Terrain Tilemap Fade to Black

[grave gulch]

Hi all,

I have been trying to work on a graphical feature of my 2D platformer which gives the effect of the terrain getting darker and darker the deeper it is from the surface. I feel like this shouldn't be too hard but my peanut brain has failed me. Below is a photoshopped example of what I am trying to achieve vs the original. Any direction would be appreciated.

[mortal whale]

Probably create another tilemap for shadows as an overlay, or just manually create a fading variants of those tiles

[solar current]

Best option may be a shader