GEOLOGY 



The general strike of the Culm Measures is east and west, but in 

 the vicinity of the Dartmoor granite these rocks, as well as the Devonian, 

 show in most cases deflections of strike in distinct accommodation to the 

 irregularities of its boundary,* a fact shown on De la Beche's map. 

 Where unfaulted their lower beds are in contact with Upper Devonian 

 rocks on the north and south. 



The uppermost beds of the Culm Measures occur in two bands on 

 the latitudes of Torrington and Eggesford, but they die out eastward 

 toward Tiverton. 



From the above it will be apparent that the Culm Measures form 

 a broad synclinal curve, or trough, from beneath which the Devonian 

 rocks emerge on the north and south. 



That this structure is shallow is proved by the repetition of the 

 upper beds in two synclinal undulations, troughing out eastward ; by 

 the innumerable small curves repeating the beds in the coast section, 

 and in all exposures continuous across the strike for any distance ; 

 and by the repetition of the Lower Culm rocks by folds between Bamp- 

 tbn and Burlescombe and on either side of Dartmoor. The Devonian 

 rocks are also repeated by innumerable small folds, or contortions, and 

 the strain has often been so great that the inverted folds have snapped, 

 or yielded to the pressure to such an extent that their axes are lost 

 on the shear of small thrust planes. There are also many faults. In 

 north Devon, in spite of these structures, the general continuity of the 

 subdivisions is unbroken and their features are well marked and con- 

 tinuous ; but in south Devon the rocks are much displaced and dis- 

 turbed and in consequence the surface is more irregular. In no part 

 of the area is the evidence of these contractile movements more apparent 

 than in the (.? pre-Devonian) mica and quartz schists and altered basic 

 rocks of the Start, Prawle and Bolt districts. These have had their 

 original bedding planes gnarled, crumpled and broken so that it is 

 generally impossible to trace the minor foldings, and strain cracks or 

 planes which simulate bedding have been produced. 



Throughout the Devonian area the cleavage planes appear to have 

 resulted from the strains to which the bedding was subjected ; in some 

 rocks making, if developed at all, a very partial appearance ; in others 

 varying in direction and amount of dip, being most horizontal where 

 gnarled, or zigzagged, bedding indications are very highly inclined or 

 vertical. The latter character is well displayed in argillaceous Upper 

 Devonian strata on the west of Dartmoor, by structures simulating even 

 bedding with low dips (as in the altered basic rocks of the Prawle 

 coast) . 



The prevalence of southerly cleavage dips and of bedding <:urves 

 inverted toward the north has made the work of piecing together the 

 complicated structures of south Devon one of great difficulty, whilst it 

 proves that the earth movements were lessening northward. 



' Pnc. Somerset Arch, and Nat. Hist. Soc. (1892), vol. xxxviii. pp. 181-93. 



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