Dorset’s Jurassic coast
In the final part of his east-to-west journey, Colin Varndell travels from Bexington to Lyme Regis
Published in July ’14
At the far west end of the Fleet lagoon, the Chesil Beach joins the mainland. Here large cubes of concrete mark the spot, affectionately known locally as the ‘tank teeth’. They were constructed at the beginning of World War 2 as a hindrance to an anticipated German invasion which was expected here. From Abbotsbury to West Bexington the coastline is less rugged with agricultural land sloping gently down to the beach. In the past there has been a spectacular wild flower display along the beach here in summer, but it appears that much of this lush vegetation may have been lost during the recent winter storms. The reedbeds which stretch from Bexington to Cogden are a haven for birds with huge starling roosts in winter and incessant music from the songs of reed, sedge and Cetti’s warblers in summer.
Cliffs appear again west of Cogden, rising gently at first before sloping down to Hive Beach at Burton Bradstock. One of the most spectacular landscape features of the Jurassic Coast can be seen at Burton, the perpendicular, yellow ochre cliff of Bridport Sands. This section of cliff between Burton and Freshwater is made up of bands of sandstone aptly named Bridport Sands, and is capped with inferior oolite and fuller’s earth. This cliff is much less stable than the east cliff of West Bay, with large rock falls occurring fairly frequently. It is generally agreed that the frequency of these rock falls is due to the narrower beach here than further west, providing less protection from the pounding sea.
Rock falls along the Dorset coast are often met with surprise, but this is a situation which has been on-going for millions of years. It is widely believed that even during the relatively recent iron age, the Dorset coastline was one mile further into the English Channel than it is today. Although rocks do fall along the section of cliff between Freshwater and West Bay, it seems to be more like individual chunks from the hard rock seams falling, after the softer layers supporting them are eroded, rather than the massive avalanches of rock which occur further east. The clay cliff between West Bay and Eype is made up from fuller’s earth with some forest marble. The name fuller’s earth originated from the technique known as ‘fulling’ when it was used to assist the cleaning of raw wool. This is a softer rock than the east cliff and is therefore much less stable and not so uniform in shape. The huge chunks of rock which are strewn on the beach here are forest marble. They have found their resting place on the shore after tumbling down from the cliff above.
Beyond Eype the cliffs become higher, rising to Thorncombe Beacon at 157 metres above sea level. The view east from here is one of the most spectacular of the Dorset Jurassic Coast, taking in Eype, West Bay, Burton, Cogden with the Chesil Beach and Portland in the far distance. The coast path from Thorncombe Beacon descends to Ridge Cliff and down to Seatown, a tiny village which can also be reached by road from Chideock.
Golden Cap is one of Dorset’s best known landmarks, and is the highest point on the south coast of England standing at 191 metres above sea level. The golden colour is due to deposits of upper greensand, but much of this is now obscured by encroaching scrub. Classic views of Golden Cap can be seen from Ridge Cliff and also looking back eastwards from Charmouth.
Often referred to as Dinosaur Land, the Spittles and Black Ven together form one of the most unstable stretches of coastline in Britain, with rockfalls and mudslides occurring with regular frequency. Thousands of huge boulders lie on the beach here, which have been deposited by mudslides. The mud having been claimed by the sea leaving these rocks to be fashioned and rounded by the constant movement of the waves.
The cliffs of the Spittles are a series of giant grey terraces, formed from blue lias clay interspersed with denser limestone layers. This landscape is constantly changing, especially in prolonged periods of wet weather when mudslides are more likely to occur. Young trees and shrubs establish themselves on these clay terraces, but as the ground shifts, they too slide down to the shore to be claimed by the sea. Stormy weather creates perfect conditions for mudslides to occur, and after such, the shore can be teeming with fossil hunters. Ammonites and fossils of marine reptiles are often uncovered here in superb detail. In fact, this stretch of coastline is regarded as one of the best fossil sites in the world.
At the Spittles end at low tide, a huge, flat area of rock is revealed aptly named Broad Ledge. This is reputedly one of the best rock-pooling sites in Britain, and it is not unusual to see scores of people looking into the pools to study the marine flora and fauna.
Like all of the river valleys which spill into the sea along the Dorset coast, the steep valley where the river Lym trickles down today would have been carved by huge volumes of water. These were the vast rivers, which resulted in the melting ice from the great northern glaciers at the end of each of the ice ages.
On Monmouth Beach, west of Lyme, we come to the end of the Dorset stretch of the Jurassic coast, with the county’s oldest rocks, the blue lias beds displayed in the cliff. But the story does not end here, as it continues even further into Devon, with rocks representing the Triassic period as far east as Exmouth. ◗
The alternating bands of hard and soft rock are thought to have settled as sediment during different environmental conditions. There is more than one theory about how this came about. One idea is that the sediment was laid down in a huge river delta, the soft rock representing prolonged periods of flood. After the flood retreated, marine animals stirred up the surface to create the harder rock layers. The theory I like best though suggests these sediments were laid down in the sea. During periods of rough weather lasting thousands of years, it was only the grains of sand which could settle on the seabed, while organic debris continued to be churned around in the sea. As grains of sand are angular in shape they could not bind tightly together and therefore formed the soft bands of rock. During calmer weather, organic matter which included the shells of tiny animals, could also settle with the sand filling in the gaps and voids and became compressed, thus forming the harder bands of rock.