Oh, What a Tunnel

That is an impressive tunnel.

My first impression is that the dough may have been rolled unevenly.  In other words, the focus on rolling the loaf tightly may have stretched the outer portion of the loaf thinner than the portion that is the core of the roll.  When baked, that thin outer layer was stiffened enough to be self supporting but the softer interior pulled away from the outer walls as it cooled.  

Note that the interior has a nearly horseshoe cross-section.  That's about 3/4 of a revolution.  You're looking for a loaf that has a spiral that represents 2 or more full revolutions.  Each layer should be even in thickness with a thin stripe of the filling at the margin.  As a serial offender in swirled loaf construction, I'm well-versed in how not to do it.  I still struggle with getting it right, though.  You may want to take a look at varda's blog, as she recently posted a picture-perfect example.

Here are some things that I think contribute to a well-made swirled loaf:

1. Less, rather than more, filling and evenly distributed.

2. Avoid or minimize butter and other fats in the filling since they are lubricants that facilitate separation of the layers.

3. Use beaten egg white painted on the dough before spreading the filling; it helps bind the filling and the dough together.

4. Aim for a dough rectangle that is twice as long as it is wide and roll from the narrow edge.

5. Strive for even pressure/tension on the dough as it is rolled.  I'm less convinced that a really tight roll is a good thing, anymore.  My impression is that the rolling process should be just firm enough to make sure that air pockets aren't created between the layers but not so tight that it could serve as the drive spring for an 8-day clock.  Sorry that I can't be more descriptive about how it should feel.

6. It's probably better to bake one of these breads low and slow, rather than hot and fast.  Maybe at 350F for nearly an hour?  The objective would be to allow enough time for the interior of the loaf to cook and set before the crust burns.

Hope this is of use.

Paul

In addition to all of Paul's excellent comments, I think it also helps to fully proof swirled loaves (indent remains), if they are underproofed (indent springs back), that can exacerbate separation of the layers.