92 
THE AVONIAN OP THE AVON GORGE 
they can be made out at the same level on the other side of the 
river and also in the repetition of the /Semmw^a-Beds south of 
the ‘ Great Fault,’ at a point which we have already passed, a 
few yards north of the bottom of the Old Zig-zag Path. The 
structure of these rocks is almost identical with that of ‘ Gotham 
or Landscape Marble,’ but on a very much larger scale. 
The upper surface of a typical bed consists of very numerous 
tall ridges, usually discontinuous, separated by equally deep 
narrow valleys. (This character is well shown on the right of 
the path which leads down, on the north side of Observatory 
Hill, from the Observatory to the Promenade.) A cross-fracture 
of the bed shows that these surface wrinkles are underlain by 
several thin layers in parallel undulations. Below this concre- 
tionary upper-portion comes the main thickness of the bed 
which consists of a very fine-grained, pale argillaceous limestone, 
mottled by black patches. In many cases these black patches 
can be seen to extend, as continuous pipes, from beneath the 
concretionary top-layer down to the under surface of the bed, 
and it is probable that most of the apparently-isolated patches 
are merely cross-sections of similar, continuous, but flexuous 
pipes. These pipes appear to be themselves concretionary and 
to be built up of thin layers which are convex upward. (Such 
pipes, bleached white by exposure, are well shown in the rock 
face just north of the Old Zig-zag, at the point already referred 
to.) The undersurface of the bed is scored by sharp grooves in 
such a way that the bottom line of a cross section of the bed is 
notched at regular intervals. Beneath such a typical bed lies a 
thin black under-layer, which is almost entirely made up of 
small black nodules. 
Compared with ‘ Landscape Marble,’ the black under-layer 
represents the ‘ hedges and bushes,’ the pipes correspond to the 
‘ trees,’ and the undulating upper-layer represents the ‘ sky.’ 
The origin of the peculiar structure described above has not 
yet been satisfactorily explained, although several theories have 
been advanced to account for it. It seems probable that such 
beds could only be laid down under water undisturbed by waves 
