534 Sir Aubrey Strahan — Geology of the Isle of Purbeck. 



neighbourhood of Swanage the whole sequence from the base of 

 Upper Chalk to the Portland Stone is open to examination, but time 

 would not admit of more than a brief inspection of the Purbeck and 

 Portland cliff-sections. The Upper Purbeck with Paludina Lime- 

 stones or " Marble-beds" and Unio beds form Peveril Point, and the 

 Middle and Lower Purbeck beds are shown more or less continuously 

 in Durlston Bay, a band composed of shells of Ostrea distoria (the 

 "Cinder Bed") forming an easily recognized horizon. About 

 30 feet below it lies the Mammal bed, a thin earthy layer which has 

 yielded the remains of several genera of marsupials. Below this 

 again are the Lower Purbeck limestones and marls, some with 

 gypsum, casts of crystals of rock-salt and insect remains, others 

 yielding a brackish-water estuarine fauna. A double fault, with 

 a downthrow of 100 feet to the south near the zigzag path, throws 

 the Cinder Bed from the top of the cliff to below the beach. The 

 junction with the Portland Stone is not well shown in Durlston Bay. 



The cliffs near Kimmeridge give a continuous section from the 

 lowest Purbeck (on the top of St. Albans Head) to a low horizon in 

 the Kimmeridge Clay, more than 1,000 feet of strata in all. From 

 the head westwards they show a descending section in gently 

 inclined strata, and at rather more than 500 feet below the top of 

 the Kimmeridge Clay the "Kimmeridge Coal" or "Brownstone" 

 emerges from below the beach. This highly bituminous layer is 

 about 2 ft. 10 in. thick and has been worked in the neighbourhood 

 from time immemorial, firstly for the manufacture of ornaments or 

 utensils, latterly as a fuel, and as a source of oil. During the War 

 it attracted much attention as a possible source of oil and other 

 products. Alum was also manufactured here. In Hobarrow Bay 

 the main anticlinal axis is reached, and thence westwards the same 

 strata are crossed in ascending order, until the beetling crag formed 

 by the Portland Stone comes down to the beach and stops further 

 progress. 



Lulworth Cove illustrates the effect of attacks by the Burf upon 

 nearly vertical strata varying in their power of resistance. The 

 Portland Stone has formed a natural breakwater, which, however, 

 has been breached in places. Stair Hole shows the first effects of 

 a breach ; the waves have worn holes through the stone and are 

 swilling the debris, the soft Upper Purbeck and Wealden strata, 

 through them. In Lulworth Cove the breakwater has been 

 completely broken through and a beautifully symmetrical natural 

 harbour formed in the outcrops of the Purbeck, Wealden, and Gault 

 formations. Everywhere the sea suffers a prolonged check on 

 reaching the Chalk. 



The coast east of Lulworth Cove shows all the formations below 

 the Chalk except the Lower Greenland, but much attenuated as 

 compared with Swanage. Here an unconformity below the Gault, 

 which becomes most pronounced at White Nothe a few miles west- 

 wards, becomes manifest for the first time. The absence of Lower 

 Greensand may be due in part to overstep by the Gault, and some of 

 the uppermost Wealden beds may be absent for the same reason. 

 The section at White Nothe shows the Gault resting on steeply 



