Why is still water an ideal environment for the formation of mold and cast fossils?

Fossil of the hard-shelled organism which shows the surface contours giving the exact 3D shape of the organism is called a mold and cast fossil. Fossils are formed in water bodies where the sediments can be deposited over the dead and decayed organisms.

When hard-shelled organism dies, still water plays important role in the formation of these fossils as: when water from sediment leaches our it decays the organic matter which creates void which is filled by sediment or mineral matter and the acidic water dissolves and breakdown the shell which gets surrounded by the mud or sand which hardens.

Thus, water plays an important role in the formation of mold and cast fossils.

Still waters allow for sediments to settle down on fossils without disturbance. This gives the minerals in the sediments time to perfuse in the fossils and petrify the fossil. The sediments also settle on the fossils without disturbance, take the form of the fossil and get cemented forming molds.