Drawing a deep shadow takes patience. I usually create them with several layers of graphite. My process goes something like this:
1. Cover the area with hatching made with the sharp point of a very soft pencil.
2. Bush the hatching with a stiff brush until it looks smooth.
3. Cover the area again with more hatching made with the sharp point of a slightly harder pencil.
4. Use an even harder pencil (again with a sharp point) to fill in all the little missed places and blotchy patches.
Shadows can’t be rushed, or the paper will be ruined. Shadows show us how to slow down and be peaceful with the drawing and with ourselves.