306 CARADOC SANDSTONE OF THE VALES OF MEIFOD AND TANAT. 



The shells most abundant here, are Orthis expansu, O. testudinaria, O. bilobata, O. canalis, fyc. 

 These latter beds with shale (near a spot called the Gryfin) pass at an angle of 20° under variegated 

 conglomerate, composed of quartz, slate, hornstone, &c, the fragments varying in size from pins' 

 heads to a man's fist. The strata, like those of the Moel-y- Garth, are traversed by joints which cut 

 through the quartz pebbles, and their fractured surfaces have often a high degree of polish. This 

 red conglomerate, overlying the whole system of the grey sandstones of the Gaerfawr, proves that 

 the similar red conglomerate and gritty beds of Welch Pool, Powis Castle, the Quakers' burial- 

 ground, and other places which rise from beneath the overlying mudstones or Upper Silurian rocks, 

 form the upper strata of the Caradoc sandstone or Lower Silurian Rocks. The conglomerate beds 

 passing into grits more or less red and occasionally calcareous, may be followed from the north- 

 western flank of the Gaerfawr for about half a mile to the north-east and about two miles to the 

 south-west, the grey and calcareous sandstones uniformly rising from beneath them. If the trans- 

 verse section from the ridge of Gaerfawr to the north-west be continued, we pass over about three 

 miles of undulating country and hilly grounds, from 700 to 900 feet above the sea, the whole com- 

 posed of the mudstone or Upper Silurian Rock, before we reach the beautiful vale of Meifod, watered 

 by the Fyrnwy, on the left bank of which are numerous ridges of Caradoc sandstone. 



This formation now begins to assume even more importance than on the flanks of 

 Caer Caradoc ; for from the banks of the Fyrnwy to those of the Tanat it occupies 

 the whole region. The first ridge is marked by its sharp and narrow form. The 

 highest part, or north-eastern end, called Alt-y-maen, is 1160 feet; Galt-yr-Ankr, its 

 south-western, is 740 feet above the sea ; the strata of the former dipping at an angle 

 of 70°, those of the latter 60° to the south-east. The direction of this ridge is there- 

 fore parallel to that of Gaerfawr, or in other words the valley of the Fyrnwy is 

 parallel to that of the Severn. These inclined strata of Caradoc sandstone are slightly 

 micaceous and occasionally calcareous, of grey and brownish grey colours externally, 

 but dark bluish grey when cut into. They are filled with the characteristic fossils, viz., 

 Orthis bilobata, PI. 19. f. 7. ; O. testudinaria, PL 20. f. 9. j O. alternata, PL 19. f. 6. ; 

 0. expansa, PI. 20. f. 19. ; 0. Pecten, PL 21. f. 9. ; and many fragments of trilobites of 

 the genus Trinucleus. The shelly matter having in general disintegrated, the impression 

 only is left in sandy ferruginous cavities. The fossils are usually arranged in irregular 

 layers, but they sometimes pass into large sub-concretionary spherical masses. In de- 

 tailing the structure of this country we must remind the reader, that to the east of the 

 Fyrnwy, the Caradoc sandstone is thrown up only at intervals, along different axes of 

 elevation, for the most part parallel, leaving between them troughs filled with undulating 

 masses of the younger deposits. (See PL 32. f. 9.) In proportion as examination is 

 extended to the north-west, the older strata are repeated with much greater frequency 

 in the numerous parallel ridges lying between the Vale of Meifod and the south-eastern 

 flanks of the Berwyn Mountains. At the south-western extremity of the Meifod Ridge 

 (Galt-yr-ankr) the strata bend round from their south-easterly inclination and dip, first 

 to the west and afterwards to the north-west, and thus beds of nearly the same age are 

 repeated in each of the sharp ridges on the banks of the Tanat, as in those which 



