The picture shows a piece of limestone rock containing fossils - Edexcel - GCSE Chemistry - Question 5 - 2011 - Paper 1

Fossils are formed in limestone through a process called sedimentation. When organisms (like shells or plants) die, their remains sink to the bottom of water bodies, such as rivers or oceans. Over time, layers of sediment—composed of tiny particles and remains—accumulate above them. The pressure from these layers compacts the sediments and the minerals in the water fill the spaces between the remains, solidifying them into limestone. This process can preserve the shape and structure of the organisms, turning them into fossils over thousands to millions of years.