Geograph - photograph every grid square

The ash trees on which these galls were visible are located at the eastern end of the bridge shown in NS3878 : Footbridge over the A82 (the trees stand in the area corresponding to the left-hand edge of that photograph).

These galls are caused by the gall midge species Dasineura fraxini, and appear as a narrow slit along (usually) the midrib of the leaf; corresponding swellings were visible on the underside of the leaves.

A purple area in the lower half of the leaf on the left is an example of a different kind of gall, caused by plant lice; see NS3976 : Leaf gall on ash for details.