This fascinating natural feature is gradually encroaching on the railway (see the cutting wall to the right of the photo below). It is being formed by the calcium-rich water that flows slowly down the side of the cutting at this location for most of the year.
The water coats the moss (which thrives in this damp, shady and sheltered location), then evaporates, leaving behind limescale (much like that found in a domestic kettle). The limescale gradually covers the moss, then new moss grows on top of the limescale. This tufa screen cannot be more than around 140 years old as the cutting was created between 1869 and 1873. The growth rate of the tufa screen can be seen by comparing the photograph immediately below (which was taken in 1978) with the video still from cab-view video footage shot in 2012 (see the Image Gallery). In the intervening 34 years, the tufa screen has completely engulfed the half-mile post.
