Dorset Field trip: Thursday October 4th 2018 Aim: To study the Lower Jurassic Lias strata (Bridport Sands and Black Ven Marl) and active mass movements. Location 1: Burton Bradstock (grid ref. SY 490888) Guide; Alan Holiday

The group walked down to Hive beach for a general view of the strata. The beach consists of pea grit shingle (chert and flint clasts) much smaller in size than the rest of Chesil beach to the southeast. Looking west there are cliffs of Bridport Sand from beach level overlain by 4m of Inferior Oolite topped by Fuller’s Earth clay. The strata dips to the East and its porosity resulted in it being a reservoir rock for the Wytch Farm

oilfield where the strata is 900m below the surface. The appearance of the Bridport Sand is very weathered with banding where the cementation and grain size differ. Undercutting is prevalent and results in frequent rock falls bringing down the Oolite and clays from above. The sandstones and clays deposited on the beach are soon removed by wave action. There are no fossils in the Bridport sand but possibly they were not preserved because of diagenetic changes (decalcification). Fossils are abundant in the Oolitic limestone above. Similar rocks to the Bridport Sand can be found further to the North, e.g. the Bath and Cotswold sands. These are a diachronous horizon indicating that environmental conditions migrated southwards as sea levels lowered. The low cliffs to the East are Fullers Earth clay also known as Frome Clay. These have been downthrown by about 20m on a normal fault which runs up the valley between the Frome clay and the Bridport Sand and was exposed on the beach after the 2014 storms

Rock armour has been placed on the beach to protect the cliffs. It comes from a local quarry and is fossiliferous. A good example of a Nautilus was pointed out. At this time the nautilus was believed to be a scavenger rather than a predator. They pre-date the ammonites. Other fossils found in fallen blocks were trace fossil burrows on the surfaces which had been

weathered out by carbonation.

Liesegang rings were found - patterns due to different iron oxide precipitates. As it was unsafe to walk along the beach close to the cliff we walked along the top and down to the beach at Burton Freshwater where it was safe to examine the cliffs. Evidence of past rock falls were apparent. Vertical fractures

which are filled with Jurassic material and are called Neptunean Dykes. The piers at the harbour entrance to West Bay reduce the long-shore drift from the west and starve the beach of new shingle.

Fallen rocks were studied and found to contain “snuff boxes” which are a distinctive horizon in the Oolite. Formation of these is not conclusive but appears to be the result of alga growth around bi-valves. Belemnites and bivalves were found.

Location 2: Charmouth (grid ref SY 364930) Guide; Alan Holiday From Charmouth there is a good view of the Spittles landslip to the NE of Lyme Regis. This is reportedly the largest landslip in Europe and is still active. A new sea defence and esplanade around Lyme Regis is preventing both marine erosion and sub-aerial mass movement. The strata are the Lower Jurassic overlain unconformably by Cretaceous sequence. They consist of Blue Lias followed by Black Ven Marl series topped by Greensand. This makes them susceptible to mud slides as the Green sand is permeable and the marls become saturated and slide seawards. The beach is chert shingle weathered out of the Greensand. Few fossils were found in the rocks at the base of the cliff mainly because of the time of year.

Location 3; Bowleaze Cove (grid ref: SY 704818) Guide; Alan Holiday From the cliffs there was a good view of the sea defences to protect Weymouth with groynes and a sea wall to prevent erosion and stop shingle being washed onto coast roads. Shingle has moved from the eastern of the beach back towards Weymouth regularly as there are no other groynes to prevent the movement. River gravel was imported to supplement the beach but it was poorly sorted and the fines were lost.

We then walked along the cliff path to inspect excellent examples of successional rotational rock falls. The cause is the underlying geology again which consists of impervious sandstone overlain by clay overlain by Osmington Oolitic limestone. Water passes through the limestone saturates the clay because of the base layer being impervious and the clay strata then fails leaving the limestone unsupported which in turn fails. Parallel fissures in the ground on top of the cliff indicate that this is an active process today.

The limestone blocks at the base of the cliff contained many fossils. Particularly some trace fossils of ‘U-shaped worm burrows (Domichnia), bivalves, ammonites and fossil wood.