ix. MARINE GEOLOGICAL EXCURSION. 



was capable of i* gape of over a yard. Durlston Bay presents the finest develop- 

 ment of the Purbecks known anywhere ; the width of the Bay, about 1 mile, 

 and the thickness of the beds, about 400ft., or twice the thickness obtained at 

 Lul worth. The following is the complete sequence : 



Upper Purbeck 

 Faludina-maxbie. 

 Burr Beds. 



Middle Purbeck 



Corbula and Beef -beds. 



Stone beds. 



Cinder-bed with Ostrea distorta. 



Stone Beds. 



Flint Bed. Black Shale, and Mammal Bed. 



Lower Purbeck 



Marls with gypsum. 

 Caps. 



Portland Stone. 



Durlston Bay has yielded the bulk of the Purbeck fossils in the various 

 museums, and these mostly come from the Middle Purbecks. It may be noted 

 generally that the changes from marine through brackish to freshwater con- 

 ditions are gradual, whilst the changes from freshwater to marine are sudden. 

 The little Mammals occur in a sort of ft dirt-bed," a few inches thick, at the base 

 of the Middle Purbecks. There is Likewise an interesting example of a forked 

 fault, which strikes the coast and may be noted about the foot of the zigzag path. 

 As one faces the cliff the hard Cinder -bed on the left (south) is brought into 

 juxtaposition with the soft beds of the Lower Purbeck on the right (north), the 

 Cinder -bed itself being lifted high up in the cliff on the north side of the fault, 

 which has a down-throw of 100 -150ft. to the south. 



Durlston Head, Tillywhim, and Anvil Point. The appearance of the hard 

 Portland Be is gives stability to the land, and thus commences a different type of 

 coast trending to the west. The junction of the Purbecks and Portlands is here 

 disturbed by many small faults. At Tillywhim there is a sequence from the 

 Portland chert -beds through the Portland freestones (Tillywhim Caves) to the 

 Oyster-bed (Perna Bouchardi), which here forms the top of the Portlandian 

 series, whilst the Purbecks occupy the surface of the promontory, and about a 

 mile west of the Light House there is a "dirt-bed" in the brow of the cliff. 

 From thence to Winspit the cliffs are very vertical, consisting of Portland Eocks 

 below with a slope of Lower Purbecks. There has been much quarrying in the 

 Portland Stone, and there are several small faults in the neighbourhood of 

 Dancing Ledge and Seacombe Cliff, and as far as the Valley at Winspit. This is 

 a very forbidding coast, yet vessels have in former times loaded with stone close 

 up to the rocks. 



