The Marrow River is such a cool concept and I love the way you've illustrated it in my mind with your description! The tar pits, a classic concept in and of itself, is so much more terrifying and interesting here thanks to your inclusion of things still being trapped beneath the surface near the Black Spine. Normally I expect to encounter a tar pit and only see the remains of things which never quite got away but coming face to face (kind of I guess) with the active struggle of things writhing is such a great aspect to it. This was a great read and I'm glad I got the chance to check it out!