Vol. 62.] TAKANNON SEKIES OF TARANNON. 651 



covered and wooded steeps into the narrow valley of the Afon 

 Iaen. 



All along the north-eastern side of the area, towards Carno, 

 however, the slopes are less conspicuous, and on the whole more 

 gentle. 



To the south the Tarannon moorland continues in an unbroken 

 stretch, far beyond the limits of this district. 



The district is well watered, and abundant rock-sections are 

 exposed both in the more open valleys of the main rivers and in 

 the gorge-like clefts of their smaller tributaries. As already indi- 

 cated, the western and northern slopes of the tableland are drained 

 by the Twymyn and Iaen respectively, these being tributaries of 

 the Dyfi. Prom the southern and western declivities, on the other 

 hand, the water flows through the rivers Clwyddog, Tarannon, and 

 Garno, and their tributaries into the Severn. Thus a line passing in 

 a direction from north-north-east to south-south-west across the 

 northern end of the Tarannon tableland forms part of the watershed 

 separating the two important river-systems of Caerdigan Bay and 

 the Bristol Channel. 



(2) General Geology. 



The rocks of the Tarannon district are, on the whole, of the 

 usual grey wacke' type. They consist mainly of shales and mudstones, 

 varying in colour from light grey to black, with occasional bands 

 of pale green and purple. These are interstratified, however, with 

 more arenaceous beds — greywacke-flags, felspathic and quartzose 

 grits — ranging in thickness from a fraction of an inch up to 

 4 or 6 feet. The grits are, for the most part, fine in texture, 

 and are nowhere coarse enough to be termed conglomerates. The 

 monotonous pale and dull colour of the beds is relieved by the 

 weathering of many of the shales, mudstones, and grits to reddish 

 brown and brilliant orange, due to the amount of iron present in 

 the rocks. 



Considered as a whole, the geology of the district is fairly simple. 

 The highest beds met with are the grits and flags of the Denbigh- 

 shire Series, which spread out in gentle undulations over the cen- 

 tral and northern parts of the tableland. These pass down into shales 

 and mudstones containing typical Wenlock fossils. Immediately 

 underlying these, apparently with complete conformity, come certain 

 pale-green mudstones with one, two, or three bands of a bright 

 maroon colour, and occasional flaggy beds. These are the local 

 Tarannon Shales, as here mapped by the Survey. The shales in 

 their turn pass down, also with apparent conformity, into a series 

 of massive quartzose grits alternating with very finely-laminated 

 shales, light to dark grey in colour, which are lettered (on the 

 Survey map) as being of Lower Llandovery age. As one descends 

 the sequence these grits gradually become less prominent, and 

 the lowest beds in the district consist almost entirely of light- 

 grey to bluish-black mudstones and shales. 



