Geological formations that shape the Dorset and East Devon landscape

The landscape we see around us here on the Jurassic Coast has been shaped over millions of years and geological formations and processes continue to determine the possible landscape changes. The Jurassic Coast hosts an array of geological formations and features. Some of the more complex underlying structures that control the way rocks are outcropped in certain areas are controlled by monclines, anticlines, faults, orogenies and / or unconformities. The geological and palaeoenvironmental heritage of the coast can also be explored by area.  

See glossary of terms and discover more on the Our Dynamic Coast Pages. 

Monoclines. Step-like folds in the otherwise horizontal underlying rocks, or ‘monoclines’ are evident in the landscape here. The Purbeck Monocline for example, has elevated the rocks to near vertical between Durdle Door and Ballard Down, where they lie almost horizontal either side of this. The chalk within this fold forms the ridgeway that runs from Lulworth to Ballard Down.

Anticlines. The Weymouth Anticline has created obvious limestone and sandstone ridges and contrasting clay valleys. The influence of the Alpine Orogeny caused this arch or dome-shaped fold in the rocks here, known as the Weymouth Anticline. Subsequent erosion of the rocks has formed the characteristic landscape of the area around Weymouth and Portland. 

Unconformities. Despite an almost continuous geological record across the range of the Jurassic Coast, there are many occurrences of rocks that were formed during one time period, resting unconformably on top of other rocks that were formed during a time that did not directly precede their time period. This phenomenon is known as an unconformity. There are several examples to choose from, but perhaps the most understood, is ‘The Great Unconformity’ which marks a major erosional event during the Cretaceous. Around 110 million years ago, earth movements tilted the rocks upwards in the West and down in the East. Those newly exposed rocks in the west were subject to erosion, removing previous rock and sediment layers before the sandstones were laid down on top. A clear example of an unconformity can be viewed at Beer, at the further west outcrop of Cretacoues chalk. Either side of Beer lie distinctive red Triassic rocks and there are no Jurassic-age rocks present. 

There are many cave features along the Jurassic Coast, most notably along the Purbeck coastline. Examples include Bacon Hole and Tilly Whim Caves. Caves on coastlines form when the sea erodes the rock surface, exploiting existing lines of weakness, such as along a fault. If the sea erodes through to the other side of the rock, an arch is formed. The most notable example of this on the Jurassic Coast, is Durdle Door but others exist, including the nearby Stair Hole and Bat’s Hole. After considerable time and further erosion, these arches are vulnerable to collapse of the horizontal section due to the force of gravity, creating sea stacks. Prime examples of sea stacks are those at Ladram Bay and Old Harry Rocks. Sea stacks will also eventually be lost to the power of the sea and the destructive erosion it causes. For example, where Old Harry Rocks now lies, there was previously a headland, then caves, arches and sea stacks. Ultimately, over time, some of these sea stacks have been lost and, as was the case for Old Harry’s ‘wife’ (as the sea stack was named) in 1896, this process will continue, eventually causing a retreat of the coastline. 

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Buried beneath our feet on the Jurassic Coast, deep in the rocks, are clues to explain what we see at the surface. For example, the red rocks along the East Devon Coast are connected to an event called the Variscan Orogeny, a period of continental collision during the Devonian-Permian Periods (~380-295 million years ago). 

At this time, the landmass on which the Jurassic Coast lies was part of a large continent called Gondwana. During the Carboniferous Period, Gondwana merged with all other landmasses on Earth to form the better known supercontinent, Pangaea. At this time, what is now the Jurassic Coast was situated much closer to the equator. This, paired with the formation of Pangaea created in an arid, desert environment where our red rocks were laid down. 

During the Mesozoic era, Pangaea slowly broke apart. Crucially for us here on the Jurassic Coast, it is this tectonic activity that set the geological framework for the evolution of the coast.

Firstly, it allowed the gradual deposition of sediments through fluctuating environmental conditions in the Triassic, through the Jurassic and into the Cretaceous creating a uniquely complete stratigraphic sequence. Enormous tectonic forces in the middle of the Cretaceous Period caused the land to tilt towards the east, lifting the rocks up in the west and exposing the sediments laid down millions of years before. Further sediments were laid down later in the Cretaceous, including the famous white Chalk. 

Since the Mesozoic Era, the modern continents have taken shape with more collisions and mountain building. The clearest example of this on the Jurassic Coast can be seen in the Lulworth Crumple. Resulting from the Alpine Orogeny that occurred roughly 30 million years ago, the rocks here are, as the name suggests, crumpled dramatically owing to the enormous pressures acting upon them causing the strata to be tilted and collapse differentially. 

In the last few hundred thousands years, as erosion has occurred and particularly during the Ice Ages, the coastline we know today emerged, exposing geology that is now world famous.