Come out of Totteridge & Whetstone tube station, on the other side of the road is a grassy/woody area. If you cross the road and walk (behind the bus stop) along a little path parallel to the railway line (railway line to your left), a path to the right descends down to the stream. Turn right again by the stream and walk back up to the main road, on your right hand side there is a large and artificial-looking mound/hill which you have just descended.
Is it just a feature of the river bank? Or was it created when the railway line was constructed?
PS I have a vague feeling I posted about this some years back - apologies if so - I have searched for that post without success, but must confess am finding it difficult to navigate this otherwise excellent site.