Jurassic Coast - Deeper into geology

The Triassic and Jurassic and Lower Cretaceous strata were deposited in a gently subsiding basin. These rocks were then tilted to the east and a period of erosion took place. As a result, the Upper Cretaceous strata lie unconformably on an eroded surface that is greatest in the west. Indeed, at the western extreme of the site the entire Jurassic is missing

The structure of the coast displays its geological interest superbly. In general, the strata dip gently to the east. The oldest rocks are therefore found in the west of the nominated Site with progressively younger strata outcropping to the east. As a direct result, most parts of the succession are readily accessible in sequential order within the cliffs and foreshore, while the continuous processes of coastal erosion mean that the exposures are constantly refreshed and new material is brought to light.

Triassic strata are the oldest rocks within the nominated Site and form the red cliffs of the East Devon coast
Triassic
The Triassic succession is a virtually continuous exposure of c. 1,100m of sediments representing most of the Triassic Period (c. 250-203 million years ago) in continental, terrestrial red-bed and, near the top of the sequence, shallow marine facies. These exposures record evidence of the gradual destruction and denudation of mountains formed in the Variscan orogeny of 330-280 million years ago, and the establishment of a widespread marine environment within a Jurassic basin, formed during the opening of the Atlantic Ocean.

Burton Cliff
A complete sequence of Jurassic strata are exposed along the Dorset coast with the oldest in the west, around Lyme Regis and Charmouth
Jurassic
The Jurassic rocks within the nominated Site have been known since the early days of geology as providing one of the finest marine sequences of this age anywhere in the World. Every stage of the Jurassic is represented; of the seventy-four ammonite zones, which have been recognised within the Jurassic, only three are definitely absent. The succession provides excellent evidence of the history of the Earth between c. 203-135 million years ago, recording six major cycles of sea level change, represented by repeated rhythms passing from clay to sandstone and then limestone. Historically, these sections have played a key role in the establishment of modern stratigraphy and biostratigraphic studies. They are internationally renowned, ‘classic’ sections on which comparative studies continue to be based.

Cretaceous
The boundary between the Jurassic and Cretaceous has still to be internationally defined, but, in Dorset, is expected to lie within the lowest beds of the Purbeck Formation. The succession within the nominated Site includes rocks of all stages of the Cretaceous Period, with the exception of the uppermost stage. The Purbeck Formation in Dorset is one of the finest late Jurassic-early Cretaceous terrestrial sequences in the world and offers a unique insight into environments and life at that time. The overlying Wealden Group is the most complete sequence of this age available at a single site in north-west Europe.

Sidmouth and the cliffs through to Beer Head. Along the East Devon coast, the Jurassic rocks have been completely eroded away and the Cretaceous Upper Greensand and Chalk lie directly on Triassic strata
Worbarrow Bay within the Army Ranges east of Lulworth Cove. The Cretaceous rocks dominate the coast wherever they are exposed

Golden Cap from Charmouth: The West Dorset cliffs display the easterly dip of the strata very clearly. The overlying Upper Greensand lies on an eroded surface in which some 80 million years of geological time is missing

At White Nothe in Purbeck, the Cretaceous rocks lie on the Upper Jurassic Kimmeridge Clay

Within Swanage Bay, there is virtually no erosion between the Lower Greensand (which is missing further west) and the Gault and Upper Greensand.
The Jurassic / Cretaceous Unconformity
Much of the nominated Site displays a spectacular example of a geological unconformity. This is an exceptionally well exposed and documented record of a world-wide Lower Cretaceous marine transgression. Uplift and erosion towards the end of the Wealden led to the erosion of the underlying Cretaceous, Jurassic, and Triassic along the length of the coast, prior to deposition of the transgressive Lower Greensand, Gault and Upper Greensand. In East Devon these strata rest directly on Triassic rocks. The unconformity becomes progressively smaller to the east, until, in the Swanage area, the Gault and Upper Greensand overlie the Wealden Formation with little angular discordance. Complex lateral changes in sedimentary environments are also recorded, allowing interpretation of the changes that took place during this important phase of sea level change.

The extensive coastal exposures provided by the Dorset and East Devon coast are complemented by modern and detailed geological maps, exceptionally well documented stratigraphy and sedimentology and an extensive subsurface database. As this knowledge has been gained, the significance of the structural geology of the nominated Site has become particularly important. Today this is one of the best understood sedimentary basins in the world, and concepts developed here have a global application.

Dorset localities have provided the names for internationally recognised stages for the Mesozoic. The Kimmeridge Clay unit gave its name to the international Kimmeridgian Stage as proposed by D’Orbigny (1846-1849). Owing to a miscorrelation at that time, only the Lower Kimmeridge Clay is now included in the modern Kimmeridgian Stage, the remainder lying within the lower Tithonian of current international use. Portland gives its name to the Portlandian Stage, named by Brongniart (1829), which is still in use in Northern Europe, though now included within the upper Tithonian. The Purbeckian, named after Purbeck, was, until recently, in international use for the lowermost stage of the Cretaceous.

Uplift
The process of plate tectonics has caused the uplift of the rocks but the sequence is complex in that several distinct periods of both uplift and subsidence have taken place during the long time period in which the rocks have formed. During this time faults within the rocks have formed,
Age
The Nominated site covers 190 million years of geological time including virtually the entire Mesozoic Period or ‘Middle Period’ of the Earth’s ancient history.




