Although indistinct nowadays, the border between Devon and Dorset lies just to the west of Lyme Regis at a place called Devonshire Head. The border between the ancient counties of Dyfnaint (Devon) and Dornsaete (Dorset) was set to follow the line of the River Lim in Saxon times. In the 8th century salt was one of the most valuable commodities in the area and the salt marshes surround Lyme was a prime source. In 774 AD the West Saxon King Cynewulf granted a triangular shaped area of land on the west bank of the Lim at Lyme to the bishopric of Sherborne. Its boundary effectively fixing the current county boundary. This western part of Lyme became known as Lyme Abbas (Abbot's Lyme) with the eastern part known as Lyme Regis (King's Lyme).

Where Dorset borders Devon in the west, the rocks date from the Blue or Lower Lias Group and are aged about 200 million years old, the boundary between the Triassic and Jurassic geological periods. As you travel east you take a walk through time with the rocks becoming progressively younger. With the odd exception such as at Black Ven (Charmouth) and Golden Cap (Seatown), the strata remains within the Lower Lias as far as Burton Bradstock. From West Bay to the Fleet at Chesil the rocks belong to the Middle Jurassic. Continuing east, around Chickerell the rocks are Oxford Clay, whilst east of Lynch Cove the strata is mainly Corallian Limestone. The barrier beach of Chesil Bank now dominates the landscape until it reaches the Isle of Portland to the southeast.

The Isle of Portland is famous for its Portland Stone, dating from the Tithonian stage of the Upper Jurassic period (approximately 152 to 145 million years old). Extensively quarried, the quarries consist of beds of white-grey limestone separated by chert beds. It has been used extensively as a building stone throughout the British Isles and beyond. It has most notably been used in the construction of St Paul's Cathedral and Buckingham Palace, as well as the United Nations headquarters building in New York.

From Weymouth to Osmington Mills, with the exception of Bowleaze Cove, the strata is mainly beds of Purbeck Stone overlying Corallian Limestone. East of Osmington Mills, near Ringstead the strata is chiefly Chalk dating from the Cretaceous Period, exhibiting itself best at White Nothe, Bat's Head, Durdle Door and Hambury Tout. Around Lulworth Cove erosion through the rocks of the Lower Cretaceous (Wealden) has given us an insight into the complex geology, faulting and deformation of the beds, especially at Stair Hole. East of Lulworth as we near Mupe Bay, with its fossil forest, Upper Chalk overlies beds of older Middle and Lower Chalk, dipping to the east on the west of Arish Mell but dipping the other way on the other side of the cove. Flowers Barrow, an Iron Age Hill Fort, offers a commanding viewpoint of the coast around here.

At the imposing limestone structure Gad Cliff, near the deserted wartime village of Tyneham, the landscape reverts to the Jurassic Period with Purbeck and Portland strata dominant. At Brandy Bay the rocks are heavily faulted and as we near Kimmeridge Bay the rocks belong to the Middle Jurassic Kimmeridge Clay Formation. Swyre Head and St Aldhelm's Head are two excellent viewpoints from which to see the coast. Between St Aldhem's Head and Durlston Head the rocks belong mainly to the Purbeck Beds of the Upper Jurassic, but as we round Durlston Head the Jurassic rocks dip to the east before disappearing under the town of Swanage.

North of Swanage the Wealden Shales are overlain by strongly dipping strata of the Lower and Middle Chalk. At Ballard Down the rocks become tilted on their end so that the Upper Chalk bedding lies vertical, with this best seen at Foreland Point at the site of Old Harry Rocks, where the rocks are now just 65 million years old.