Vol. 67.] ASSOCIATED ROCKS OF NORTH-EAST MONTGOMERYSHIRE. 439 



of Cyrtograptus are common in each of these zones, but many of 

 them could not be determined. 



The beds pass up into strata of Ludlow age, the ' zone of 

 Monograptus nilssoni being observed near Moel-y-garth Farm. 



(5) Yr A lit Group. — The Ludlow Shales form the most wide- 

 spread division of Paloeozoic rocks in the area. They are, as a 

 rule, uniform in character, and consist, on the lower horizons, 

 chiefly of pale thinly-bedded shales, soft and muddy, but tend to 

 become hard, flaggy, and calcareous above. The highest beds are 

 thin flaggy sandstones, but these are only seen in one locality, the 

 top of the Yr Allt escarpment. 



A surprising feature of these soft shales is the way in which 

 they occupy the crests of the highest hills ; but this is explained by 

 the fact that, wherever they so occur, they are much faulted and 

 crumpled. They appear to have suffered greatly in the movements, 

 which have affected the district, and in several places small over- 

 thrusts can be detected in the Wenlock and Ludlow. In the Cefn 

 area, the beds have already been described by Miss Elles and 

 Miss Wood (Mrs. Shakespear). 



These beds form a long line of escarpment running north-north- 

 eastwards from Belan to the Severn, broken in the middle by the 

 Welshpool Gap, 1 in which they are hidden by drift and alluvium. 

 The strata are folded against the Bron-y-Buckley Fault into a long 

 narrow anticline and syncline. From the core of the anticline 

 the Wenlock Shales crop out ; but against the Bron-y-Buckley Fault 

 a thin strip of Ludlow Shales is usually found. This is seen in 

 the quarries on the top of the hill at Belan (where Ludlow Beds 

 succeed Wenlock against the fault), and at the southern end of 

 Bron-y-Buckley Wood. 



Below Cwm Caethro the zone of Monograptus scanicus occurs 

 opposite Lower Gungrog Farm, where the strata consist of 

 strongly-jointed dark-grey mudstones and shales weathering brown 

 (fig. 5, p. 437). They are broken by a couple of faults between 

 the farm and the orchard. This zone extends into the orchard, 

 where beds yielding Monograptus varians, var. /3 Wood, come in. 

 Above the orchard, the zone of M. nilssoni, consisting chiefly of 

 soft earthy shales, with occasional bands of harder mudstones, can 

 be traced right up to the waterfall, where the stream leaves the 

 uppermost wooded dingle. 



Farther north, the strata are more flaggy and calcareous. They 

 form a magnificent and bold escarpment in the cliffs of Yr Allt. 

 The lower part of that escarpment is concealed beneath screes ; 

 the upper part, consisting of hard false-bedded flagstones with a 

 few thin shaly horizons, has been well quarried in places. Few 



1 See rny ' Note on the Glacial Geology of the Area,' to be published 

 separately. 



