If you go back to the entrance of Dragonspire, there's a path that you follow (furthest to the left, I think) that eventually is blocked by a pair of huge boulders. That's the spot in question...
I usually access it from the other side and use the Sigil on the boulders so I can get out [img]smile.gif[/img] I jump into the water from the area where you place the dragonshards, and I generally have a hard time squeezing through the other exit. There's a small cave opening you can get into from there.
Remember to check out the sigil, too... it has a rather nice property