VOL. XVII. (I) EXCURSION— BUILTH-WELLS & RHAYADER 25 



After a bridge over the brook that enters the reservoir to the west of the 

 dam had been crossed, a shallow cutting was entered that showed the Rhayader 

 Pale Shales. The section showed very well indeed slaty cleavage, the 

 cleavage-partings being inclined but a httle from a right-angle from those of 

 the bedding. 



Crossing the Pont-ar-Elan and climbing the steep and bare hillside, the 

 Members halted on the old Aberystwyth Road to look up the upper reaches 

 of the Elan River, which they saw pursuing a braided course at the bottom 

 of a desolate-looking valley. 



Crossing Pen Rhiw-wen (1600 feet), whence the Members saw Plinhmmon, 

 where Wye and Severn rise and then diverge, the descent was commenced. The 

 Gwynllyn Cascade was seen and passed, but near Glanllyn the botanists 

 of the party disembarked, to join the others later in the day at Rhayader. 



Rhayader was reached at 2.30, the round of some 15 miles being accom- 

 plished since 10 a.m. After tea the Members returned to Builth- Wells by the 

 4.57 p.m. train. 



During the evening some of the Members visited Builth Castle, now only 

 a collection of grass-grown mounds, but once a formidable fortress. 



On Thursday morning the Members travelled by the 9.35 a.m. train 

 to Aberedw. The object of visiting this locality was to study certain glacial 

 phenomena and the historic spots connected with Llywelyn. 



The Hon. Secretary first led the way to a small gravel-pit close to 

 the Wye, and near to where that river is joined by the Edw (text-fig. i). He 



Moral Tie ^^ , ^ 



Pre. 



Post G/aaal^M__m^'^l(^ castLe- 



Graue/pir ^Rcdlv^au. 



Fig. I.— Sketch map of the meeting-place of the Edw with the Wye 



said that he would shortly tell them why he had brought them to see it. At 

 present all he wanted them to do was to note its aspect and constituents. 

 The gravel was composed of rock-fragments of all sizes, confusedly inter- 

 mingled, some well-smoothed and rounded, others angular and tabular. The 

 rock-fragments coming under the former category were pieces of the distinc- 

 tive diabase-porphyrite from the Carneddau, and of Caban Conglomerate, 

 probably from the neighbourhood of Rhayader; those coming under the 

 latter, pieces of Ludlow and Wenlock shaly rock from more local sources. 

 Crossing the railway-line and climbing the steep crag up to the crowning 

 mound, the Members sat down while the Hon. Secretary gave his interpretation 

 of the river-features. He held that in closing Glacial times washed material 

 derived from the lateral moraine of the Wye Glacier and from the terminal 

 moraine of the Edw Glacier blocked up the Aberedw Valley. When the 

 cUmate became less rigorous, and the snow and ice melted, the Edw found 

 Its pre-glacial channel blocked. A lake was therefore formed, which— as its 



