1.6 Shallow lake

From about 80 to 50 million years ago, in the late Cretaceous and early Tertiary, the climate became wetter and a broad shallow lake formed.

During each wet season, a thin layer of silt was deposited on the lake floor. Then in the dry season, as the lake evaporated, salts precipitated on the silt. These annual cycles appear today in the rocks on Ping Chau as countless thin layers of light and dark siltstone.

On the land surrounding the lake, a great variety of plants became established, along with many insects. Thus today the siltstone layers contain leaves, pollen, spores and insect fossils.

Many of the siltstone layers also display ripple marks formed by wave action in shallow water, and desiccation cracks formed when the lake dried out.

Credit : History Museum of HK.