Mupe Bay provides a prime example of coastal erosion in progress, the sea undermining the chalk cliff and slowly dissolving the subsequent falls with every tide. The Mupe Rocks march out across the Bay, followed closely by the Mupe Ledges, an exposed section of folded Purbeck Beds that form an area in the sea where the waves meet from two different directions. This bay is used as a fair weather anchorage for small boats. This area lies within the MODís Lulworth Firing Range, but visitors are free to explore when the range walks are open to the public.
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Worbarrow Bay and Mupe Bay Geology
An aerial view of the geology of Worbarrow Bay and Mupe Bay
Chalk, Upper Greensand, Gault, Wealden Beds, Purbeck Beds, Portland Stone
Coastal Features: The Great Unconformity and The Cretaceous Blanket, Chalk Cliff Erosion, Mupe Bay and Worbarrow Bay, The Mupe Rocks, Mupe Reef
Mupe Bay and Worbarrow Bay
The geology of Worbarrow Bay and Mupe Bay is very similar and they almost mirror one another, with the mirror being where the two bays meet at Arish Mell Gap. They both consist of Upper Chalk, Middle Chalk, Upper Greensands, Gault, Lower Greensands, Wealden Beds, Purbeck Beds and Portland Stone that have been subject to complex folding. All the sediments have been tilted vertically, around 45 degrees for Worbarrow and 60 degrees for Mupe. In effect both the cliff sections have been buckled upwards to form a horseshoe shape with the bottom of the horseshoe hidden from sight deep beneath the ground. The cliff sediments have also been twisted horizontally. This is why the Chalk is found at the back of the bay, while the Portland Stone and Purbeck Beds take the brunt of the seas at the front, forming the peninsula of Worbarrow Tout in the east and the Mupe Rocks in the west.
The Great Unconformity and The Cretaceous Blanket
The sediments which make up the Jurassic Coastline were deposited in the following order, Triassic, Jurassic and Cretaceous, with the oldest rock, the Triassic sitting at the at the bottom of the pile.
Part way through the Early Cretaceous Period the sea levels dropped and the layers of rock were tilted to the East by earth movements. Overtime the exposed rocks were eroded before sea levels rose again, depositing more Cretaceous sediments.
The combined tilting and erosion explains why the Jurassic Period is completely missing in East Devon and why areas of cliff all along the coastline are capped with Cretaceous rocks which are millions of years younger than the sediments laying directly beneath them.
Running through the cliff alongside the Mupe Rocks are bands of 147 million year old Purbeck Beds. These Beds lead down onto the beach and out into the bay where they form a reef. The waves here meet from two different directions.
This area is in the heart of the MODís Lulworth firing range, the large number boards mark the firing zones.
Chalk Cliff Erosion
Erosion of coastal Chalk cliffs is slow. Chalk is relatively hard, and only becomes unstable when the sea has carved a cave into its base, destabilising the weight of cliff above. The sea then has to wash away the cliff fall before the process can start all over again. The milky coloured seawater found at the base of Chalk cliffs, particularly around fresh falls, is evidence of coastal cliff erosion in progress.
The Mupe Rocks
The Mupe Rocks consist of 150 million year old Portland Limestone that have been left as sea stacks to bear the brunt of the seas natural forces. Their peculiar angle is evidence of the intensive folding which affected the area around Lulworth when the continents of Africa and Europe collided around 30 million years ago.
Coastal Visitor Centre
Lulworth Heritage Coast Centre
Town/Village or Area:
Tourist Info Centres
in this Area: Bindon Hill, Rock Pools
The wildlife living on the chalky soil of Bindon Hill is exceptionally rich. It is home to a host of plants and animals, a great place to see butterflies and moths.
The geology of the area around the Mupe Rocks provides a large rocky beach with ledges to explore. The nearest and easiest access to this site is from Lulworth Cove.
Interest: MOD Lulworth Firing Range and Walking, Bindon Hill Fort
Bindon Hill Fort
The earthworks of an Iron Age hill fort follow the summit of Bindon Hill for 1 ľ miles.
At 168 meters above sea level, it offers far reaching views across the Lulworth coastline.