448 



STEATIGRAPHICAL GEOLOGY 



by a bed of black phosphatic nodules, the " coprolite bed," and 

 the clays above this contain Ammonites which resemble Portlandian 

 species, so that the beds have been classed by some as Portlandian. 

 As, however, the Ammonites are not identical with Portlandian 

 forms, and as the Neocomian Belemnite (B. lateralis) occurs 

 throughout, with Hoplites regalis in the higher part, the beds are 

 probably of Lower Cretaceous age (see p. 484). 



5. The Purbeck Beds. The typical Purbeck Beds are only 

 found where the Portland Beds are completely developed. They 

 are a variable set of beds exhibiting alternations of terrestrial, 

 freshwater, brackish -water, and estuarine conditions, the changes 

 from brackish to freshwater deposits being generally gradual, while 

 the reverse changes are abrupt, indicating sudden inroads of the 

 sea. In Durlston Bay, near Swanage, these beds are 400 feet thick, 

 and have been divided into Lower, Middle, and "Upper Groups, 

 characterised respectively by Cypridea purbeckensis, C. granulosa, 

 and 0. punctata (see Fig. 149). The following is a summary of 

 the cliff section : 



f Grey and purple marls with Vimparus cariniferus 

 Upper, | Shales with beds of Paludina limestone (Purbeck 



80 feet 1 marble), Cypridea abundant 



I.Unio Beds and shelly limestones with Vimparus . 

 Alum-shales and thin limestones with layers of gyp- 

 sum, Corbula, Cyrena, Cyclas, etc 



Hard shelly limestones with partings of shale, fossils 

 Middle, of marine and estuarine species .... 



155 feet I Cinder bed, a mass of Ostrea distorta .... 



Thin limestones, marls, and shales with remains of 

 insects, fish, and freshwater shells ; at the base is 

 a black shale with mammalian remains 

 ''Marly freshwater beds, Physa and Planorbis 

 Marls and marly limestones with Cardium, Corbula, 

 Cyrena, and Cypridce ...... 



Marly limestones and shales, with Cypridce . 



Beds of broken slaty limestone 



Brown bituminous limestones with a layer of dark 

 earth (dirt bed) resting on Portland stone 



Lower, 

 170 feet 



Feet. 

 14 



45 

 15 



64 



50 

 9 



43 



7 



92 

 36 



15 



19 



409 



The same series can be seen in Worbarrow Bay, Mupe Bay, 

 and Lulworth Cove, but it becomes thinner to the westward, 250 

 feet at Mupe Bay, only 176 feet at Lulworth, and at Ridge way 

 Hill 190 feet. 20 



The lowermost beds are well exposed in the quarries at 

 Portland, where the most remarkable stratum is the upper "dirt 

 bed," which is a dark-brown loamy earth an actual soil or terres- 

 trial surface, in which cycads and coniferous trees are still rooted. 



