Slickenside on Fault planes in Borrowdale

Geological faults are approximately planar discontinuities in rocks where the rocks on one side of the plane has moved with respect to the rocks on the other side. The relative motion is parallel to the plane. If the motion were perpendicular the plane would be an open crack or, perhaps, if it was filled with minerals, a vein.
New minerals can form on fault surfaces and, if they grow while the fault is moving, they often grow as fibres with their long axes parallel to the direction of fault motion. These fibre-coated surfaces are called slickensides and they can be very useful kinematic indicators - telling you which direction the fault moved. Rub your finger back and forwards along the fibres. One direction will feel quite smooth and the other will feel rough. Your direction of movement with respect to the plane is the same as the fault's movement when it feels smooth and opposite to the sense of movement when it feels rough.
Slickensides are very common in the UK's mountains once you get your eye in.

[@robert-wr9xt]

Wonderful example of slickensides. The illustrations were spot on.
This one minute thing has legs. Great presentation and I’ll be watching more.
‘The world around us.’ might be a tagline?
Thanks from America.

[@OneMinuteGeology]

Cheers Robert. Much appreciated.