

sedimentary Sedimentary rocks are types of rock (geology), rock that are formed by the accumulation or deposition of mineral or organic matter, organic particles at Earth#Surface, Earth's surface, followed by cementation (geology), cementation. Sedimentati ...

strata In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...

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

monocline A monocline (or, rarely, a monoform) 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 (see diagram) * ...

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

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...

and early

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...

epochs In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured. The moment of epoch is usually decided by ...

, 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 The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...

chalk Chalk is a soft, white, porous, sedimentary carbonate rock. It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor. Chalk ...

which now forms the

Purbeck Hills The Purbeck Hills, also called the Purbeck Ridge or simply the Purbecks, are a ridge of chalk downs in Dorset, England. It is formed by the structure known as the Purbeck Monocline. The ridge extends from Lulworth Cove in the west to Old Har ...

. This chalk band runs from Swyre Head via Flower's Barrow to

Old Harry Rocks Old Harry Rocks are three chalk formations, including a Stack (geology), stack and a stump, located at Handfast Point, on the Isle of Purbeck in Dorset, southern England. They mark the most eastern point of the Jurassic Coast, a UNESCO World H ...

. From here the fold continues under the sea to

The Needles The Needles is a row of three stacks of chalk that rise about out of the sea off the western extremity of the Isle of Wight in the English Channel, United Kingdom, close to Alum Bay and Scratchell's Bay, and part of Totland, the westernmo ...

and forms the central spine of the

Isle of Wight The Isle of Wight ( ) is a county in the English Channel, off the coast of Hampshire, from which it is separated by the Solent. It is the largest and second-most populous island of England. Referred to as 'The Island' by residents, the Isle of ...

. 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 The Isle of Purbeck is a peninsula in Dorset, England. It is bordered by water on three sides: 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 n ...

. The resistant beds of chalk and

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

form two ridges and the softer Wealden 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 is a small cove located just west of Lulworth Cove in Dorset, southern England. The folded limestone strata known as the ''Lulworth crumple'' are particularly visible at Stair Hole. There are several caves visible from the seaward s ...

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

Arish Mell Arish Mell is a small embayment and beach between Mupe and Worbarrow Bays in Dorset, England and is part of the Jurassic Coast and the South West Coast Path passes just to the north. It is about due south of Lulworth Castle and East Lulworth ...

and at

Peveril Point Peveril Point is a promontory on the east-facing coast of the Isle of Purbeck in Dorset, England, and is part of the town of Swanage. It forms the southern end of Swanage Bay. It is located at OS Grid Ref: SZ 041 787. The rocks that make up ...

further east. These features also include the polygonal thrust ridges developed in the harder rock bands at

Kimmeridge Bay Kimmeridge Bay () is a bay on the Isle of Purbeck, a peninsula on the English Channel coast in Dorset, England, close to and southeast of the village of Kimmeridge, on the Smedmore Estate. The area is renowned for its fossils, with The Etches ...

and related to the growth of the monocline is the fault at

Ballard Down Ballard Down is an area of chalk downland on the Purbeck Hills in the English county of Dorset. The hills meet the English Channel here, and Ballard Down forms a headland, Ballard Point, between Studland Bay to the north and Swanage Bay to the s ...

References

{{Jurassic Coast Isle of Purbeck Jurassic Coast Geology of Dorset