The Jurassic Coast, near The Casterbridge Guest House, Dorchester

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The Casterbridge Hotel is perfectly poised about the mid-point of the Heritage Coast. From here any part can be reached in an hour and an excursion to the oil bearing strata and shale-pennies of Kimmeridge can be followed next day by fossil collecting beneath the Golden Cap or the equally golden capped Stonebarrow near Charmouth.

A fossil from the Jurassic CoastWest BayLyme Regis

95 miles of gently tilted rock strata display 185 million years of geological time. The Jurassic Coast is so named because every stage of the period can be found at some point on the coast. The west end of the site close to the Exe estuary is the oldest. Here the rich red cliffs are formed from Triassic desert sand and the greater part of the Triassic period is represented from 250 million to 203 million years ago.

Around 203 million years ago the deserts slipped beneath a shallow sea and the Jurassic period began. The remarkable fossils in the grey limestones at Lyme Regis show how different Dorset had become. From Lyme to Swanage the strata are progressively younger until about 135 million years ago the Jurassic passed into the Cretaceous and the brilliant chalk cliffs of Ballard Down and Old Harry Rocks mark the eastern end of the World Heritage Site.

Lyme Regis

Instead of following the crowds onto The Cobb turn right and walk west to Monmouth Beach.

Scattered beneath your feet on Monmouth Beach are huge ammonites. They are far too big to collect and would probably shatter if you tried but if you look around you will find examples showing internal structure, others with external shell details and yet more with mineral deposits within their chambers.

It is a breathtaking introduction to the Jurassic, for introduction it is; these grey stones are among the oldest of the Jurassic series. Only a little further west of here at Pinhay Bay the last of the Triassic strata disappear beneath the surface. Here is where the Jurassic begins.

Chesil Beach

The long bank of the Chesil protects the Fleet Lagoon and the low lying country behind.

The pebbles of the beach, almost all of flint and chert, contain samples from many points to the west including distinctive stones from the far side of Lyme Bay. All sorts of elaborate theories have been proposed to explain how they got there. In fact the answer is probably that the beach is old. The pebbles formed an ancient beach across what is now Lyme Bay which was rolled back by a rise in sea level. The beach is still moving a little, old charts have it a little further offshore and big storms push material over the crest to the inshore side and continue the rolling motion of the stones.

The Fleet Lagoon may occupy a drowned valley. These days it looks much more like a one sided river valley than a coastline. Now there are only a few small streams draining into the Fleet, the lands forming its original catchment must be beneath the sea. Despite the estuary-like appearance of the opening into Portland Harbour at Small Mouth the water there is salt, not fresh.

Lulworth Cove

It was probably the tiny stream which now discharges into the northwest corner of the cove that made the first breach through the tough Portland stone that guards the soft cretaceous chalk behind. The energy of the waves then pushed through and enlarged the gap into the perfect circle we see today.

The near vertical band of stone was twisted into position by the forces that built the Alps and the Pyrenees to the south. Evidence of the forces involved can be seen in the folded strata of the Lulworth Crumple seen at its best at Stair Hole nearby.

Kimmeridge Bay

Soft shale cliffs are eroding fast. Soon the sea will break into the lower land behind the existing cliff line and the bay will widen dramatically.

The Clavell Tower on the clifftop was in danger of falling into the sea and has now been moved a short way inland. This picture was taken before the move. For more evidence of rapid erosion look out for the WWII pillbox that was set snugly in the opening in the cliff where the stream emerges but now is exposed and crumbling on the beach.

The beach here is comprised of chunks of limestone toppled from the cliff along with the shale. The fragile shale breaks into thin sheets which rapidly wear into discs known as shale 'pennies' and then are ground away by the waves. Some of these rocks are oil bearing. Quite often a sharp petroleum smell arises from the debris of a cliff fall and it is possible to find limestone fragments with a strong natural odour.

Kimmeridge oil is extracted by the 'Nodding Donkey' on the clifftop. Despite being at the culmination of a visible anticline the source of the oil is a mystery for the drill string terminates in the cornbrash which cannot be the source. During the seepage from its secret reservoir to the well the oil is naturally filtered and emerges partially refined. Kimmeridge oil is pale green, translucent and as one oil-man put it, "Ready to drink."