For anyone else trying to find the fossils at Devil’s Den: there are tons but the easiest ones to find are large fossil sea biscuits on the dark/back side. If you stand on the platform facing the stairs there is one platform to your left and one to your right. If you go to the LEFT platform and descend, the corner that points towards the wall points DIRECTLY at a large grouping of fossil sea biscuits. Look a few feet above the floor - once you see the first few you’ll start seeing them everywhere (even in the swim-throughs!).