Shading the negative spaces

This week as I explore marks, I've been looking at shading. negative spaces, and what gives a 3D effect, among other things.

There are three examples here, first of all shading with a 4B graphite pencil between the outline of an image I've been playing with quite a lot recently.

Then there is the unexpected result of using a black Conté stick flat on its side and pulling it down cartridge paper twice to shade gently, and leaving a small space between each pull. The 3D effect was a great surprise to me as in the original, the negative space between the Conté marks seems to jump forward dramatically especially with half-focused eyes. 

And in the last, the result of moving the Conté in an arc and then overlaying again with broken layers to give texture and depth.

Nothing earth-shattering here but lots of enjoyable thinking in progress ...