Knopper Gall.

At one time we used to keep a "blip bank" a selection of small interesting items which could be blipped when all else failed.  It is a long time since this slipped away.  However tonight I spotted this gall which I had brought back from the canal to identify.

It is a Knopper Gall.  Galls are caused usually by an insect which lays an egg on the tree.  In this case it is a Cynipid wasp (ANdricus quercuscalicis).  I was trying to remember if the Oak was Sessile or Pedunculate, however identifying the gall decided things for me.  The wasp specialises in Pedunculate Oaks.

The wasp is a comparative new comer having arrived in the UK in the 1960s and since having spread throughout England and Wales having even reached Scotland.