The Ffestiniog Pumped Storage Scheme, in Merioneth, the first in Britain, is situated on Upper Cambrian and Ordovician sediments and metasediments, on Ordovician volcanics and fine-grained intrusions and on a granite laccolite. The nature and structure of these rocks and the glacial history were major factors in the choice of site and in the design and construction of works which include two large dams, deep vertical shafts, pressure tunnels and penstocks, and a power station of 360 MW capacity.

The presence of a glaciated rock-lip of Ordovician igneous rocks located the upper dam site, but the effects of glacial drag necessitated deeper excavation than expected; faulting also had to be taken into account. Twin 640 ft vertical shafts pass down through the igneous rocks into Ordovician sediments. Four pressure tunnels and the power station site were excavated in flaggy Upper Cambrian rocks, subject to a varying degree of meta-morphism; in spite of well-marked bedding and cleavage and the presence of several faults (forecast from site investigation) these strata proved satisfactory both mechanically and hydraulically. Hornfelsed sediments provided sound foundations at shallow depth for the Lower Dam; deeper excavation was necessary in granite cut by minor faults and intrusions and affected by weathering.

Site exploration included nearly 4000 ft of core boring.

No new geological views are put forward but observations are recorded bearing on the relation of the granite to its aureole, on the nature of the Ordovician volcanics and intrusions, on the Cambrian/Ordovician boundary and on the fault and joint systems.

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