Turn around and head back across the bridge of Wizard's Crossing. Turn to the right and head on up the slope toward the gate you see The Wizard Claptrap. Approach him and wait for him to open the gate and move into the next area. He will clear the way into Stonecrag Ridge. Now talk with him to begin the next part of the quest.
Claptrap: "No way! That's like my third favorite type of magic! Look, you're going to need the secret Dwarven Passphrase to open the door. Go find some weird runes scattered around the mines -- I bet those are the letters making up the passphrase! If you can find the letters, you can probably get outta here!"