440 THE LLANDOVERY ROCKS OP MONTGOMERYSHIRE. [Aug. 19H. 



fossils occur, and these suggest the upper zones of the Lower 

 Ludlow. 



At Derwen-deg the shales are so well cleaved that the bedding- 

 planes are hard to distinguish, but fragments of Monograptus nihsoni 

 were found on the edges of splintered pieces. A thin series of 

 calcareous sandy flags rests upon the hard flags of Yr Allt cliffs, 

 in the lane-section about 300 yards north of Spout House. These 

 beds yield myriads of Dayia navicula, together with an assemblage 

 •of fossils which suggests the horizon of the Aymestry Limestone 1 

 or the uppermost beds of the Lower Ludlow [10, p. 438]. 



In the Park of Powis Castle, the Ludlow Shales are seen on the 

 west side of the Castle, resting almost directly upon the Powis- 

 Oastle Conglomerate Series. They form the highest ground in the 

 Park, and are much crushed and contorted. They evidently 

 overlap the Wenlock Series, or are brought in by a fault parallel 

 and subsidiary to the Bron-y-Buckley .Fault. 



The beds form a high ridge, which runs north-north-eastwards 

 to Nant-y-caws Broo*k. The country here is much covered by drift; 

 but the Ludlow strata appear to swing round in a great curve to 

 the west, overlapping the underlying beds on the axis of the Guils- 

 field Valley anticline. They form an escarpment running almost 

 due east and west as far as Glyn, where it again takes up the 

 north-easterly direction. North of Glyn, Monograptus colonus occurs 

 in extraordinary abundance in some exposures. The graptolites are 

 preserved in limonite probably after pyrite, and so their presence 

 renders the beds quite rotten and crumbly. 



The Lower Ludlow Beds cover a very wide tract on the west of the 

 area, and numerous excellent exposures occur, notably near Trefnant 

 and The Park. The strata are much folded ; they consist of brown 

 flags and shales, which pass upwards into blue flags containing an 

 extraordinary number of calcareous concretions or septaria, locally 

 called ' cannon-balls.' These differ considerably from the limestone- 

 blocks seen in the Wenlock Shales at Ty-brith. The blue flags often 

 contain numerous specimens of Monograptus colonus arranged in 

 star-like patterns on the bedding-planes. Nearly the whole of this 

 outcrop belongs to the zone of M. nihsoni, a species which is very 

 abundant. This zone is evidently very thick — not less than 400 to 

 500 feet in the neighbourhood of Trefnant. The underlying zone 

 is rarely exposed, and is apparently not so well developed. The 

 rolling of the beds, however, renders accurate measurement difficult. 



Higher zones are doubtfully present, although the barren flags of 

 Tir-newydd are similar in character to the flaggy beds of the 

 upper part of the Yr Allt escarpment, and may represent the upper 

 division of the Lower Ludlow. 



1 Miss G-. L. Elles & Miss I. L. Slater, ' The Highest Silurian Eocks of the 

 Ludlow District ' Q. J. G-. S. vol. lxii (1906) p. 197. 







