The little crack is an Agility shortcut found north of the Trollheim mountain. An Agility level of 60 is required to use the shortcut. The shortcut allows access to the God Wars Dungeon for people without 60 Strength, which is the requirement for boulder next to the little crack. The fastest way to get to the shortcut is to use the Ghommal's hilt teleport to Trollheim.
BEWARE that the north part of the mountain is covered with Thrower trolls throwing rocks at you as you pass, it's advised to use Protect from Missiles as you pass.