Palaeontological sites and fossils include both surficial and bedrock deposits and the fossilized remains of both living and extinct species of plants and animals. They range from invertebrate fossils and fossil localities found in rocks of Palaeozoic age, through Mesozoic-aged plants and animal remains, particularly those of large land and marine reptiles, to plants and mammals of the Cenozoic. Significant mammalian fossils, such as mammoth or extinct species of bison, can also occur in glacial outwash and post-glacial deposits, generally in gravels. The locations of such isolated finds are extremely difficult to predict.