The Triassic and Jurassic and Lower Cretaceous strata were deposited in a gently subsiding basin. These rocks were then tilted to the east and a period of erosion took place. As a result, the Upper Cretaceous strata lie unconformably on an eroded surface that is greatest in the west. Indeed, at the western extreme of the site the entire Jurassic is missing
		The structure of the coast displays its geological interest superbly. In general, the strata dip gently to the east. The oldest rocks are therefore found in the west of the nominated Site with progressively younger strata outcropping to the east. As a direct result, most parts of the succession are readily accessible in sequential order within the cliffs and foreshore, while the continuous processes of coastal erosion mean that the exposures are constantly refreshed and new material is brought to light.

Triassic strata are the oldest rocks within the nominated Site and form the red cliffs of the East Devon coast		Triassic The Triassic succession is a virtually continuous exposure of c. 1,100m of sediments representing most of the Triassic Period (c. 250-203 million years ago) in continental, terrestrial red-bed and, near the top of the sequence, shallow marine facies. These exposures record evidence of the gradual destruction and denudation of mountains formed in the Variscan orogeny of 330-280 million years ago, and the establishment of a widespread marine environment within a Jurassic basin, formed during the opening of the Atlantic Ocean.



A complete sequence of Jurassic strata are exposed along the Dorset coast with the oldest in the west, around Lyme Regis and Charmouth		Jurassic The Jurassic rocks within the nominated Site have been known since the early days of geology as providing one of the finest marine sequences of this age anywhere in the World. Every stage of the Jurassic is represented; of the seventy-four ammonite zones, which have been recognised within the Jurassic, only three are definitely absent. The succession provides excellent evidence of the history of the Earth between c. 203-135 million years ago, recording six major cycles of sea level change, represented by repeated rhythms passing from clay to sandstone and then limestone. Historically, these sections have played a key role in the establishment of modern stratigraphy and biostratigraphic studies. They are internationally renowned, ‘classic’ sections on which comparative studies continue to be based.

		Cretaceous The boundary between the Jurassic and Cretaceous has still to be internationally defined, but, in Dorset, is expected to lie within the lowest beds of the Purbeck Formation. The succession within the nominated Site includes rocks of all stages of the Cretaceous Period, with the exception of the uppermost stage. The Purbeck Formation in Dorset is one of the finest late Jurassic-early Cretaceous terrestrial sequences in the world and offers a unique insight into environments and life at that time. The overlying Wealden Group is the most complete sequence of this age available at a single site in north-west Europe.
Sidmouth and the cliffs through to Beer Head. Along the East Devon coast, the Jurassic rocks have been completely eroded away and the Cretaceous Upper Greensand and Chalk lie directly on Triassic strata		Worbarrow Bay within the Army Ranges east of Lulworth Cove. The Cretaceous rocks dominate the coast wherever they are exposed

Golden Cap from Charmouth: The West Dorset cliffs display the easterly dip of the strata very clearly. The overlying Upper Greensand lies on an eroded surface in which some 80 million years of geological time is missing At White Nothe in Purbeck, the Cretaceous rocks lie on the Upper Jurassic Kimmeridge Clay Within Swanage Bay, there is virtually no erosion between the Lower Greensand (which is missing further west) and the Gault and Upper Greensand.		The Jurassic / Cretaceous Unconformity Much of the nominated Site displays a spectacular example of a geological unconformity. This is an exceptionally well exposed and documented record of a world-wide Lower Cretaceous marine transgression. Uplift and erosion towards the end of the Wealden led to the erosion of the underlying Cretaceous, Jurassic, and Triassic along the length of the coast, prior to deposition of the transgressive Lower Greensand, Gault and Upper Greensand. In East Devon these strata rest directly on Triassic rocks. The unconformity becomes progressively smaller to the east, until, in the Swanage area, the Gault and Upper Greensand overlie the Wealden Formation with little angular discordance. Complex lateral changes in sedimentary environments are also recorded, allowing interpretation of the changes that took place during this important phase of sea level change.

		The extensive coastal exposures provided by the Dorset and East Devon coast are complemented by modern and detailed geological maps, exceptionally well documented stratigraphy and sedimentology and an extensive subsurface database. As this knowledge has been gained, the significance of the structural geology of the nominated Site has become particularly important. Today this is one of the best understood sedimentary basins in the world, and concepts developed here have a global application. Dorset localities have provided the names for internationally recognised stages for the Mesozoic. The Kimmeridge Clay unit gave its name to the international Kimmeridgian Stage as proposed by D’Orbigny (1846-1849). Owing to a miscorrelation at that time, only the Lower Kimmeridge Clay is now included in the modern Kimmeridgian Stage, the remainder lying within the lower Tithonian of current international use. Portland gives its name to the Portlandian Stage, named by Brongniart (1829), which is still in use in Northern Europe, though now included within the upper Tithonian. The Purbeckian, named after Purbeck, was, until recently, in international use for the lowermost stage of the Cretaceous. Uplift The process of plate tectonics has caused the uplift of the rocks but the sequence is complex in that several distinct periods of both uplift and subsidence have taken place during the long time period in which the rocks have formed. During this time faults within the rocks have formed, Age The Nominated site covers 190 million years of geological time including virtually the entire Mesozoic Period or ‘Middle Period’ of the Earth’s ancient history.