Yol. 68.] GEOLOGICAL STKUCTURE OF CENTRAL WALES. S37' 



appears to separate in a general way the tracts of Silurian and 

 Ordovician rocks. 



The Aberystwyth Grits. — Fortunately the base-line of these^ 

 grits can be traced with greater precision than that of most of the 

 other groups, and it affords useful information regarding the 

 structure. The grits form two belts, one lying between the Teifi 

 Yalley and the coast of Cardigan Bay, and extending from Berth to 

 near Llangranog. This outcrop is indicated on the Geological 

 Survey map, but it is possible that, in certain cases (near Mydroilyn 

 and Dihewid), other grits occupying a lower horizon have been 

 included with them. The range and characters of these grits have 

 been described by Keeping. 



The other outcrop occupies the high tableland east of the Teifi 

 Valley. Until recently these grits were confounded with those of 

 Plynlimon, which are separated from them by a thickness of nearly 

 a mile of rocks. The discovery of characteristic late Gala grapto- 

 lites close to the base of the eastern grits near Llyn Rhuddnant and 

 in the western grits near Aberystwyth, together with the great 

 similarity in their lithological characters, conclusively proved their 

 identity. 



On the Geological Survey map the eastern grits are represented 

 by long lines of yellow dots following the prominent strike-ridges ;. 

 on the whole, their distribution appears to be fairly correctly 

 indicated. 



The grits are best developed on the hills east of Tregaron and 

 Llanddewibrefi, and there extend considerably farther west than is 

 shown on the Survey map. They appear to succeed shaly strata 

 with sandy seams, which at Pont Llanio station yielded numerous; 

 graptolites, namely : Monograptus Jiolmi, M, marri,, M. turriculatus,. 

 M. resxirgens^ M. cf. exiguus, M. nodifer, 31. nudus, and early types 

 of 31. priodon^ an assemblage corresponding closely with that of 

 the Blaen-Myherin Mudstones which occur not far below the base- 

 of the Aberystwyth Grits farther north. ^ 



East of Llanddewibrefi, massive dark-grey grits are repeatedly 

 thrown into shallow folds, and it is impossible to make out 

 in detail the stratigraphical arrangement of the beds. About 

 3 miles in that direction occur the massive conglomerates which 

 form the canspicuous ridge of Craig Twrch ; while still farther east 

 there is a considerable thickness of pale-green mudstones with thin 

 gritty seams, associated (at a place called Draenllwyndu) with a band 

 of maroon-coloured mudstones. The lithological type is precisely 

 that of the Dolgau Mudstones, which in the Tarannon area succeed. 

 the great grit group of Talerddig, and there is little doubt that they 

 form the highest strata exposed in Central Wales west of the 

 Wenlock boundary. They appear to occupy the core of a deep 

 syncline, and to this fact owe their preservation in this remote spot. 

 The credit of discovering these rocks belongs to Mr. Jenkin Lloyd,, 



^ Q. J. G. S. vol. kv (1909) p. 510. 



