I was always taught that a sketchbook was a place where you needn't finish a picture. When you have achieved what you set out to do that is when you stop. I find this a useful practice, it allows me to make mistakes, to push, to warm up. I sketch every day, sometimes working from memory and sometimes on location.
These sketches are about the Thames path between Marlow and Henley.
Long walk, bright light on the Thames