I made a brick!

This is supposed to be vollkornbrot and yes, i'm supposed to wait at least 24 hours before i can eat it. But really it looks inedible...literally, heavy as a brick.
the recipe called for only 6g of SD in the perferment which is only to sit for 1.5 hours. After that all igredients are mixed, patted into a loaf pan and proofed for 2 hours. I followed the recipe. But it lookes as though it's gone haywire?
total rye flour was 560g with 480g water. Any idea what went wrong?