The secret of age - the oldest sedimentary rock in Ma Shi Chau

The oldest rock in Ma Shi Chau is located on its southeastern coast. It was formed during the Permian period (about 280 to 225 million years ago). The rock types are black shale, grey mudstone and siltstone. Do you know how geologists determined the age of these rocks?

The answer lies inside the rocks themselves. Their ages are primarily determined by the fossils they contain. Fossils are naturally preserved remains or evidence of past life. Some organisms died and their bodies were covered by sediments. After billions of years, the bodies turned into fossils while the surrounding materials became sedimentary rocks.

Some organisms are known to have existed within a specific period and had a broad geographical distribution. Their preserved bodies are called index fossils specifically because they can indicate the age of rocks. The key index fossils found here are ammonites, bivalves, brachiopods, and lamellibranches, which only survived up to the Permian era, one of the earliest geological age for sedimentary rocks formed in Hong Kong. In fact, what you are stepping on now is almost the oldest sedimentary rock in Hong Kong!

Apart from fossil dating, do you know any other ways to identify the age of rocks?