From @MagnusRevang on April 2, 2016 8:54

I kerp stumbling on pockets of air with a lot of exposed ore - some even floating, yet I never encounter gravel while underground? This leads me to believe that gravel fails to generate - is this the case? Mining in a mushroom island biome

Copied from original issue: Team-RTG/1.8.9-Realistic-Terrain-Generation#65

Gravel gen seems to be working fine for me in all biomes, including Mushroom Islands.

@MagnusRevang If this is still happening for you, post your fml-client-latest.log file and I'll take a look.