Fossilisation processes. Animation of the processes involved in forming a fossil of a fish. The opening part of the clip shows biological tissue and detritus formed of calcium-containing minerals drifting down from the upper parts of the ocean to the abyssal floor. Over geological time, layers of sediment build up, eventually forming sedimentary rocks such as limestone. The following part of the clip shows a fish dying and the body sinking to the seabed. Over time, the skeleton is covered by sediments. Given the right conditions, a fossil may then form as minerals replace the bones as the rock forms. This preserves the shape of the bones. Over millions of years, tectonic forces may then elevate the rock above sea level, here forming part of a mountain range. Weathering and erosion then removes the overlying rock layers, exposing the fossil, sometimes at high altitudes and long distances from the nearest coast.