Route 42 connects Ecruteak City to Mahogany Town, and it also provides several entrances to Mt. Mortar. Keep your eyes peeled for the second entrance to the mountain, found on the stretch of road that's surrounded by two ponds—it's very easy to miss but very important!
You'll pass three entrances to Template:~ as you cross Route 42, but you can't get to the caves' ultimate prize (an exclusive Pokémon) without HM 07 (Waterfall), so Mt. Mortar will be covered later on. For the time being, focus on reaching Template:~
Route features
Three rare Apricorn trees
You'll need to Cut your way into a private Apricorn grove along the middle of the route, but it's well worth it. It's the only place in the game where you can get Pink, Green, and Yellow Apricorns, which can be made into Love, Friend, and Moon Balls.
A free Water Stone
Trade phone numbers after beating Tully, and he'll give you a call when he fishes up a free Water Stone. You can use that to evolve Staryu, Cloyster, Eevee, or Poliwag.
More headbutting fun
Remember when you Headbutted all those trees to get a Pineco and Exeggcutor? Well it's time to start Headbutting again, because starting with Route 42, there's a whole new assortment of Pokémon to dislodge. You'll get a lot of Spearow, but you'll also knock down new Pokémon Heracross and Aipom. If you've lost the Pokémon that knew how to Headbutt, you can re-but the TM (#02) in Goldenrod City.
Fighting Pokémon are still few and far between. Mankey was underpowered in Red, Blue and Yellow, but a lot's been done to improve it in Gold (it can't be found in Silver or Crystal). It finally gets a strong late game Fighting move (Cross Chop), and early moves like Karate Chop have been powered up.
Marill is a lot easier to get in Crystal. They still only come out at night, but you don't need to go into Mt. Mortar and wait for swarms like you do in Gold and Silver. Marill is one of the weakest new Pokémon in Gold, Silver and Crystal. Its stats are pretty poor (even after evolution) and its selection of moves is pretty bad.
#214 Heracross
#190 Aipom
Heracross is an interesting mix of Bug and Fighting types, and it alone learns Bug's new super attack: Megahorn. The Bug equivalent of Lightning and Fire Blast, this attack could K.O. a Mewtwo in one shot. Too bad the Fighting type makes it too vulnerable to Psychics to last that long.
This disturbing Pokémon is one of 32 pure-Normal Pokémon in Gold, Silver and Crystal. Aipom is quick, but has otherwise unimpressive stats ans a poor collection of moves (although it can learn a bunch of TM's). It's fun to catch, but it isn't really worth using.
960
Mankey
Mareep
Flaaffy
Spearow
Fearow
Ekans
Arbok
Rattata
Raticate
Zubat
Golbat
Marill
Goldeen
Seaking
Magikarp
