The Cambrian Coast Line, which runs along the coast from Dyfi Junction to Pwllhelli, was built on a narrow shelf cut into a steep cliffside at Friog Rocks, between Llwyngwril and Fairbourne. Following two major incidents, where trains were derailed and tumbled to the beach below, and numerous minor rock falls, Friog Rocks avalanche shetler was built in about 1935 to protect the railway. The shelter is 56.3m (185 feet) long and built of reinforced concrete.
There is a detailed illustrated plan of the area in C.C.Green, The Coast Lines of the Cambrian Railways, Vol 2, Wild Swan, 1996.
B.A.Malaws, RCAHMW, 10 February 2017.