GEOLOGY 



with drab bands has been encountered in a shaft sunk at the Clay Works 

 between Lower Marland and Moor Hill farms to a depth of 80 feet 

 without reaching its base. At 56 feet from the surface, veins (or irre- 

 gular beds) of lignite, seldom exceeding a foot in thickness, were en- 

 countered. These clays were described and correlated with the Bovey 

 clays in 1879/ About a quarter mile from Petrockstow church fine 

 gravel (chiefly of worn quartz and occasionally bound in ferruginous 

 cement) and coarse sand contains a few fragments of whitish sandy rock, 

 in one of which a portion of Echinus was found, denoting derivation in 

 part from Selbornian materials. Towards Huish gravel is also worked. 

 These gravels may be marginal deposits of, or overlie, the clays. At 

 about 1 1 miles north-west of Haldon Belvidere and about 14 miles from 

 the Petrockstow depression, a patch of gravel rests on the Lower New 

 Red breccia, at a quarter of a mile west of Colford (near Yeoford) . A pit 

 5 feet deep showed (twenty-seven years ago) pale buff gravel of small 

 subangular quartz stones with broken well-worn chalk flints and a few 

 pieces of dark grey slaty rock. 



This cordon of observations points to the north-westerly extension 

 of Cretaceous rocks and of Tertiary deposits between Haldon and Bide- 

 ford Bay, and it is not improbable that a detailed survey of the Culm 

 area would afford further evidence. 



The large tracts of disintegrated granite in the drainage depressions 

 of Dartmoor may be, in part, the relics of Tertiary gravels and sands, 

 dating to a period when the lakes of Petrockstow and Bovey were fed 

 by streams rising in the granite. 



The sloping surface of the Cretaceous plain from Black Down south- 

 ward, coupled with the difficulty in distinguishing sand associated with 

 Tertiary gravels from Greensand which may underlie them, in view of 

 the great thickness of sand said to underlie the Bovey pipeclays in that 

 depression, calls for additional and protracted investigations in the Bovey 

 valley and on its borders, before we can reconstruct the former extension 

 of the Cretaceous and Tertiary rocks, and assign a definite reason for 

 discrepancies in level with reasonable hope of success.' The simple ex- 

 planation given in the prefatory notes of deposits left at different stages 

 in the processes of Tertiary denudation has been provisionally adopted. 



The planed surfaces of the Devonian limestone hills of the Torquay, 

 Brixham and Plymouth area probably date from Permian times, as out- 

 liers of Lower New Red on the surface and in fissures are found near 

 Waddeton and Brixham, etc. ; but, as in the case of the potholes in the 

 limestone north of Kingsteignton, these materials may have been re- 

 moved during, or prior to, the deposition of Tertiary gravels and sands, 

 and subsequently corroded hollows may have retained, very locally, traces 

 of Tertiary deposition. 



On the Hoe, Plymouth, averaging 100 feet above mean tide, 



' Ussher, 'On the Deposits of Petrockstow in Devon,' Trans. Devon Assoc, for 1879. 

 * Godwin-Austen attributed the discrepancies in level to subsequent movements {^art. Jouni, 

 Geol. Soc, vol. vi. p. 91). 



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