A HISTORY OF DURHAM 



(i) Lower Group, consisting of 



(a) a conglomerate at the base and 



[b) compact limestone. 



(2) Middle Group, consisting of 



(f) shell limestone and 

 {a) cellular limestone. 



(3) Upper Group, consisting of 



(e) botryoidal limestone and 



{J) upper yellow limestone. 

 It is better to have a classification such as this, confessedly open to 

 improvement but more useful, so far as it goes, than none at all. 



One striking result of the changeable nature of the Magnesian 

 Limestone is, naturally enough, constant difference in the degree of 

 resistance which its component parts offer to denuding action both 

 mechanical and chemical, and, as a consequence of this, extraordinarily 

 diverse weathering features. Where hard and soft, crystalline and earthy, 

 calcareous rock is as it were commingled in a kind of omniform mosaic, 

 it is not surprising to find caverns, ravines, stacks, promontories of all 

 kinds to be the rule, and all such features are eminently characteristic of 

 the coast of Durham from South Shields to the Hartlepools. One of 

 these features is deserving of special mention. This is the occurrence in 

 some of the cliff sections and in some of the adjoining sea stacks — 

 especially in Marsden Bay — of ancient caverns, V-shaped, and evidently 

 at one time subterranean waterways (like those in the Mountain Limestone 

 of Craven), the roofs or vaults of which have in course of time collapsed, 

 filling the underground ravine with angular fragments of the overlying 

 limestone. These fragments, wholly unrounded, have at a subsequent 

 period been cemented together by secondary dolomitic matter, and now 

 appear as portions of a solid mass of breccia — so solid that several have 

 resisted the waves and the weather better than the unbroken rock from 

 which the original caverns were eroded and now stand out as great sea 

 stacks on the beach. Such a mass is the fine stack known as Lot's 

 Wife near the well-known cave-drilled islet named Marsden Rock. 

 These peculiar breccias, the occasional formation of which even at the 

 present day gives rise to violent but of course quite local earth shakes, are 

 known as ' breccia gashes.' 



'1 HE RED BEDS OF SOUTH DURHAM OR SALT MEASURES 



A great series of red coloured sandstones and clayey arenaceous 

 beds, miscalled ' marls,' follows immediately upon the topmost portion of 

 the massive Magnesian Limestone. Qiiite a thousand feet of these strata 

 are met witli in south Durliam, and form the floor on which the Pleis- 

 tocene or Drift deposits have been laid in that region. The latter more 

 often than not conceal the former to so great an extent that no very 

 certain line can be drawn to indicate their lower boundary. Roughly 

 it may be said that the Durham side of tlie Tees from the mouth of 



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