VOL. XVII. (i) EXCURSION— BUILTH-WELLS & RHAYADER 23 



On Wednesday the Members left Builth at 8.26 a.m., and arrived at 

 Rhayader at 9.2 a.m., the objective being the Cwm Elan Reservoirs that 

 supply Birmingham. 



On the drive from Rhayader, the first stop was just above the great 

 Caban Coch Dam. 



The Hon. Secretary said it would be unnecessary for him to say more 

 about the geology than that they were now in the heart of a district whose 

 rocks prior to the construction of the reservoirs were practically unknown to 

 geologists. But, fortunately, one of the engineers, Herbert Lapworth, who 

 was engaged upon a section of the water-mains, had applied himself with the 

 skill that characterized his father's (Prof. C. Lapworth) epoch-making 

 researches to the task of unravelling the stratigraphy of the district around 

 Rhayader, and had made three main sub-divisions of the unpromising-looking 

 Silurian rocks— in descending order (i.) the Rhayader Pale Shales, (ii.) the 

 Caban Group, and (iii.) the Gwastaden Group. The determination of the 

 true succession had been complicated by numerous foldings and faultings. 

 Shales, flags, mudstones, grits, and conglomerates composed these groups, 

 and had it not been that a few graptolites had been procured from certain of 

 the shale-beds, the correlation of the deposits would have been difficult 

 in the extreme. 



The following table will help to elucidate the remarks on the strati- 

 graphy, and show the relations of the beds in the Rhayader and Builth 

 Wells districts : — 



Rhayader Builth-Wells 



Ludlowian 

 Wenlockian 

 I r Rhayader Pale Shales, thinning out to Tarannon Shales (a few 

 ■^ feet thick) 



- Caban Group, 1500 ft: thinning out to U. Llandovery Grit (a 



few feet thick, & con- 

 Gwastadan Group (1800 feet) i Absent taining P nblongus 



'Cf PBala r"^^ -f"g unconform: 



-05 • ^aia ( ably upon the 



"H S I [Llandeilo] Llandeilo 



0>: [ [Arenig] Arenig 



In the cliffs bordering the road at Caban Coch, and in the great quarry 

 close to, no better teaching-specimens of conglomerate could be desired— the 

 white quartz-pebbles standing out in pleasing contrast to their dark matrix 



The scheme of making six reservoirs in the Elan and Claerwen Valleys 

 and of constructing an aqueduct 73 miles long to Birmingham, was originated 

 by the late James Mansergh, F.R.S , Past President Inst. C.E. At present 

 the three reservoirs on the Elan have been constructed, and the foundations 

 of one dam on the Claerwen, but the others on the Claerwen will not be made 

 until the necessity arises. 



From an excellent descriptive book, entitled, "The City of Birmingham 

 Water Department, Elan Supply: A Description of the Works" by E 

 Anthony Lees, it appears that the total extent of the gathering ground is 

 over 70 square miles. Its minimum elevation is 700 feet O.D., while several 

 of the hills round the rim of the watershed rise to elevations of over 2000 feet 

 The average yield of the gathering ground has been calculated on the basis of 

 36 mches per annum, on which basis there is an available yield of 99 million 

 gallons per diem. Of this 27 millions a day go to the Wye as compensation 

 water, and 72 million gallons for supply. The completed scheme provides a 

 total storage capacity of 17,960 million gallons, of which 11,320 million 

 gallons are provided by the works already executed. The aqueduct is 



l< 



