TILESTONE FORMATION. 



181 



3. Tikstone. (See Sections, PI. 31. fig. 1. PI. 33. figs. 4 and 7. and PI. 34. figs. 1, 

 3, 6, 6, 7, 8, and 9. PL 36. figs. 5 and 8. and g. of wood-cut, p. 179.) 



Course of the Tilestone from Caermarthenshire, through Brecknockshire and Radnorshire 



into Shropshire. 



This lower division of the Old Red System, though of much smaller dimensions 

 than the overlying formations, has very marked characters both in structure and fossil 

 contents, and is very clearly defined by occupying a position in which it passes upwards 

 into the cornstone and marls, and downwards into the Silurian rocks. In this relation 

 it has been already alluded to at Pont-ar-lleche (bridge on the tiles) , near Llangadock in 

 Caermarthenshire, from whence it is seen to run in a nearly rectilinear course, from the 

 Tri-chrug on the south-west, to near Builth on the north-east, occupying the loftiest 

 part of the escarpments of the wild tracts of Mynidd bwlch-y-groes and Mynidd Epynt, 

 at heights of fifteen hundred and sixteen hundred feet. In this range, the tilestones 

 are extensively quarried, and the strata, which are inclined at seventy and eighty degrees 

 near Pont-ar-lleche, diminish to forty and forty-five degrees at the north-eastern end of 

 the Mynidd Epynt, the dip being invariably to the south-east. After a great flexure 

 on the Wye, to the east of Builth, the tilestones are again found in similar relations 

 overlapping the Silurian rocks in the Begwm and Clyro Hills, Radnorshire, and ex- 

 tending thence to Kington in Herefordshire ; in which part of their range they are 

 much less inclined. (See PL 31. figs. 1 and 6, and PL 33. fig. 2.) Throughout their 

 course from Caermarthenshire to Kington, the distinguishing beds are finely laminated, 

 hard, reddish or green, micaceous, quartzose sandstones, which split into tiles. 

 Although the greenish colours prevail, these beds are usually associated with reddish 

 shale, and the decomposition of the mass uniformly produces a red soil, by which 

 character alone the outline of the division is easily defined ; being always clearly sepa- 

 rable from the upper beds of the Silurian System, which decompose into a grey surface. 

 In Shropshire and the contiguous parts of Herefordshire, this lower member of the Old 

 Red System rarely occupies high ground, (except in the instance of the outlier of Clun 

 Forest, hereafter to be described,) and being for the most part recumbent on the talus 

 of the upper Silurian rocks, where the latter sink down into valleys, it is generally much 

 obscured by alluvial detritus. In the gorge of the Teme, however, between Ludlow and 

 Downton Castle, it is well laid open, particularly at a spot called the Tin Mill. (PL 31. 

 fig. 2.) Flaglike, micaceous, dark red sandstone "Bur Stones" rise there at an angle 

 of about fifteen degrees from beneath the red argillaceous marls of Oakley Park, and 

 pass down into a lightish-coloured grey, yellowish, and greenish grey freestone, of 

 which Downton Castle is built, which will presently be described as constituting the 

 upper stratum of the Silurian System. Similar relations are visible at Ludlow, and at 

 Richard's Castle to the south of Ludlow. 



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