Geology of Ma Shi Chau

The 61-hectare Ma Shi Chau Special Area is located in Tolo Harbour in the northeast New Territories.

檢視較大的地圖

It is made up of Ma Shi Chau, Yeung Chau, Centre Island and an unnamed islet to the northeast of Sam Mun Tsai New Village. It has high value of conservation because of its spectacular flora, fauna, geological and coastal features. Let's walk along the Nature Trail to see what geological features can be seen.

When we look back from the starting point, there is a tombolo that is a piece of land connecting Ma Shi Chau with the headland. It is formed over the years by sediments accumulating in one end of the island and extends to link with headland. The tombolo is perhaps 50 feet wide and the difference in water quality on both sides of it is marked. The harbour side, more open to the elements is more turbid and gets a lot of wave action, whereas the bay side is distinctly cleaner and calmer, the latter no doubt to the sheltering effect of the islands deflecting winds and large wave action.

Tombolo

Next, we can see the unconformity of tuff and conglomerate. These two types of rock are different in colour and texture, but they come in contact. This is called unconformity. Originally, conglomerate, a kind of sedimentary rock was formed by the accumulation and consolidation of sediments. It is composed of rounded pebbles cemented by sand and mud. Since the sediments are mostly brought down from the upstream parts of rivers, we can see why the edges of pebbles in conglomerate are rounded! It is reddish brown in colour and coarse-grained. Later on, volcanic ash was ejected during volcanic eruption. It cooled and settled on the conglomerate to form tuff, a kind of igneous rock It is grey in colour and has a fine texture.

Unconformity of tuff and conglomerate

A bit further away, a shell beach is seen. In the sheltered bay, the sea wave is weak, favouring the deposition of shells. Where do the shells come from? In the past, there was the rearing of pearls in the Tolo Channel. In the 1960s and 70s, this industry declined, but the shells remained. Another reason is that in the sheltered area, other shellish organisms also survive.

Shell Beach

In Shui Mong Tin, there was an illegal development. The trees were cut and cleared, the shell beach was levelled, various levels of platform were built, concreteretaining and boundary walls and drainage connections were constructed. The construction also blocks the geo-appreciation walking path during high tide. Why was it built there? What were the impacts on this area? Let's watch the video.

Further away, the oldest rock in Ma Shiu Chau appeared. it was formed during Permian period (about 280 to 225 million years ago).

The oldest rock in Ma Shi Chau

We can see the quartz veins again, but on a large scale. Under high temperature and pressure, the quartz dissolved in underground water. It intruded into the fracture. When temperature became lower, it cooled and solidified to form quartz porphyry. During weathering, the quartz was resistant, but other minerals were easily decomposed and became dark in colour. The quartz remained as quartz veins.

Quartz veins

Different types of rock have different resistance to weathering and erosion. After repeated wave erosion, the soft mudstone has been eroded and a depression appears. On the other hand, the hard sandstone or quartzphric dyke remains.

Differentaial erosion at Lung Lok Shui

When we looked further away, there was a wave-cut platform. How did it fom? The wave attacked the coast between high tide level and low tide level. A notch was formed by undercutting. Overhanging part collapsed, forming a sea cliff. As cliff retreated, a wave-cut platform is formed.

Wave-cut platform

Look! A sausage structure was being found. Originally, there are three layers of rock. The middle layer consist of the hard, but brittle sandstone which has darker colour, while the upper layer and lower layer are composed of soft siltstone and mudstone respectively. Their colour is lighter. Due to the deposition of sediment, the lower rock layer is compressed. This creates tensional force. The cracks develop in the sandstone. Continuous compression causes the soft mudstone to seep into the cracks in the sandstone. Then the sandstone breaks into different pieces. This looks like a broken line on the ground.

Sausage structure

Source:www.geopark.gov.hk/en_landforms1l.html


Folding in Ma Shi Chau was very intense. Sedimentary rocks are seriously folded because of compressional forces which were very great here.

Folding

Along the shoreline, the reddish brown rock is commonly seen. The sedimentary rock is broken and has been laid underground for many years. The groundwater which contains ferric oxide seeped into the joints of the rock, so the joints are reddish brown in colour. Later on, the overlying rock was weathered. The sedimentary rock has been exposed. Under strong sunlight, the rock is subjected to weathering, The joints are widened.

Endowing with invaluable geological heritage of over 280 millions years of age, this special area becomes an important part of Hong Kong Geopark. Any destruction or damage might be irreparable to the scientific, educational and aesthetic values of our geological heritage. So it is essential to consider how to balance between economic development and environmental conservation while tourism industry is blooming here.