602 



ME. D. C. EVANS ON THE ORDOVICIAN [JSToV. I906, 



is across the strike, and they have thus cut up the country into a 

 series of ridges and hummocky hills, few of which reach an altitude 

 of 600 feet. These dip-valleys afford numerous rock-exposures, 

 but none too many to enable the geologist to interpret so complex 

 a structure with any degree of confidence. 



The peculiar hollow extending west of Caermarthen by way of 

 Llanllwch traverses the whole length of the district with a few 

 slight variations, falling in with the Tave Valley west of St. Clear's. 

 On the south, the ground rises towards the Old-Red-Sandstone ridge ; 

 and on the north it forms a broken upward slope to the high ground, 

 Avhich here may be considered as the eastward continuation of the 

 Prescelly Hills. 



(6) Drifts. — On the whole, the country is fairly free from 

 surface-accumulations, such as Boulder-Clay and other drifts. There 

 is some alluvium in the valleys, especially as they approach the sea ; 

 but in their upper reaches, where they are generally very narrow, 

 the quantity is practically negligible. 



In the vicinity of. St. Clear's, some Boulder-Clay and gravel are 

 found. A patch of the former near Morfa-bach is in places 

 impregnated with ferruginous matter, deposited probably by spring- 

 water rising from the underlying pyritous shales. This feature has 

 led to much fruitless endeavour to find coal. 



Near Trefanty, Morfa-bach, and Mylet, a considerable quantity 

 of sand and gravel occurs. In a pit near the first-named place there 

 is a fair sample of oblique and contorted bedding. 



Gravel is also dug on the high ground north-east of Glyntaf, as 

 well as near Maesgwynne, and at two or three points in the Gynin 

 Valley. 



III. The Succession and detailed Description of the Rocks. 

 Table I. — The Succession in the Northern Limb of the Anticline. 



Bala-Caradoc. 



(?)• 



f Brown-grey mudstones, shales, and grits. 



Llanvirn. 



y Black slaty beds, with Dicranograptus. 

 I 



Llandeilo. ■{ 



Dicranograptus- 

 Beds. 



Beds. 



,-Murchisoni 

 Beds. 



D.-bifidus 

 Beds. 



Ashy black shales, with lenticular bands of 

 limestone, containing Dicranograptus, 

 i Leptograptus, etc. 



■{ Black shales and mudstones weathering 

 ! buff and yellow, containing graptolites 

 I and trilobites. 



' Black shales, with crowds of Diplograptus 

 ^- and shells. 

 AsapJms-ti/r annus- f Ashy beds and sandy flags, with Asaphus 

 \ tyv -annus, etc. 



f Striped gritty flags, with abundance of 

 ( Didymograptus MurcMsoni. 

 \ Blue-grey ash, weathering rusty brown. 

 I Soft mudstones, with crowds of Didymo- 

 ^_ graptus Murchisoni. 

 J Shales, mudstones, and bands of ash, with 

 \ graptolites and trilobites. 



1 ' Bala-Caradoc ' includes beds of the Shropshire type and Sedgwick's ' Upper 

 Bala.' 



