A to Z of North Wales Geology
F is for..............FAULT (Continued)
Little Fault on Anglesey The major fault seen on the previous page has a number of smaller faults running in a parallel alignment up to 10 metres away (right). The displacement, or throw, of the fault in this picture is clearly demonstrated by the hard, central bed of limestone. Where the fault cuts mechanically weak mudstones above and below, it is much less obvious. Measurement of the throw depends upon correlation of the adjacent rocks, which is often difficult - especially when the throw is large. The throw of this little fault is about 50mm. The fault plane can then be seen as a crack running diagonally across the lower rock unit in the foreground, and we are able to get a 3D impression of its trend.

Fault Mineralisation on Anglesey Further inland, the major fault plane is exposed as a wall 20-30m high. The existence of a serious structural weakness has exerted a powerful influence on the erosion of the local rocks by the sea. The presence of faults which are not necessarily exposed can often be inferred by careful observation of the landscape - even from space by satellites.

The presence of a major system of cracks in the rocks allows the concentration of fluids which flow naturally through the buried rock units. These fluids are an important lubricant in the regime of movement along the fault. They also have the capacity to carry dissolved minerals, particularly where mechanical forces create extreme pressure in localised areas.

All the rocks in this exposure are limestones and carbonate-rich mudstones, rocks whose Calcium Carbonate content is easily mobilised in water. The result has been the deposition of extensive sheets of crystalline carbonate, or calcite. This can be seen clearly in the second picture where the pale colour of the crystalline material contrasts with the darker grey limestone. You can also see a number of North Wales Geology Association members posed for scale.

©Jonathan Wilkins, 11.00