A to Z of North Wales Geology
F is for..............FAULT
Fault on Anglesey A FAULT is a break in a rock unit. Faults occur because the rock units have been subjected to stress greater than they can endure, and the end result is not simply a breakage, but also movement between the two rock masses on either side of the break. Faults exist on every scale from millimetres to hundreds of kilometres. When exposed at outcrop, faults appear as a line which will be a plane when considered in three dimensions.

In the picture, the horizontal layers of limestone end suddenly at a more or less vertical break. The dark vertical crack is the outcrop of the fault plane. On the right of the fault the original layering of the limestone has been destroyed by the crushing and shearing associated with the earth movements. Also displayed in the centre of the picture is 'drag-folding' where the layering has been bent by the movement - which is quite clearly downwards on the right (or upwards on the left, which is the same thing). Click here for more on the faulting at this location.

FAULTING is usually caused by tectonic forces - those relating to the movement between the 'plates' which comprise the Earth's surface. When the plates grind together they cause compression which results in the crumpling and thickening of the local rock units, which will eventually be displaced as the rocks fail and the stress is relieved by movement along lines of weakness. Separating plates create tensions which will be relieved as blocks are pulled apart and slip downwards in response to crustal thinning. Many names are applied to different forms of fault, depending upon whether the movement direction is horizontal or vertical, and the shape of the fault plane.

FAULTS often occur in parallel sets or zones which allow repeated movement over multiple episodes of tectonism. An example is the Menai Strait or Afon Menai, which has certainly been grinding along since the Caledonian period 400 million years ago, and still gives off the occasional shudder which we feel as an earthquake.

P.S. Only trendy news copywriters refer to fault lines. Geologists understand their own terminology.

©Jonathan Wilkins, 11.00