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fact that the Llandovery Beds here 

 rest directly upon the Trilobite-Dingle 

 Shales, the throw is only to the extent 

 of the thickness of the Llandovery 

 and Wenlock — probably about 500 feet 

 on the east. 



On the western side of the district, 

 the Moel-y-garth Fault lets down the 

 Silurian to the west, and the throw seems 

 to be of greater amount than that of the 

 Dron-y-Buckley Fault. The Moel-y- 

 garth Fault is a dip-fault, and causes a 

 displacement in the outcrop of upwards 

 of half a mile, bringing Wenlock zones 

 against the Caradocian of Moel-y-r th. 

 The direction of this fault seems to be 

 the normal one of the district, and several 

 subsidiary faults run parallel to it. 



The two main faults would appear to 

 come together under the alluvium to the 

 north of Guilsfield, and this effect is 

 what might be called a ' keystone-fault.' 

 or ' inverted trough-fault ' : the former 

 term is perhaps the more expressive. 



I am of opinion that some of the 

 apparent overlap of the Silurian beds, 

 especially the Ludlow Shales, is pro- 

 bably due to overthrust accompanying 

 normal faulting. This is difficult to 

 prove, on account of the nature of the 

 beds ; but it is a significant fact that, 

 wherever these soft and rotten Ludlow 

 Shales occur, they occupy high ground 

 and are much contorted and crushed. 

 Small overthrusts can readily be distin- 

 guished, especially in Cwm Caethro ; but 

 the overthrusting suspected would be on 

 a larger scale. It seems probable that, 

 in the movements which brought about 

 the folding and faulting of the area, the 

 lower and more consistent beds bent or 

 broke under the pressure, while the soft 

 shaly beds above would tend to move 

 forward over the beds beneath. This 

 phenomenon can be seen on a small 

 scale, in almost any quarry where hard 

 beds including shaly bands are folded. 

 Such overthrusting in the Silurian beds 

 has been noticed by Prof. Reynolds in 

 the Tortworth Inlier, 1 and by the present 



1 ' On the Fossiliferous Silurian Eocks of the 

 Southern Half of the Tortworth Inlier ' F. R. 0. 

 Reed & S. H. Reynolds, Q. J. O. S. vol. lxiv 

 (1908) p. 535. 



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