This stretch of coastline on the Devon-Dorset border provides superb exposures through the Triassic-Jurassic transition and Cretaceous rocks that overlie them. The Undercliff area is a large and still active landslip.

The area is a designated SSSI and a National Nature Reserve (NNR) and is part of the Dorset and East Devon Coast World Heritage Site. Public access is restricted to the South West Coast Path, which runs through the undercliff, and along the beach. Parking is available at Lyme Regis and also above the Cobb, for access to eastern areas of the Reserve (coastal footpath and shore to Pinhay Bay). At the western limit of the Reserve, limited parking for cars is available just north of the access road to Axmouth Harbour and also within Seaton. There is no public access to any other area of the Reserve.

At the eastern end of the coastal section the cliffs provide excellent sections in the late Triassic Mercia Mudstone Group (c. 220 - 210 million years old). These desert plain mudstones give way to the coastal lagoon and shallow marine shales, muds and limestones of the Penarth Group, which formed on a hot arid coastline not unlike the modern Persian Gulf. Cliff exposures in Pinhay Bay show features in these deposits such as tidal channels and mud cracks (which formed as pools dried up). Deepening waters led to the deposition of the Blue Lias, which marks the base of the Jurassic (205-142 million years ago).

The lower part of the Lias exposed here comprises alternating limestones and shales. The limestones are well known for the fossil ammonites that they contain. Large ammonites are most spectacularly seen on the foreshore on either side of Seven Rock Point. The Blue Lias is also famous for the skeletons of large marine reptiles, such as ichthyosaurs and plesiosaurs. Overlying and cutting across the Jurassic deposits are the sands and sandstones of the Upper Greensand of Lower Cretaceous age (around 107-95 million years old) and the Upper Cretaceous Chalk. Large blocks of a hard part of the Middle Chalk are found on the beach in Pinhay Bay. The large landslips of the Undercliff have probably been occurring for thousands of years. One of the most famous of these landslips occurred on Christmas Day 1839 when a vast area of land slid seaward leaving a chasm to form Goat Island.