Unlike Mt. Moon, the Rock Tunnel is mostly underground, and very little, to no light leaks in from outside. It's a rough path to Lavender town, but the walk is only about three hours time.
Available Pokemon:
Zubat (Lots of Zubat)
Golbat
Geodude
Graveler
Onix
Sandshrew
Diglett
Ghastly (Rare)