Identification: Orange/red colouring on live tree bark

I write from Hertfordshire, England.
Over many years we have observed an orange/red "dusting" on various trees in our garden. It occurs particularly on two oaks and a willow. Never on silver birch of which we have many. It is there all year round and appears from ground level upwards for about 3/4 yards and is only on one quarter of the trunk which faces south west.
Any ideas?
Peter Hodge

 

Some sort of algae or lichen.

Can you post a photo?

 

Maybe several weeks but when I get an image I will post it.