what's on the bottom of my loaf

Lots and lots of questions I realize but have something unusual on the bottom of my loaf and figured someone would recognize it. This is Ru007's seeded loaf--though I used a rye starter this time and did a 3 stage build with a combination of rye/wheat. The bottom is coated in oat bran which is my norm. These were fermented for ~4-1/2 hours, pre-shaped, shaped then straight to the refrigerator for ~13 hours proof...onto the stone in a 500f oven (cold). I had the dough in a towel lined banneton and then enclosed in plastic for the proof.

So the question is, what caused it/how to avoid it in the future? One other question is about the scoring...the oven spring was good but didn't get much action on this...are the cuts too shallow or at a bad angle? I didn't have any blow-outs elsewhere on the loaf.

Thanks!