The Rock Tunnel Connects Route 10 to Lavender Town. To get through, you need to have a Pokémon that knows the HM05 Flash. There is also a Move Teacher who will offer to teach Rock Slide to one of your Pokémon on the second floor.
If you picked Squirtle or Bulbasaur as your starter and leveled it up decently, you'll have little problem getting through. Those who picked Charmander as their starter will struggle though, as the rock types here are super effective against fire types. Pick up as many super potions as you can in Vermillion City, as you'll be using them. Worry about getting to the other side and come back for the items/Pokemon later once you have a stronger team.
On the second level, in southwest corner, there is a guy who will teach your Pokémon Rock Slide.
As the second Fighting type Pokemon you can catch in the game Machop is a great replacement for a Mankey if you caught one earlier. He gets tons of fighting type moves and evolves twice, first into Machoke at level 28, and then to Machamp after being traded. The only thing working against him is that once you gain access to Saffron City, you'll probably replace him with Hitmonlee or Hitmonchan.
Onix is the second Rock type pokemon you encounter in the game following the Geodudes you run into inside Mt. Moon. Although not as well rounded as a fully evolved Golem, Onix's Defense stat is unmatched. In addition, he receives Dragon Breath at level 30 and a Steel type move, Iron Tail, at level 45, which makes him a contender for use against Lance's Dragons in the Elite Four.
Zubat
Mankey
Machop
Geodude
Onix
Ashton
Cubone 23
Slowpoke 23
1104
Martha
Oddish 22
Bulbasaur 22
Charmander 22
Allen
Graveler 21
Jigglypuff 21
Meowth 21
Pidgey 21
Bellsprout 22
Clefairy 22
Rattata 19
Pidgeotto 21