Why is still water an ideal environment for the formation of mold and cast fossils? Sediment builds up over organisms.
Sediment turns organisms into rock.
Hard tissue decays under sediment.
Soft tissue is preserved under sediment.

Geothermic gradient is the rate of increasing temperature with respect to increasing the depth in the earths interior. away from tech tonic plates boundaries, it is about 25-40 c/km (72-87 degrees f/mi) of depth near the surface in most of the world.