LLANDEILO FLAGS (RANGE AND DISLOCATIONS OF). 



355 



On the high road from Caermarthen to Swansea, where pebbles of white quartz of the size of small 

 beans are disseminated in a base of dark grey schist, with some felspar and green earth, the strata 

 being vertical, and their strike from north-east to south-west. It is no easy task to distinguish 

 these sandstones from the overlying and underlying formations in their progress westward from the 

 right bank of the Towy to St. Clears, on account of great denudations, and the apparent absence 

 of fossils and calcareous strata ; but before we enter Pembrokeshire, the Llandeilo flags reappearing 

 in great force, are again clearly separated from the Upper Silurian Rocks by thick masses of yellow 

 sandstone, largely quarried at Llandwror, and containing fossils of the true Caradoc formation. 

 (See Map.) 



** Llandeilo Flags." 



Llandeilo flags, distinguished by the presence of the Asaphus Buchii and A. tyr annus, 

 and underlying the great mass of Caradoc sandstone, are exhibited on the left banks of 

 the rivers Sowdde and Towy below Llangadock. They extend thence by Tan- yr- alt, 

 Pen-y-banc and Pompren-arreth, to the low hills of Pentref and Tir-wyn-fach, where 

 they strike across the Towy, occurring in great force at Llandeilo and in Dynevor 

 Park. From Llandeilo to Caermarthen, this formation is seen on the right bank 

 of the river, chiefly in detached and broken masses, near Llangathen and Llanegwad, 

 the only places where I have detected the rock on the left bank being at Golden Grove 

 and at Capel dewi. Beyond the latter point the calcareous matter thins out, and does 

 not reappear till we reach Clog-y-frain, on the borders of Pembrokeshire, though the 

 course of the formation is traceable at intervals by its organic remains, as at Pensarn, 

 &c, near Caermarthen. 



By consulting the map, it will be seen that between Llangadock and Caermarthen, a 

 space about fifteen miles in length; and from half a mile to two miles in width, these 

 Llandeilo flags have been singularly thrown about with divergent strikes and reversed 

 dips. At their north-eastern end a transverse section (PI. 34, fig. 5.) from Tan-yr-alt, 

 to Blaen-dyffrin-garn, passes over the low hills of Pen-coed, Pen-llan, and Tyr-y-garn s 

 exposing beds of black calcareous flagstone, occasionally very pyritous, and more or 

 less charged with trilobites, alternating with thickish bedded strata of grey, quartzose 

 sandstone and dark shelly grits 1 . These flagstones have a prevailing direction to the 

 south-west, and dip in opposite directions, i. e. both north-west and south-east, at high 

 angles. In following the strata to the south-west, they are subjected to several breaks, 

 by one of which they are deflected to the north-north-west, or nearly at right angles to 

 their prevailing strike ; but they resume their south-westerly direction, and range with 

 tolerable regularity along the western flanks of Cairn-goch and Carreg-cegin, plunging 

 at one or two spots at high angles beneath the overlying sandstone. (PI. 34, fig. 6.) 

 At Pompren-arreth, the subordinate beds of grit thinning out, the blackflags and shale 



» Encrusting springs issue from some of the more calcareous beds. 



