North Ninepin includes Sai Mei Chau and Hok Tsai Pai.
The outcropped volcanic rock with columnar joints of the Ninepins is a type of acidic rhyolite volcanic rock. The rock is relatively consistent and has significant contents of potash feldspar and quartz phenocrysts. This rock set, known as the High Island Formation, is part of the Early Cretaceous Kau Sai Chau Volcanic Group.
Rocks of the Ninepins date back to the age of dinosaurs about 140 million years ago. It is believed that this group of islands were created by a major volcanic eruption which produced about 70 cubic kilometres of volcanic ash. A series of peaks emerged abruptly on the broad plain as a result. Given that Mount St. Helens spewed out only 2 cubic kilometres of ash when it exploded while the catastrophic Krakatau eruption of Indonesia in 1883 produced 12 cubic kilometres of volcanic substances, one can imagine the devastating magnitude of this ancient eruption. After rising, these peaks faced severe erosion over millions of years. Gradually they became the magnificent rugged landscape we see today.