Trầm tích thực hiện bởi nước hoặc gió có thể xây dựng trong lớp sâu, trên đất liền hoặc phổ biến hơn trên đáy biển. Khi vật liệu được thêm vào, trọng lượng của nó nén các lớp thấp hơn. | L p SEDIMENTARY ROCKS Sediments carried by water or wind may build up in deep layers either on land or more commonly on the seabed. As more material is added its weight compresses the lower layers. Over millions of years minerals dissolved in seawater or groundwater cement the compressed grains of sediment together to form sedimentary rocks. Most of these are made of rock debris but typical limestones consist of the skeletons and shells of marine organisms while coal is made of the remains of long-dead plants. o LIMESTONE Marine organisms absorb dissolved chalky minerals from seawater and use them to build their skeletons and shells. When they die the chalky calcareous material survives decay and builds up in layers on the seabed. Over time the layers may be compressed into chalk or limestone which often contains visible shell fragments. o FLINT Chalk is a soft white limestone made of the calcareous skeletons of countless microscopic marine organisms that lived roughly 100 million years ago in a shallow tropical sea. It often contains nodules of hard black flint which probably formed from the glassy skeletons of other organisms such as sponges. Flint is a lot like glass fracturing to produce razor-sharp edges and was widely used by early humans to make stone tools. o COAL If plant remains accumulate in waterlogged airless conditions they turn into peat. If the peat is buried deep beneath more sediments it can be compacted and heated to form coal a black shiny rock that can be burnt as fuel. The oldest hardest coal formed from plants that lived some 300 million years ago is almost pure carbon. FOSSILS Spider in amber is perfectly preserved down to every tiny detail of its body Only the hard shell of this ancient sea creature is preserved as stony fossil A FOSSILIZATION Most living things are destroyed after they die but a very few may be smothered by something that preserves them. Insects and spiders drowned in sticky tree sap millions of years ago are perfectly .
