64 GEOLOGICAL NOTES ON T1IE ISLE OF PORTLAND. 



Clifton, and a few seat to the Geological Society. The red 

 loaoi can be traced nearly a mile to the Old Quarry near the New 

 Prison Church. This deposit may be referred to the High-level 

 Platform Drifts, which of ten occur upon hill plateaux at consider- 

 able heights above the level of the Higher Yalley Gravels and 

 do so here and there on the opposite, range between Hardy's 

 monument and Swanage. 



This drift contains some pebbles of Chert from the 

 Upper Greensand, and of iron stone and Sarsen-stone with 

 chalk flints from the hills between Upwey and Dorchester, which 

 are separated from Portland by a low plain of Jurassic beds 

 a nd although at a greater distance from the Tertiaries are 

 more freely furnished with these relics than are the more recent 

 drifts of the intervening space. 



Professor Prestwich argues that during the deposition of the 

 Drift, there was a gradual sloping plain from the Greensand and 

 Tertiary area to the Bill of Portland, and that at that period Port- 

 land vas joined to the main land and the materials conveyed by a 

 stream which flowed from north to south, bringing down the 

 Greensand and Tertiary relics with which it was charged. Sub- 

 sequently an anticlinal running east and west bringing up the 

 underlying beds, as low down as the Forest Marble, between 

 Broadway and Bucklaud Eipers, raised the south end of Portland 

 as it now appears, and with it the corresponding beds at Eidg- 

 way. The great fault in this locality however is of an older date. 

 A simultaneous denudation of the Weymouth district took place 

 when it was temporarily submerged and during a period of grad- 

 ual emergence, with intervals of short oscillations of varied form 

 and strength, brought down the soil charged with land shells and 

 light materials alternating with the coarser ones. The final 

 emergence must have been attended with some violence, proba- 

 bly volcanic, denuding the island of the Middle 

 Purbeck beds, sweeping them into the sea. 

 At Cliesilton the debris was nat carried out saa-warl bat 

 spread out beyond the foot of the escarpment, as may be 



