Yol. 56.] SILTJEIAN SEQUENCE OF KHAYADER. 9& 



Micaceous Elags and Grits (AbJ. A small exposure of the 

 uppermost beds of the latter division has been swept bare in a 

 small gully about 150 yards south of Ty'n-y-graig. The rock 

 shows intermixed blue flags and shales, weathering to a brilliant 

 scarlet and orange. The flags are striped with thin white grit- 

 bands and lustrous calcite-films may be noticed in certain beds. 

 I have obtained from here a few graptolites (Diplograptus sp.) 

 and a specimen of Ortlioceras. About 200 yards higher up the 

 hill, approximately in the same line of strike, certain shales lining 

 the sides of the path to Gro-isaf yield Climacogrcqitus parvulus, a 

 fairly conclusive index of the true horizon of these beds ; and any 

 uncertainty is dissipated on continuing the section, for blocks of 

 the soft brown beds of the Eottenstone Group (Ab 2 ) strew the 

 sides of the lane immediately above Ty'n-y-graig Cottage. About 

 150 yards farther on, the Conglomerate-bed (Ba.) is again met 

 with. 



(4) Nant-y-Gro. 



The fourth and last section is by far the most interesting, as 

 regards the Gwastaden Group, that is to be found in the western 

 areas. For, not only are the exposures continuous for nearly a 

 mile, but they show that even at a distance of over 4 miles from 

 the typical section the divisions of the lower portions of the 

 Gwastaden Group have maintained, to all intents and purposes, 

 their lithological characters, and after a careful traverse can be 

 separated out. The section figured here (fig. 9, p. 92) is taken 

 down the Nant-y-Gro, which discharges its waters into the Elan r 

 about ^ mile below its confluence with the Claerwen. 



The rocks in this area are much faulted and folded, but with 

 detailed measurements of the dip the beds can be fairly well pieced 

 together and the true succession determined. Erom Allt Goch 

 Hill the Cerig Gwynion Grits (Aa) are easily traced westward 

 through Crugiau Bach, and, after a slight dislocation, make their 

 appearance on the right bank of Nant-y-Gro, whence they may be 

 carried on by the eye to the bold terraces of Gro Hill. We 

 again find the cleaved Blue-black Shales underlying the grits in the 

 Xant-y-Gro itself, some 400 or 500 yards above the ford. Some 

 distance down, the grits appear in the form of an anticline : 

 and immediately below the ford a strike-fault brings down the 

 overlying Micaceous Elags and Grits (AbJ. In the latter 

 group, however, the individual grit-beds are much thicker than 

 one has been accustomed to find hitherto. Erom a few flaggy beds 

 in the basement-grits well-preserved specimens of Climacograptus 

 normalis may be extracted. 



Below the footbridge, another fault running north-north-east and 

 south-south-west cuts out a considerable thickness of rock, the strata 

 between the dislocations lying in a small syncline. The rocks are 

 found to be somewhat crumpled and folded farther down stream, and, 

 in consequence, cover a greater breadth of outcrop; and thus it is not 

 until we come within about 200 yards of the concrete dam, which 



